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From: David Farber on 18 Jun 2010 18:47
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/farberbear/Repair/st-1700b/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


From: Michael A. Terrell on 19 Jun 2010 10:50

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/farberbear/Repair/st-1700b/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.
From: Michael A. Terrell on 20 Jun 2010 00:58

David Farber wrote:
>
> "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote in message
> news:T-ednVZAeKq1RIHRnZ2dnUVZ_s6dnZ2d(a)earthlink.com...
> >
> > 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/farberbear/Repair/st-1700b/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?


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