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

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.

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

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,

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

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.

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.