David Farber wrote:

"Michael A. Terrell" wrote in message
...

David Farber wrote:

One of my ~30 year old Sound Technology distortion/power analyzers has a
problem. It's been sitting around for a number of years because I had a
spare. The symptom is that when you are measuring distortion and move the
rotary selector switch one step from the 1% range to the .3% range, the
meter goes from a near zero reading to full deflection and then some. If
I feed the signal output to my other analyzer, the distortion is very low
so I know the oscillator is ok.

Here is a copy of the schematic:
http://members.dslextreme.com/users/.../st-1700b.html

At the output of U202, pin 6, the signal goes from zero (meter is working
properly) to a nice sine wave (meter pegs) when the switch is rotated.to
the .3% range and below. The signal is too low to measure at the input
of U202 no matter where the switch is. There is a very detailed circuit
description in the owner's manual. However I have a general sense that
there's an open circuit somewhere causing the gain to go full blast. I
cleaned the switches but it wasn't of any help. Anyone have any clever
ideas as to how to pinpoint the trouble?

Troubleshoot it. Is the switch part of an attenuator, or does it
switch in more gain for the last range? Look to see if the op amp is
oscillating. Look for bad electrolytics on the supply rails. It isn't
rocket science. A distortion analyzer is a tunable notch filter and
attenuator, followed by an AC voltmeter.


The first thing I did was to check all the caps. They're ok.

You can see from the schematic that the switch is part of the attenuator and
that U202 is before the attenuator switch. So my question is why does U202
suddenly have a wild signal swing when switched to the next lower step?

Thanks for your reply.


You have a lot of DC coupled stages, and some are not very good op
amps. A dc offset can make a string like that unstable. Did you look
at pin 6 of U202 with a low capacitance scope probe? How does the DC
voltage there compare to the working unit?

Are the op amps marked 2605 Harris HA2605? If so, they have been out
of production for some time. Metal cased op amps, (and other metal
cased ICs) started disapearing 10 years ago.

You might luck out and fix it with a couple .1 caps to ground of the
supply pins of U202 if it's oscillating. Another thing to check is all
the mounting screws for the PC board and any shields. Loosen the screws
and tighten them up to remove any oxide. Generally op amps oscillate
from defective bypass caps, or signals being coupled from another
circuit. Also, did you test the resistance for the contacts in that
mode after you cleaned the switches? An open or high resistance contact
cn upset the circuit.

One question. Is the sine wave close to the frequency the filter is
tuned to?


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