Sound Technology ST-1700B distortion analyzer measurement pegs meter on low range.

[David Farber]

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

[Michael A. Terrell]

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.

[David Farber]

"Michael A. Terrell" <mike.t...@earthlink.net> wrote in message
news:T-ednVZAeKq1RIHR...@earthlink.com...

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?

[Michael A. Terrell]

> 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?

[David Farber]

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,

[Michael A. Terrell]

Pushbutton switches are a pain in the ass. From the age of the
equipment I would guess that they were made by ALPS? I have had a lot
of bad interlocking pushbutton switches over the years. The silver
plating is low grade, and I've even seen it flake off. The contact lube
used in some switches is a low grade grease that washes away with
contact cleaner, exposing the contacts to contaminants. Mouser carried
some push button switches the last time I looked. I always used some GC
Tunerlube to replace the lube in pushbutton switches when I had to clean
them. Check out the http://www.mouser.com site to see if you can find
new switches, or turn the unit of and press the buttons 50 to 100 times
each to wear away any oxide, then relubricate the switches.

If you can't find what you need, post some pictures of the switches
and I'll see if I have any in my collection.

Switchcraft made my favorites, but they are quite expensive.

[David Farber]

Hi Michael,

Since I now can get good readings on the 100 x 10 range, I am not going to
worry about the 10 x 100 range. The switches are buried under some pc
boards. Not worth the trouble to fix.

Thanks for your reply.

[Michael A. Terrell]

David Farber wrote:
>
> Hi Michael,
>
> Since I now can get good readings on the 100 x 10 range, I am not going to
> worry about the 10 x 100 range. The switches are buried under some pc
> boards. Not worth the trouble to fix.
>
> Thanks for your reply.

You're welcome.