Sound Technology ST 1701A Not Settling In Distortion Mode

[jblmar]

My 1701A works fine except that in Distortion mode, the pointer will not settle completly. It will waver +/- 2 or 3 lines. After selecting a frequency and reading the distortion on the meter, if I press the 100 Hz or X10 or X100 switches, the set reacts as if a new frequency was selected. This is in the LOW DISTORTION mode. The FAST mode LED lights (as it would changing frequency), the meter pegs to the right (as it should) then the LED goes off and the pointer settles. It doesn't fully settle. There are times when the pointer swings to 2/3 of the scale then back. Pressing the above switches a few times returns the set to somewhat normal operation.
Anyone know of a good contact cleaner to use?

[1audio]

What you describe is correct behavior. When any change is made in the frequency or the switches are pressed it starts a timeout on the oscillator, part of the fast settling process.

How low does it go in self test 1 KHz 3V with the 400 and 30K filters on? If you get to .0015% you are near the performance limits. its possible to improve it from there but its a lot of work for a snall improvement.

ST warn's against cleaning the switches since the chemicals could cause leakage etc. I don't know what to say about a 20 year old switch array. If it was in LA you may need to. I would look at what Caig Labs DeoxIT® D-Series has that could be used.

[jblmar]

Takes about 3 seconds to go back to LOW DISTORTION mode. Same as when it works properly.
I think I corrected the problem. The capacitors located on the Ratio switch or a bad ground connection most likely. Removed the top and bottom panel and the set works fine now.
I'll check and clean the ground connections.
Last resort would be to clean switches

[john curl]

Just to contribute here, I have 3 ST 1700's in my lab at the moment, 2 are Audio1's and one is my own. They ALL have problems developed over decades. The switches stop working properly, the null circuits stop nulling, etc. Still, they are OK units, better than most stuff out there.

[jblmar]

I suspect the Sprague Tantalum caps on the RATIO switch as being the cause. Connecting my DMM for voltage test to the caps shows the following when the meter swings wildly.
Withe the DMM connected to - of the caps, the side leading to the ratio switch, the voltage rises slightly as the meter swings. It remains stable oyjerwise.

With the + side of the cap connected (this side is directly coupled to the output of the chip), voltage remains stable in either state.
Just a first look, but I may be on the right track.

Anyone knows where I can get those Sprague caps?

[1audio]

I don't have one handy to look at. Perhaps you can post a photo of the affected area. What options does it have?

The caps attached to the opamps at the output of the amplitude and phase null detectors are all large values. Are those what you are referring to? They are used for driving the photocells in the notch filters. They are used in integrating around the opamps that drive the leds connected to the photocells.

A good quality low leakage electrolytic would work OK. However I think you are looking at a symptom, not a cause. The network around the notch has the resistors from the tuning in it. An intermittent connection there would cause all kinds of grief. many of the connections in the box are using .025" square posts with push on berg connectors. These are quite reliable, for the first 10 years. Try remove/replace any around the switches to clean and renew the connections.

Do you have a scope to look at the unit?

[jblmar]

Demian,
The caps are,
C313
C314
C315

C324
C325
C326

From 1 uF to 100 uF. 10%. 150 is printed on the cap.
I'll get some pic for you.
BTW. The distortion reading you asked about (post 2) is .001%.
All direct to the RATIO switches.

[jblmar]

Originally Posted by 1audio

I don't have one handy to look at. Perhaps you can post a photo of the affected area. What options does it have?

The caps attached to the opamps at the output of the amplitude and phase null detectors are all large values. Are those what you are referring to? They are used for driving the photocells in the notch filters. They are used in integrating around the opamps that drive the leds connected to the photocells.

A good quality low leakage electrolytic would work OK. However I think you are looking at a symptom, not a cause. The network around the notch has the resistors from the tuning in it. An intermittent connection there would cause all kinds of grief. many of the connections in the box are using .025" square posts with push on berg connectors. These are quite reliable, for the first 10 years. Try remove/replace any around the switches to clean and renew the connections.

Do you have a scope to look at the unit?

Yes, I checked the oscillator output on the scope as well as frequency checks on my Fluke 1900A Counter. The pattern was excellent as well as the frequency on the 1900A. Rock solid over a few hour period.
The problem is either on the analyzer the switches or the caps.

[1audio]

Let me recap before further exploration-
1) Oscillator is stable (What happens to it when you press a button?)
2) Analyzer intermittantly jumps around but will settle to .001% or so.

Please confirm the following:
1) pressing a button on the frequency array that is already selected doesn't produce a massive transient on the oscillator.
2) The range switch causes the instability on the analyzer- Going down range sometimes causes fluctuations.

Places to check- first are the optocouplers. They are soldered in (unless on later units they used sockets). See if they are microphonic or connections around them are. They may need replacement, they do age a lot. Check that they are in the proper operating range, look at the current through the LED (voltage on series resistor) to see that its right and stable.
The rotary switch may have dirty contacts. Its silver plated and Deoxit should work well on it.
Do you have the IM option or any of the other options?

[jblmar]

emian,

Originally Posted by 1audio

Let me recap before further exploration-
1) Oscillator is stable (What happens to it when you press a button?)
2) Analyzer intermittantly jumps around but will settle to .001% or so.

Please confirm the following:
1) pressing a button on the frequency array that is already selected doesn't produce a massive transient on the oscillator.
2) The range switch causes the instability on the analyzer- Going down range sometimes causes fluctuations.

Places to check- first are the optocouplers. They are soldered in (unless on later units they used sockets). See if they are microphonic or connections around them are. They may need replacement, they do age a lot. Check that they are in the proper operating range, look at the current through the LED (voltage on series resistor) to see that its right and stable.
The rotary switch may have dirty contacts. Its silver plated and Deoxit should work well on it.
Do you have the IM option or any of the other options?

Demian,
To answer your questions.
Q1: When a preselected button is pressed (x10, 100 or x100 especially), the meter pegs in Distortion mode.
Q2: Never really settles and holds there. Always moving.

Q1A: Yes, produces 100% deflection of the meter in Distortion mode.
Q2A: Instability on all ranges that show a display.

I plan to view the output of the oscillator & analyzer sections on the scope over the weekend. I also plan to connect an ext oscillator to the 1701A and measure its distortion. This will indicate if the 1701A is reading the distortion of the 1701A oscillator or the problem is within the analyzer section.
Here's how to connect your 'scope to the 17xx;

From 17xx
Generator Out to Balanced Inputs.
Distortion Out to scope.
Monitor to scope.

I pulled and cleaned all Op Amps on both boards. Swaped like OP Amps to check if they were the issue. Also substituted a new NE5534H as a final check. Same results.
P.S. Mark and return OP Amps to original location.

When I first bought the 1701A (used) it arrived with little or no output from the oscillator as wll as a distorted pattern. Replaced U1 on the oscillator board. I had to by 10 pieces @ $8 each. Needless to say, I wasn't happy. On the up side, the oscillator now worked.

Cleaned INPUT, RATIO & ATTEN switches first with CRC then 100% Deoxit. ST warning relates to cleaners that were available 35 years ago.

I'll post my findings.

The 1701A has Option 003 (SET LEVEL).