• webcor B-124

    From MarkS@21:1/5 to All on Mon Aug 17 15:37:54 2020
    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit
    has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

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  • From Peter Wieck@21:1/5 to All on Tue Aug 18 03:59:23 2020
    In rank order of non-invasive options - most of which I expect you have done already:

    a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
    b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
    c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
    d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
    e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war
    radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.


    Best of luck with it!

    Peter Wieck
    Melrose Park, PA

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  • From tabbypurr@gmail.com@21:1/5 to Peter Wieck on Tue Aug 18 20:21:43 2020
    On Tuesday, 18 August 2020 11:59:25 UTC+1, Peter Wieck wrote:

    In rank order of non-invasive options - most of which I expect you have done already:

    a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
    b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
    c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
    d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
    e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war
    radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.


    Best of luck with it!

    Peter Wieck
    Melrose Park, PA

    pickup wires connected the wrong way round? Or passing too close to mains. Original OP transformer replaced with a non-humbucking one?
    Deck not connected to chassis (safely/indirectly!)


    NT

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  • From MarkS@21:1/5 to tabb...@gmail.com on Wed Aug 19 16:32:14 2020
    On Tuesday, August 18, 2020 at 11:21:45 PM UTC-4, tabb...@gmail.com wrote:
    On Tuesday, 18 August 2020 11:59:25 UTC+1, Peter Wieck wrote:

    In rank order of non-invasive options - most of which I expect you have done already:

    a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
    b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
    c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
    d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
    e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war
    radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.


    Best of luck with it!

    Peter Wieck
    Melrose Park, PA

    pickup wires connected the wrong way round? Or passing too close to mains. Original OP transformer replaced with a non-humbucking one?
    Deck not connected to chassis (safely/indirectly!)


    Seems all is in order. All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap. Reversed the cartridge leads, no change, and the lead integrity is uncompromised right to the RCA phono plug. I believe the OPT original as it
    still has rivets holding it to the chassis. I did replace the cartridge with one that's close to the original but not identical. Wondering if the output of this cart isn't as high as the original meaning could it be the original's output put it above the
    amps SN level?

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  • From MarkS@21:1/5 to MarkS on Wed Aug 19 17:17:05 2020
    On Wednesday, August 19, 2020 at 7:32:15 PM UTC-4, MarkS wrote:
    On Tuesday, August 18, 2020 at 11:21:45 PM UTC-4, tabb...@gmail.com wrote:
    On Tuesday, 18 August 2020 11:59:25 UTC+1, Peter Wieck wrote:

    In rank order of non-invasive options - most of which I expect you have done already:

    a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
    b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
    c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
    d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
    e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-
    war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.


    Best of luck with it!

    Peter Wieck
    Melrose Park, PA

    pickup wires connected the wrong way round? Or passing too close to mains. Original OP transformer replaced with a non-humbucking one?
    Deck not connected to chassis (safely/indirectly!)


    Seems all is in order. All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap. Reversed the cartridge leads, no change, and the lead integrity is uncompromised right to the RCA phono plug. I believe the OPT original as it
    still has rivets holding it to the chassis. I did replace the cartridge with one that's close to the original but not identical. Wondering if the output of this cart isn't as high as the original meaning could it be the original's output put it above the
    amps SN level?

    Looking at the Sams, there is a circuit version at has a 12SJ7 pentode connected in place of the 12SQ7, higher gain for sure. Was it there for a lower output cartridge perhaps?

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  • From Peter Wieck@21:1/5 to All on Thu Aug 20 10:04:31 2020
    What happens if you short the cartridge inputs?
    What is the voltage across the filaments? |
    And, have you checked the rectifier on a tester (if tube), or measured AC over DC on the output(s)?

    Peter Wieck
    Melrose Park, PA

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  • From John Robertson@21:1/5 to MarkS on Thu Aug 20 12:08:55 2020
    On 2020/08/17 3:37 p.m., MarkS wrote:
    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit
    has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!


    Does the hum vary up/down with the volume control?

    If yes, then it is pre-amp or cartridge (wiring) related.

    If the hum does not change with the volume setting then something is
    either wrong with the output tube(s) or the B+ supply.

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

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  • From MarkS@21:1/5 to John Robertson on Thu Aug 20 14:52:49 2020
    On Thursday, August 20, 2020 at 3:09:01 PM UTC-4, John Robertson wrote:
    On 2020/08/17 3:37 p.m., MarkS wrote:
    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams
    circuit has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

    Does the hum vary up/down with the volume control?

    If yes, then it is pre-amp or cartridge (wiring) related.

    If the hum does not change with the volume setting then something is
    either wrong with the output tube(s) or the B+ supply.

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

    OK,

    Peter: Sorry I didn't update the rectifier but I swapped the 35Z5 with several other known good with no change. I shorted the cart lead at the cart and no change so I went to the amp phono plug and shorted there- no change! So I'm sure its in the amp.

    John: The hum changes volume with the volume control.

    So, is it possible there is something with either the volume or tone pots or associated?

    Thanks,

    Mark

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  • From John Robertson@21:1/5 to MarkS on Thu Aug 20 17:19:07 2020
    On 2020/08/20 2:52 p.m., MarkS wrote:
    On Thursday, August 20, 2020 at 3:09:01 PM UTC-4, John Robertson wrote:
    On 2020/08/17 3:37 p.m., MarkS wrote:
    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams
    circuit has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

    Does the hum vary up/down with the volume control?

    If yes, then it is pre-amp or cartridge (wiring) related.

    If the hum does not change with the volume setting then something is
    either wrong with the output tube(s) or the B+ supply.

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

    OK,

    Peter: Sorry I didn't update the rectifier but I swapped the 35Z5 with several other known good with no change. I shorted the cart lead at the cart and no change so I went to the amp phono plug and shorted there- no change! So I'm sure its in the amp.

    John: The hum changes volume with the volume control.

    So, is it possible there is something with either the volume or tone pots or associated?

    Thanks,

    Mark


    Unlikely to be any of the pots, but usually this sort of hum is either
    injected (noise from wire placement for example) or B+ hum that is not
    well filtered. If the rectifier tube was bad chances are the hum would
    be loud enough even with the volum turned down.

    Can you unplug the input to the amplifier? Is that what you called the
    'amp phone plug'?

    Might there be some sort of Mute circuit that has a short to the frame?

    Otherwise might be a pre-amp tube that has an internal short - should
    show up on a half-decent tube tester.

    If you post the schematics one of us will probably take a look and
    advise better. Pretty much guessing right now!

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

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  • From tabbypurr@gmail.com@21:1/5 to MarkS on Fri Aug 21 02:43:01 2020
    On Thursday, 20 August 2020 00:32:15 UTC+1, MarkS wrote:
    On Tuesday, August 18, 2020 at 11:21:45 PM UTC-4, tabby wrote:
    On Tuesday, 18 August 2020 11:59:25 UTC+1, Peter Wieck wrote:

    In rank order of non-invasive options - most of which I expect you have done already:

    a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
    b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
    c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
    d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
    e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-
    war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.


    Best of luck with it!

    Peter Wieck
    Melrose Park, PA

    pickup wires connected the wrong way round? Or passing too close to mains. Original OP transformer replaced with a non-humbucking one?
    Deck not connected to chassis (safely/indirectly!)


    Seems all is in order. All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap. Reversed the cartridge leads, no change, and the lead integrity is uncompromised right to the RCA phono plug. I believe the OPT original as it
    still has rivets holding it to the chassis. I did replace the cartridge with one that's close to the original but not identical. Wondering if the output of this cart isn't as high as the original meaning could it be the original's output put it above the
    amps SN level?

    Show us the circuit, then we can see more precisely where potential issues lie & what to test. There were some amps where the main reservoir cap did not return to chassis.


    NT

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  • From Peter Wieck@21:1/5 to All on Fri Aug 21 08:26:32 2020
    All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap.

    Yabbut.... are you sure that there is only one (1) B-? Please check the schematic very carefully, as quite often, one-or-more filter caps is 'above ground', and connecting it/them to chassis B- will cause hum. And, as it happens, more often than not,
    that particular hum will be VC -dependent.

    Peter Wieck
    Melrose Park, PA

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  • From MarkS@21:1/5 to Peter Wieck on Fri Aug 21 12:38:15 2020
    On Friday, August 21, 2020 at 11:26:34 AM UTC-4, Peter Wieck wrote:
    All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap.
    Yabbut.... are you sure that there is only one (1) B-? Please check the schematic very carefully, as quite often, one-or-more filter caps is 'above ground', and connecting it/them to chassis B- will cause hum. And, as it happens, more often than not,
    that particular hum will be VC -dependent.
    Peter Wieck
    Melrose Park, PA

    They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?

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  • From Michael_A_Terrell@21:1/5 to MarkS on Fri Aug 21 16:24:02 2020
    MarkS wrote:

    They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?

    Create a free Dropbox account. Post it there as a public file, then
    post a link here.

    --
    Never piss off an Engineer!

    They don't get mad.

    They don't get even.

    They go for over unity! ;-)

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  • From John Robertson@21:1/5 to MarkS on Fri Aug 21 13:17:40 2020
    On 2020/08/21 12:38 p.m., MarkS wrote:
    On Friday, August 21, 2020 at 11:26:34 AM UTC-4, Peter Wieck wrote:
    All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap.
    Yabbut.... are you sure that there is only one (1) B-? Please check the schematic very carefully, as quite often, one-or-more filter caps is 'above ground', and connecting it/them to chassis B- will cause hum. And, as it happens, more often than not,
    that particular hum will be VC -dependent.
    Peter Wieck
    Melrose Park, PA

    They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?


    I don't know about the old binary usenet lists - if they still work, but probably best to put it on a free host that doesn't want emails, etc for harvesting...and give us the link.

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

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  • From MarkS@21:1/5 to terrell....@gmail.com on Fri Aug 21 15:22:38 2020
    On Friday, August 21, 2020 at 4:24:08 PM UTC-4, terrell....@gmail.com wrote:
    MarkS wrote:

    They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?
    Create a free Dropbox account. Post it there as a public file, then
    post a link here.

    --
    Never piss off an Engineer!

    They don't get mad.

    They don't get even.

    They go for over unity! ;-)

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is different (as stated before), the model I have mostly resembles the 12SQ7 version. I'm thinking mfg was done of the fly depending on available components. I want to get this damn thing off my
    bench so I can dig into an Ampex F44. I've never had something so simple give me so much grief (doing this for a friend of course). Been around tube electronics for >44 years. Restored many radios; built many amps. I have a decent scope if need be.

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  • From Fox's Mercantile@21:1/5 to MarkS on Fri Aug 21 18:38:38 2020
    On 8/21/20 5:22 PM, MarkS wrote:
    OK thanks! see if it works.

    I'm almost tempted to suggest a heater cathode short.


    --
    "I am a river to my people."
    Jeff-1.0
    WA6FWi
    http:foxsmercantile.com

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  • From Michael_A_Terrell@21:1/5 to MarkS on Fri Aug 21 20:26:11 2020
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is different (as stated before), the model I have mostly resembles the 12SQ7 version. I'm thinking mfg was done of the fly depending on available components. I want to get this damn thing off my
    bench so I can dig into an Ampex F44. I've never had something so simple give me so much grief (doing this for a friend of course). Been around tube electronics for >44 years. Restored many radios; built many amps. I have a decent scope if need be.

    What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you
    don't have another tube, you could use a resistor in place of its heater
    84 ohms at 2Watts would give a 12.6 drop in the heater string. 82 is the closest standard value. That, or use a Variac set to about 105 volts and
    short the two heater pins with the tube removed. One other possibility
    is a damaged tube socket that is leaking AC to the grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

    --
    Never piss off an Engineer!

    They don't get mad.

    They don't get even.

    They go for over unity! ;-)

    --- SoupGate-Win32 v1.05
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  • From Jim Mueller@21:1/5 to MarkS on Sat Aug 22 05:49:59 2020
    On Mon, 17 Aug 2020 15:37:54 -0700, MarkS wrote:

    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
    rid of. All caps replaced, leads dressed, you name it. According to the
    Sam's photofact, the circuit has several variants. The only difference I
    can see is the OPT. The Sams circuit has an OPT that has what looks like
    a bucking winding in it for the 50L6GT output tube cathode. This one
    does not. Any ideas or a mod that can minimize the hum? Thanks!

    Not to be obvious, but did you try replacing the tubes with known good
    ones? Or trying these tubes in a known working radio? Sometimes audio
    output tubes develop leakage from the heater to the control grid. This
    is more common in 50C5 and 35C5 types because of their small size, but
    octals can do it too. It's an easy test.

    The extra winding on the output transformer is for negative feedback; it
    isn't a hum bucking winding.

    --
    Jim Mueller wrongname@nospam.com

    To get my real email address, replace wrongname with eggmen.
    Then replace nospam with expressmail. Lastly, replace com with dk.

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  • From MarkS@21:1/5 to Jim Mueller on Sat Aug 22 04:42:31 2020
    On Saturday, August 22, 2020 at 1:50:01 AM UTC-4, Jim Mueller wrote:
    On Mon, 17 Aug 2020 15:37:54 -0700, MarkS wrote:

    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
    rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit has an OPT that has what looks like
    a bucking winding in it for the 50L6GT output tube cathode. This one
    does not. Any ideas or a mod that can minimize the hum? Thanks!
    Not to be obvious, but did you try replacing the tubes with known good
    ones? Or trying these tubes in a known working radio? Sometimes audio
    output tubes develop leakage from the heater to the control grid. This
    is more common in 50C5 and 35C5 types because of their small size, but
    octals can do it too. It's an easy test.

    The extra winding on the output transformer is for negative feedback; it isn't a hum bucking winding.

    --
    Jim Mueller wron...@nospam.com

    To get my real email address, replace wrongname with eggmen.
    Then replace nospam with expressmail. Lastly, replace com with dk.

    Yeah, I've replaced the tubes with at least 2 sets of known good ones, no difference. Agree, it does seem the extra winding is cathode feedback which is kind of cool in such a small amp. But this one doesn't have it.

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  • From John Robertson@21:1/5 to All on Sat Aug 22 17:02:44 2020
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is different
    (as stated before), the model I have mostly resembles the 12SQ7
    version. I'm thinking mfg was done of the fly depending on available
    components. I want to get this damn thing off my bench so I can dig
    into an Ampex F44. I've never had something so simple give me so much
    grief (doing this for a friend of course). Been around tube
    electronics for >44 years. Restored many radios; built many amps. I
    have a decent scope if need be.

       What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you
    don't have another tube, you could use a resistor in place of its heater
    84 ohms at 2Watts would give a 12.6 drop in the heater string. 82 is the closest standard value. That, or use a Variac set to about 105 volts and short the two heater pins with the tube removed. One other possibility
    is a damaged tube socket that is leaking AC to the grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.


    If R2 is the volume control, and the hum goes away when reduced then you
    have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.

    Is the phono input ground shell connection compromised? Any chance it is actually not tied to ground?

    John :-#)#
    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

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  • From tabbypurr@gmail.com@21:1/5 to MarkS on Sat Aug 22 16:18:03 2020
    On Friday, 21 August 2020 23:22:41 UTC+1, MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is different (as stated before), the model I have mostly resembles the 12SQ7 version. I'm thinking mfg was done of the fly depending on available components. I want to get this damn thing off my
    bench so I can dig into an Ampex F44. I've never had something so simple give me so much grief (doing this for a friend of course). Been around tube electronics for >44 years. Restored many radios; built many amps. I have a decent scope if need be.

    post it as an image file maybe

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Michael_A_Terrell@21:1/5 to John Robertson on Sat Aug 22 23:26:51 2020
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is
    different (as stated before), the model I have mostly resembles
    the 12SQ7 version. I'm thinking mfg was done of the fly depending
    on available components. I want to get this damn thing off my bench
    so I can dig into an Ampex F44. I've never had something so simple
    give me so much grief (doing this for a friend of course). Been
    around tube electronics for >44 years. Restored many radios; built
    many amps. I have a decent scope if need be.


    Webcor built some models for several years, so ECOs would occur.


    What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you
    don't have another tube, you could use a resistor in place of its
    heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
    82 is the closest standard value. That, or use a Variac set to about
    105 volts and short the two heater pins with the tube removed. One
    other possibility is a damaged tube socket that is leaking AC to the
    grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.


    If R2 is the volume control, and the hum goes away when reduced then
    you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


    He said he had tried multiple tubes. A bad pot can cause problems,
    as well. Some need the case soldered to ground, because the thing isn't
    built on a metal chassis. The tip about eliminating a tube for a test is
    a general suggestion for troubleshooting. If it uses wafer sockets,
    leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is actually not tied to ground?

    Is the shield intact? I replaced the lead on more than one turntable because the repeated flexing had worn through the braid.


    --
    Never piss off an Engineer!

    They don't get mad.

    They don't get even.

    They go for over unity! ;-)

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jim Mueller@21:1/5 to Jim Mueller on Mon Aug 24 00:52:22 2020
    On Mon, 24 Aug 2020 00:49:03 +0000, Jim Mueller wrote:

    On Mon, 17 Aug 2020 15:37:54 -0700, MarkS wrote:

    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
    rid of. All caps replaced, leads dressed, you name it. According to the
    Sam's photofact, the circuit has several variants. The only difference
    I can see is the OPT. The Sams circuit has an OPT that has what looks
    like a bucking winding in it for the 50L6GT output tube cathode. This
    one does not. Any ideas or a mod that can minimize the hum? Thanks!

    Somewhere in this thread, you said that turning the volume down
    eliminated the hum but shorting the phono input jack did not. Someone
    else said that means that the problem is in C4, C5, or R3. I would add
    the phono jack itself to that list. But my guess at this point is C4.
    Is it the correct value? Is it good (not the same as being new)? Is it connected correctly, including the ground connection? Try using your ohmmeter from B- to the bottom end of C4 and the shield of the phono
    jack. They should both show 0 ohms. Use the ohms function of your
    meter, not the continuity function.

    OOPS. The shield of the phono jack should NOT show continuity to B- but
    should show the capacitance of C4 if you have a capacitance tester.



    --
    Jim Mueller wrongname@nospam.com

    To get my real email address, replace wrongname with eggmen.
    Then replace nospam with expressmail. Lastly, replace com with dk.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Jim Mueller@21:1/5 to MarkS on Mon Aug 24 00:49:03 2020
    On Mon, 17 Aug 2020 15:37:54 -0700, MarkS wrote:

    This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
    rid of. All caps replaced, leads dressed, you name it. According to the
    Sam's photofact, the circuit has several variants. The only difference I
    can see is the OPT. The Sams circuit has an OPT that has what looks like
    a bucking winding in it for the 50L6GT output tube cathode. This one
    does not. Any ideas or a mod that can minimize the hum? Thanks!

    Somewhere in this thread, you said that turning the volume down
    eliminated the hum but shorting the phono input jack did not. Someone
    else said that means that the problem is in C4, C5, or R3. I would add
    the phono jack itself to that list. But my guess at this point is C4.
    Is it the correct value? Is it good (not the same as being new)? Is it connected correctly, including the ground connection? Try using your
    ohmmeter from B- to the bottom end of C4 and the shield of the phono
    jack. They should both show 0 ohms. Use the ohms function of your
    meter, not the continuity function.



    --
    Jim Mueller wrongname@nospam.com

    To get my real email address, replace wrongname with eggmen.
    Then replace nospam with expressmail. Lastly, replace com with dk.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Robertson@21:1/5 to All on Sun Aug 23 18:45:08 2020
    On 2020/08/22 8:26 p.m., Michael_A_Terrell wrote:
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is
    different  (as stated before), the model I have mostly resembles
    the 12SQ7 version. I'm thinking mfg was done of the fly depending
    on available components. I want to get this damn thing off my bench
    so I can dig into an Ampex F44. I've never had something so simple
    give me so much grief (doing this for a friend of course). Been
    around tube electronics for >44 years. Restored many radios; built
    many amps. I have a decent scope if need be.


       Webcor built some models for several years, so ECOs would occur.


        What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you
    don't have another tube, you could use a resistor in place of its
    heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
    82 is the closest standard value. That, or use a Variac set to about
    105 volts and short the two heater pins with the tube removed. One
    other possibility is a damaged tube socket that is leaking AC to the
    grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.


    If R2 is the volume control, and the hum goes away when reduced then
    you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


       He said he had tried multiple tubes. A bad pot can cause problems,
    as well. Some need the case soldered to ground, because the thing isn't built on a metal chassis. The tip about eliminating a tube for a test is
    a general suggestion for troubleshooting. If it uses wafer sockets,
    leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is
    actually not tied to ground?

       Is the shield intact? I replaced the lead on more than one turntable because the repeated flexing had worn through the braid.



    Good point, that fits equally well with my thought the ground connection
    at the RCA jack is possibly compromised.

    So, Mark, what is the word?

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From MarkS@21:1/5 to John Robertson on Sun Aug 23 19:26:33 2020
    On Sunday, August 23, 2020 at 9:45:14 PM UTC-4, John Robertson wrote:
    On 2020/08/22 8:26 p.m., Michael_A_Terrell wrote:
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is
    different (as stated before), the model I have mostly resembles
    the 12SQ7 version. I'm thinking mfg was done of the fly depending
    on available components. I want to get this damn thing off my bench
    so I can dig into an Ampex F44. I've never had something so simple
    give me so much grief (doing this for a friend of course). Been
    around tube electronics for >44 years. Restored many radios; built
    many amps. I have a decent scope if need be.


    Webcor built some models for several years, so ECOs would occur.


    What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you
    don't have another tube, you could use a resistor in place of its
    heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
    82 is the closest standard value. That, or use a Variac set to about
    105 volts and short the two heater pins with the tube removed. One
    other possibility is a damaged tube socket that is leaking AC to the
    grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.


    If R2 is the volume control, and the hum goes away when reduced then
    you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


    He said he had tried multiple tubes. A bad pot can cause problems,
    as well. Some need the case soldered to ground, because the thing isn't built on a metal chassis. The tip about eliminating a tube for a test is
    a general suggestion for troubleshooting. If it uses wafer sockets,
    leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is >> actually not tied to ground?

    Is the shield intact? I replaced the lead on more than one turntable because the repeated flexing had worn through the braid.


    Good point, that fits equally well with my thought the ground connection
    at the RCA jack is possibly compromised.

    So, Mark, what is the word?
    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

    Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From MarkS@21:1/5 to MarkS on Mon Aug 24 16:08:30 2020
    On Sunday, August 23, 2020 at 10:26:34 PM UTC-4, MarkS wrote:
    On Sunday, August 23, 2020 at 9:45:14 PM UTC-4, John Robertson wrote:
    On 2020/08/22 8:26 p.m., Michael_A_Terrell wrote:
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is
    different (as stated before), the model I have mostly resembles
    the 12SQ7 version. I'm thinking mfg was done of the fly depending >>>> on available components. I want to get this damn thing off my bench >>>> so I can dig into an Ampex F44. I've never had something so simple >>>> give me so much grief (doing this for a friend of course). Been
    around tube electronics for >44 years. Restored many radios; built >>>> many amps. I have a decent scope if need be.


    Webcor built some models for several years, so ECOs would occur.


    What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you >>> don't have another tube, you could use a resistor in place of its
    heater 84 ohms at 2Watts would give a 12.6 drop in the heater string. >>> 82 is the closest standard value. That, or use a Variac set to about >>> 105 volts and short the two heater pins with the tube removed. One
    other possibility is a damaged tube socket that is leaking AC to the >>> grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.


    If R2 is the volume control, and the hum goes away when reduced then
    you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


    He said he had tried multiple tubes. A bad pot can cause problems,
    as well. Some need the case soldered to ground, because the thing isn't built on a metal chassis. The tip about eliminating a tube for a test is a general suggestion for troubleshooting. If it uses wafer sockets, leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is
    actually not tied to ground?

    Is the shield intact? I replaced the lead on more than one turntable because the repeated flexing had worn through the braid.


    Good point, that fits equally well with my thought the ground connection at the RCA jack is possibly compromised.

    So, Mark, what is the word?
    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."
    Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

    OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .0022
    cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp like
    this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From John Robertson@21:1/5 to MarkS on Mon Aug 24 17:46:41 2020
    On 2020/08/24 4:08 p.m., MarkS wrote:
    On Sunday, August 23, 2020 at 10:26:34 PM UTC-4, MarkS wrote:
    On Sunday, August 23, 2020 at 9:45:14 PM UTC-4, John Robertson wrote:
    On 2020/08/22 8:26 p.m., Michael_A_Terrell wrote:
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is
    different (as stated before), the model I have mostly resembles
    the 12SQ7 version. I'm thinking mfg was done of the fly depending >>>>>>> on available components. I want to get this damn thing off my bench >>>>>>> so I can dig into an Ampex F44. I've never had something so simple >>>>>>> give me so much grief (doing this for a friend of course). Been
    around tube electronics for >44 years. Restored many radios; built >>>>>>> many amps. I have a decent scope if need be.


    Webcor built some models for several years, so ECOs would occur.


    What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you >>>>>> don't have another tube, you could use a resistor in place of its
    heater 84 ohms at 2Watts would give a 12.6 drop in the heater string. >>>>>> 82 is the closest standard value. That, or use a Variac set to about >>>>>> 105 volts and short the two heater pins with the tube removed. One >>>>>> other possibility is a damaged tube socket that is leaking AC to the >>>>>> grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.


    If R2 is the volume control, and the hum goes away when reduced then >>>>> you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


    He said he had tried multiple tubes. A bad pot can cause problems,
    as well. Some need the case soldered to ground, because the thing isn't >>>> built on a metal chassis. The tip about eliminating a tube for a test is >>>> a general suggestion for troubleshooting. If it uses wafer sockets,
    leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is >>>>> actually not tied to ground?

    Is the shield intact? I replaced the lead on more than one turntable
    because the repeated flexing had worn through the braid.


    Good point, that fits equally well with my thought the ground connection >>> at the RCA jack is possibly compromised.

    So, Mark, what is the word?
    John :-#)#

    --
    Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

    OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .0022
    cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp like
    this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.


    Is the Volume Control (VC) floating from the chassis? Perhaps the hum is
    coming in via either the Common connection on the VC or perhaps the VC
    is mounted to something metal that is not common to the amplifier chassis?

    Is the RCA input to the amp unplugged while you have been doing these tests?

    John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From MarkS@21:1/5 to John Robertson on Wed Aug 26 08:42:52 2020
    On Monday, August 24, 2020 at 8:46:47 PM UTC-4, John Robertson wrote:
    On 2020/08/24 4:08 p.m., MarkS wrote:
    On Sunday, August 23, 2020 at 10:26:34 PM UTC-4, MarkS wrote:
    On Sunday, August 23, 2020 at 9:45:14 PM UTC-4, John Robertson wrote:
    On 2020/08/22 8:26 p.m., Michael_A_Terrell wrote:
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is
    different (as stated before), the model I have mostly resembles >>>>>>> the 12SQ7 version. I'm thinking mfg was done of the fly depending >>>>>>> on available components. I want to get this damn thing off my bench >>>>>>> so I can dig into an Ampex F44. I've never had something so simple >>>>>>> give me so much grief (doing this for a friend of course). Been >>>>>>> around tube electronics for >44 years. Restored many radios; built >>>>>>> many amps. I have a decent scope if need be.


    Webcor built some models for several years, so ECOs would occur.


    What happens when you short the wiper of the volume control to
    ground? Jeff may be right about the Heater to Cathode leakage. If you >>>>>> don't have another tube, you could use a resistor in place of its >>>>>> heater 84 ohms at 2Watts would give a 12.6 drop in the heater string. >>>>>> 82 is the closest standard value. That, or use a Variac set to about >>>>>> 105 volts and short the two heater pins with the tube removed. One >>>>>> other possibility is a damaged tube socket that is leaking AC to the >>>>>> grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube. >>>>>>

    If R2 is the volume control, and the hum goes away when reduced then >>>>> you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


    He said he had tried multiple tubes. A bad pot can cause problems,
    as well. Some need the case soldered to ground, because the thing isn't >>>> built on a metal chassis. The tip about eliminating a tube for a test is
    a general suggestion for troubleshooting. If it uses wafer sockets, >>>> leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is
    actually not tied to ground?

    Is the shield intact? I replaced the lead on more than one turntable >>>> because the repeated flexing had worn through the braid.


    Good point, that fits equally well with my thought the ground connection >>> at the RCA jack is possibly compromised.

    So, Mark, what is the word?
    John :-#)#

    --
    Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

    OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .
    0022 cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp
    like this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

    Is the Volume Control (VC) floating from the chassis? Perhaps the hum is coming in via either the Common connection on the VC or perhaps the VC
    is mounted to something metal that is not common to the amplifier chassis?

    Is the RCA input to the amp unplugged while you have been doing these tests? John :-#)#

    --
    (Please post followups or tech inquiries to the USENET newsgroup)
    John's Jukes Ltd.
    MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
    (604)872-5757 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."

    Hi, the VC terminals are floating from the metal chassis but the VC body is mounted to the chassis. One thing about the hum I hear, the frequency seems to change as you rotate the VC from min to max. When at minimum, no hum- rotate towards max and ~ mid
    span there are higher frequency harmonics then back to lower frequency hum at max volume. Phono RCA is disconnected & connected through out these tests, doesn't seem to effect the hum. Going to poke around more tonite, maybe hang a scope on it.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From MarkS@21:1/5 to MarkS on Wed Aug 26 17:10:33 2020
    On Wednesday, August 26, 2020 at 11:42:55 AM UTC-4, MarkS wrote:
    On Monday, August 24, 2020 at 8:46:47 PM UTC-4, John Robertson wrote:
    On 2020/08/24 4:08 p.m., MarkS wrote:
    On Sunday, August 23, 2020 at 10:26:34 PM UTC-4, MarkS wrote:
    On Sunday, August 23, 2020 at 9:45:14 PM UTC-4, John Robertson wrote: >>> On 2020/08/22 8:26 p.m., Michael_A_Terrell wrote:
    John Robertson wrote:
    On 2020/08/21 5:26 p.m., Michael_A_Terrell wrote:
    MarkS wrote:

    https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

    OK thanks! see if it works. I should tell you 1) The OPT is >>>>>>> different (as stated before), the model I have mostly resembles >>>>>>> the 12SQ7 version. I'm thinking mfg was done of the fly depending >>>>>>> on available components. I want to get this damn thing off my bench
    so I can dig into an Ampex F44. I've never had something so simple >>>>>>> give me so much grief (doing this for a friend of course). Been >>>>>>> around tube electronics for >44 years. Restored many radios; built >>>>>>> many amps. I have a decent scope if need be.


    Webcor built some models for several years, so ECOs would occur.


    What happens when you short the wiper of the volume control to >>>>>> ground? Jeff may be right about the Heater to Cathode leakage. If you
    don't have another tube, you could use a resistor in place of its >>>>>> heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
    82 is the closest standard value. That, or use a Variac set to about
    105 volts and short the two heater pins with the tube removed. One >>>>>> other possibility is a damaged tube socket that is leaking AC to the
    grid.

    http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube. >>>>>>

    If R2 is the volume control, and the hum goes away when reduced then >>>>> you have something sneaking in via C5, R3 or C4.

    Or the wiring for same components.

    Can't see the 12SQ7 as the cause as it is after the pot.


    He said he had tried multiple tubes. A bad pot can cause problems, >>>> as well. Some need the case soldered to ground, because the thing isn't
    built on a metal chassis. The tip about eliminating a tube for a test is
    a general suggestion for troubleshooting. If it uses wafer sockets, >>>> leakage is a real possibility.


    Is the phono input ground shell connection compromised? Any chance it is
    actually not tied to ground?

    Is the shield intact? I replaced the lead on more than one turntable >>>> because the repeated flexing had worn through the braid.


    Good point, that fits equally well with my thought the ground connection
    at the RCA jack is possibly compromised.

    So, Mark, what is the word?
    John :-#)#

    --
    Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

    OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .
    0022 cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp
    like this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

    Is the Volume Control (VC) floating from the chassis? Perhaps the hum is coming in via either the Common connection on the VC or perhaps the VC
    is mounted to something metal that is not common to the amplifier chassis?

    Is the RCA input to the amp unplugged while you have been doing these tests?
    John :-#)#

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    (Please post followups or tech inquiries to the USENET newsgroup)
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    Hi, the VC terminals are floating from the metal chassis but the VC body is mounted to the chassis. One thing about the hum I hear, the frequency seems to change as you rotate the VC from min to max. When at minimum, no hum- rotate towards max and ~
    mid span there are higher frequency harmonics then back to lower frequency hum at max volume. Phono RCA is disconnected & connected through out these tests, doesn't seem to effect the hum. Going to poke around more tonite, maybe hang a scope on it.

    Update- the VC measured over 3 meg when it should be 1 meg. Also, R 10 is connected to pin 3 of the 35Z5 versus pin 2 as shown in the schematic. R5 might be an issue. So, new components ordered, will update when they are received and in place.
    Thanks, Mark

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  • From MarkS@21:1/5 to All on Sun Aug 30 08:22:04 2020
    Update- the VC measured over 3 meg when it should be 1 meg. Also, R 10 is connected to pin 3 of the 35Z5 versus pin 2 as shown in the schematic. R5 might be an issue. So, new components ordered, will update when they are received and in place.
    Thanks, Mark

    OK, we have success, thing works great. Replacing the VC with a new 1M and replacing R10 & R5 did the trick. Regarding R10; the Sams has a picture of the chassis showing component location and in the pic, R10 is obviously connected to pin 3, not pin 2 so
    I left it that way. I didn't do it one component at a time so not sure which made the biggest improvement. Shot gunned so to speak 'cause I just wanted to be done with it.

    Want to thank you all for the inputs, really helps when sorting things out. Also great to see some of the old names I remember back here in a single thread.

    Very best regards,

    Mark

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