Michael A. Terrell wrote:
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?

Hi Michael,

The oscillating is at the same frequency of the signal output (1kHz) so I
assume that is not a parasitic oscillation. I make a mistake in identifying
the correct IC, I should have said U203 is where the wild voltage swing
takes place.

Here are some readings I made with my low capacity 10:1 probe:

U202, pin 6 output, selector set to 1% range, 4.8 Vp-p.
U203, pin 6 output, selector set to 1% range, 0.0 Vp-p.

U202, pin 6 output, selector set to .3% range, 3.8 Vp-p.
U203, pin 6 output, selector set to .3% range, 2.8 Vp-p.

I was about to end my message at this point when I went back and re-read
your message to make sure I didn't leave anything out. I noticed the part
where you mentioned comparing the good analyzer to the bad one. So I
disassembled the good one and checked the dc voltages first. They were ok as
were the dc voltages on the bad one. On the AC side, of course the voltages
were zero at the output of U203 on the good one. Here comes the fun part.
While probing around the bad one again, I could hear the meter pegging as I
was touching different pins trying to hold my hand steady. All of a sudden,
the signal was gone and the meter stopped pegging. It seems to be working
correctly now. BUT if I move the frequency select switches from X100 on the
top row, X10 on the next row to X10 on the top row, X100 on the next row
(still maintaining a 1kHz signal), the distortion is much higher, over 3%.
If I gently tap the X100 switch on row 2, the meter jumps around like a bad
tape monitor switch but never drops below 3%. There is a yellow sticker on
the shield that covers the frequency selector switches that says, "WARNING
DO NOT SPRAY SWITCHES. PUSHBUTTON SWITCHES ARE PERMANENTLY LUBRICATED." etc.
etc. Perhaps the switches are causing all of these symptoms?

Thanks for your reply,

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