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?

Thanks for your reply.
--
David Farber
David Farber's Service Center
L.A., CA