I had two Sound Technology 1700B analyzers. One worked fine. The other,
which I gave away to a fellow tech friend, had a problem only when you
were trying to make a distortion measurement that was less than 1% (e.g.
0.3%, o.1% etc.). When you enter those smaller ranges, the analog meter
pegs. I searched for a service manual online and fortunately was able to
find it though now I have to troubleshoot from long distance. You can
download it from here:
https://app.box.com/s/1l0s5z4zlah43krmfmeonhn1srva1fha (pages 43-44
contain the schematic of the distortion analyzer section)

When trying to analyze what circuitry changes occur when shifting from
the 1% distortion scale to any level below, I am stumped by the
schematic diagram of the Ratio/dB switch. In one area on page 44,
labeled "ATTENUATOR," it is self-explanatory that the wiper on one of
the wafers steps through a voltage divider network that changes the
level by +/- 10dB for each turn of the switch. I was thinking that maybe
one of the resistors in the voltage divider opened up to upset the
balance of the divider network however any resistor (R219-R233) that
opens up in that network would effect the higher range as well as the
lower range. So far, everything in the schematic makes sense to me.

Next if you scroll down and to the left you will come across the, "PHASE
NULL CONTROL" and the AMPLITUDE NULL CONTROL." Again the RATIO switch is
tied into these circuits. Here is where I cannot understand how the
circuit changes as the switch is moving the contacts from one position
to the next. Let's start with the PHASE NULL CONTROL circuit. On the
left side of the diagram, you see S2AF and underneath it is a vertical,
solid black line. In the middle of the black line there is a terminal
labeled, "6." which appears to be connected to the .1 contact of the
Ratio switch. If that vertical, solid black line means that those top 5
nodes are all in common, then the positive ends of C314 and C315 should
be tied together. Using my working machine, a quick check with my
multimeter verifies those capacitors ore now wired in parallel however
if I move the switch two clicks to the 1% range, then according to the schematic, those capacitors should not be in wired in parallel. My
multimeter says they are still wired in parallel. Now either the
schematic drawing is incorrect or I am misinterpreting how the RATIO
switch is shifting its connections as it steps through the different
ranges. The same situation arises in the very similar AMPLITUDE RANGE
CONTROL CIRCUIT. Capacitors C325 and C326 are wired in parallel no
matter where the function switch is turned.

In summary, I think something is throwing off the DC input voltage to
one of the op amps and I was suspecting a leaky or shorted cap however
since I now have doubts about the circuit wiring in the schematic as
compared to what I see inside the analyzer, I am not so sure.

Thanks for your replies.

--
David Farber
Los Osos, CA

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