I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in both directions on an ohm-meter !!!
You have a new type of device! Save it. It just might be worth
something... someday... maybe...
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
both directions on an ohm-meter !!!
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
both directions on an ohm-meter !!!
On 2024/03/14 1:56 p.m., Liz Tuddenham wrote:
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in both directions on an ohm-meter !!!
Can digital ohm-meters be fooled by ESR?
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
both directions on an ohm-meter !!!
On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the >>finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in >>both directions on an ohm-meter !!!
Used to keep a special box for spectacularly wrong parts, but never
found a situation where their re-use was justified, even as a joke.
Everybody takes themselves so seriously these days. At least each part
had a story behind it . . . .
RL
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
both directions on an ohm-meter !!!
On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the >finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in >both directions on an ohm-meter !!!
Was it a light blue Sanyo?
Chuck <chuck23@dejanews.net> wrote:
On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It
is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in
both directions on an ohm-meter !!!
Was it a light blue Sanyo?
It was indeed!
On Sat, 16 Mar 2024 20:44:58 +0000, liz@poppyrecords.invalid.invalidThose Sanyo caps caused many weird problems in Pioneer receivers. Back
(Liz Tuddenham) wrote:
Chuck <chuck23@dejanews.net> wrote:
On Thu, 14 Mar 2024 20:56:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier. It >>> >is a complex beast and one channel had gone down to almost nothing.
Luckily the service sheet was available and I traced the fault to the
board which houses the tone controls and filters.
The gain is in two stages with the tone controls around the second
stage. On one of the channels, the voltages on the single transistor
for that stage were all wrong. After lifting the transistor off the
board, it became apparent that the base bias was far too high and the
finger of suspicion pointed to the inter-stage coupling capacitor
feeding in leakage current from the preceding stage.
I tested the capacitor, which was marked "4.7". It read exactly 4.7k in >>> >both directions on an ohm-meter !!!
Was it a light blue Sanyo?
It was indeed!
Sounds like a known issue, then. Unless Chuck is psychic, of course.
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