Where can I get an old board re-built?

[synchro]

Looking at some datasheets, the letters GR below the 2SH13 part number on the unijunction transistors designate that they've been sorted out for having a higher "intrinsic standoff ratio" η of 0.63-0.75, vs. the 0.47-0.75 range of unsorted parts.
The 2SH13 is a Japanese part that uses the JIS device numbering standard.
The closest part I found with a North American JEDEC designation is 2N494B, with the 2N493B a close second. I'll spare everyone of the details unless someone wants to know.

If a repair shop finds that they need to replace one of these devices, it would be best to also replace the other so that they'll be better matched. But the board would probably be functional if only one of them is replaced. Hopefully they will not need replacement, but it could still be wise to acquire a few of these devices because they're obsolete technology and becoming less and less available. I would try to get "new old stock" made by a well known manufacturer (GE, Motorola, TI, etc) rather than what might be China sourced parts.

 

[nuckythompson]

I finally got access to another card so I did what troubleshooting I could.

-Resistors all tested fine, just used a multimeter to measure resistance.
-Transformer continuity test, tested OK.
-Traces all look good.
-Not sure how I can test capacitors without removing from the circuit, same with the diodes, IGBT, and IC op amp.

-Found some discrepancies with the pots... the blue pots on the bad board read 9.4kΩ and 53kΩ while the other board had values of 14.5kΩ and 62kΩ. I gave the pots a few turns back and forth and put them back to resistances I measured originally.

-Also found differences in the single turn pots. Values below. Again, I gave the dial some turns and placed the pots back to original resistance readings. I did play a bit with the dial and noticed that I could not get the pots on the bad board to read as high of resistances as the good board.

Bad board values were:
VR1 1-2: 0.742kΩ VR2 1-2: 0.746kΩ
1-3: 1.58kΩ 1-3: 1.565kΩ
2-3: 0.847kΩ 2-3: 0.826kΩ
Good board read:
VR1 1-2: 0.866kΩ VR2 1-2: 0.913kΩ
1-3: 1.669kΩ 1-3: 1.727kΩ
2-3: 0.812kΩ 2-3: 0.824kΩ

Single turn pot datasheet: https://tocos.com/uploads/media/products/0001/01/4d257d0e6000c1dbc7d2a4c98a7f60fc16ec997c.pdf

I read that you can test capacitors in circuit with an ESR meter. The op amp was not removable so I am not sure how to test that in circuit, same with the IGBT and diodes. I don't want to risk any damage by taking the multimeter to them.

Any ideas how to test the rest of these devices in circuit?
Should I adjust the pots to match the working board as best I can?

Thanks!

 

[steve66]

The diodes can be tested by measuring resistance both directions (measure once, and swap the leads and measure again).

They should have very high (probably infinite) resistance in one direction, and low resistance in the other direction. But some of that depends on if anything is in parallel with them.

An analog ohm meter can make a quick check of larger capacitors, although they are polarity sensitive. If you connect the leads in the right direction, they should read low and then the resistance should increase as they charge up. Again, somewhat depends on what if anything is in parallel. Easiest to see on an analog meter, but for large caps you can even see it on a digital meter.

I think you are unlikely to damage anything with the low voltage and current of an ohmmeter.

 

[ptonsparky]

SEWAG is the values for the pots are close enough. You would want to adjust them to the values of the good boards if you can.

 

[synchro]

The single turn POTs are for nulling out the input DC offset of the OP amp that they're connected to, and so their appropriate setting will be unique to each individual OP amp.

The multi-turn POTs adjust the closed loop gain of the OP amps, and so they are likely used for compensating out the tolerances of parts in their corresponding signal channel. At this point I wouldn't change the settings of the POTs from where you found them.

Given the apparent age of the design, the two gray electrolytic capacitors really should be replaced in any case. They do degrade over time.
A reliable test of them would be difficult to make while on the board, especially if you don't have a schematic showing how they connect to other parts. And if they are removed they should be replaced anyway.
However, you could temporarily place a jumper across their leads to remove any possible residual charge. And then measure their resistance. You might be able to distinguish whether one is a dead short, but otherwise the measurement will not be very meaningful. It would not be unexpected that a good capacitor would show a resistance reading that changes vs. time.

 

[electrofelon]

A wild guess is that it's a demodulator for processing the two quadrature AC outputs of a "resolver" that's used to sense the angular position of a shaft. The two transformers on the board would step down these two AC signals which could be over 100 volts down to lower levels that the electronics can easily handle. The two demodulated signal outputs would represent the sine and cosine of the angular position. The two 1-turn POTs would be adjusted to get equal gains on the in-phase and quadrature channels, and the blue multi-turn POTs would be used to eliminate any DC offsets that might be present.

https://www.digikey.at/en/articles/...angular-position-and-velocity-with-a-resolver

That's exactly what I was about to say.

I thought it was a device for testing the amplitude linearity of a single sideband phone transceiver using two non-harmonically related audio frequency tones.....

 

[synchro]

I thought it was a device for testing the amplitude linearity of a single sideband phone transceiver using two non-harmonically related audio frequency tones.....

Oh, you mean a two-tone intermodulation test to determine the 3rd order intercept point?

https://en.wikipedia.org/wiki/Third-order_intercept_point

 

[nuckythompson]

Update:

-I tested the Zener diodes, they all look OK reading MΩ one way and OL the other.
-The diodes read MΩ both ways.
-Not sure about the capacitors... Some had resistance changing over time and some were a steady resistance. For the ones that changed over time, some would be increasing in resistance, some would decrease to a point and then increase, not too sure what this tells me. The two gray electrolytic capacitors changed over time.

Is the changing resistance supposed to go up and down?
For the capacitors that are not changing, does this tell me anything?

Thank you everyone!

 

[electrofelon]

Oh, you mean a two-tone intermodulation test to determine the 3rd order intercept point?

https://en.wikipedia.org/wiki/Third-order_intercept_point

Exactly...... did I really need to say two tone intermodulation test?? Pretty sure everyone knew that's what I was talking about

 

[ptonsparky]

Update:

-I tested the Zener diodes, they all look OK reading MΩ one way and OL the other.
-The diodes read MΩ both ways.
-Not sure about the capacitors... Some had resistance changing over time and some were a steady resistance. For the ones that changed over time, some would be increasing in resistance, some would decrease to a point and then increase, not too sure what this tells me. The two gray electrolytic capacitors changed over time.

Is the changing resistance supposed to go up and down?
For the capacitors that are not changing, does this tell me anything?

Thank you everyone!

Sounds like you are having fun and with all the help you are getting such as with this description:

Oh, you mean a two-tone intermodulation test to determine the 3rd order intercept point?

https://en.wikipedia.org/wiki/Third-order_intercept_point

You should have it up and running in... well, eventually, maybe.

 

[gadfly56]

Sounds like you are having fun and with all the help you are getting such as with this description:

You should have it up and running in... well, eventually, maybe.

Did you mean the heat death of the universe?

 

[electrofelon]

Did you mean the heat death of the universe?

I find the heat death scenario to be quite depressing. The Big rip possibility, although also not happy, is at least a little more interesting.

 

[zbang]

Not sure about the capacitors...

Basic rule for electrolytic caps- replace them (always). For the cost, it's seldom worth the effort of testing them.

For non-electrolytics- you can lift one lead and properly test, but may find that across-the-board LOL replacement is still easier and faster.