"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.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

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.

--
David Farber
David Farber's Service Center
L.A., CA