Where can I get an old board re-built?

[W@ttson]

These guys do repairs on legacy devices:

Cygnus LLC - Electronic & Electromechanical Design & Manufacturing

Over 30 yers of experience, full range of electronic design and manufacturing services, trusted vendor for a variety of industries including defense, transporation, medical, telecom, and others.

www.cygnusnj.com

From their site:

"
REPAIR / REFURBISH

We have extensive experience in the repair and refurbishing of all kinds of electronic devices. Our specialty is bringing back to life critical legacy devices, long out of production and support, for the transportation and military industries."

 

[nuckythompson]

Is there evidence that the board itself (very likely made of epoxy-fiberglass like FR-4) or its circuit traces are damaged? If not, perhaps one or more failed components could be identified and replaced. It appears to be maybe a 1970s or 1980s vintage design based on what look like TO-8 hermetic packages for the 4 semiconductors on the upper right. The upper two are likely to be OP-amps or comparators, with signal levels adjusted by the two single-turn potentiometers, and the two blue multi-turn POTs perhaps adjusting an offset or threshold.The semiconductors labeled Q1 and Q2 are likely to be transistors if they have three leads.

The two electrolytic capacitors on the lower right are the most likely components to have failed given the age of the board. The semiconductors would be most vulnerable to any overvoltages / surges. Of course, a visible inspection should be done to identify any apparent physical damage on the components. At least continuity tests could be done on the transformers. Since there appear to be two identical signal channels on the board, resistance measurements across corresponding points on the two circuits could be made (including across individual components) to see whether any significant differences can be found. Such differences could help narrow down where a failed component might be.

Can you tell us what the basic function of the board is, or what type of equipment that it's in? What kind of symptoms did the board failure present?

I see that gar has just posted similar comments,

Thanks for the response. It is part of a motor governor system. Sends raise and lower commands when the unit is in auto. When in manual the raise and lower commands are controlled by external push buttons. The manual mode still functions.

I can't really see any visible damage and it has a hard lacquer coating making it difficult to troubleshoot. I was able to check continuity on the transformers and they appear to be OK. I am reluctant to attempt any repairs because I do not have any spare boards. I do have spare components from old failed boards but I am worried I will risk damaging our only unit.

Any advice on how I would go about adjusting/testing the pots?

 

[steve66]

What do the POT's adjust?

Testing them should be fairly easy with an ohmmeter. Measure the resistance between the terminals and see if it changes as the pot is turned.

Hopefully, there isn't much parallel resistance on the POTs to affect the readings.

 

[nuckythompson]

What do the POT's adjust?

Testing them should be fairly easy with an ohmmeter. Measure the resistance between the terminals and see if it changes as the pot is turned.

Hopefully, there isn't much parallel resistance on the POTs to affect the readings.

Good question. I am waiting on a schematic from OEM so I can understand this myself.

 

[synchro]

Near the POTs are what appear to be two semiconductor devices in TO-8 packages. Their higher lead count indicates they contain integrated circuits. They also appear to be inserted into white colored IC sockets. That notion is confirmed by the lack of a yellowish conformal coating on the device and within small circular area surrounding the socket.

And so if they are available, these semiconductors might be replaced without any soldering involved. The location of the metal tab on the package should be marked on the socket to insure that a replacement is properly oriented.

Can you read the labeling on these semiconductors, and/or provide a picture that's zoomed into their location so that we might read any markings?

 

[synchro]

And if you haven't already done so, I would refrain from adjusting the POTs at least initially. Because if a different failed part is identified and replaced, having to realign the POTs will complicate getting everything to work properly. At a minimum, you could mark the existing setting of the one-turn POTs, and also note how many turns the present setting of the multi-turn POTs is from either fully clockwise or counter-clockwise.

 

[synchro]

Thanks for the response. It is part of a motor governor system. Sends raise and lower commands when the unit is in auto. When in manual the raise and lower commands are controlled by external push buttons. The manual mode still functions.

I can't really see any visible damage and it has a hard lacquer coating making it difficult to troubleshoot. I was able to check continuity on the transformers and they appear to be OK. I am reluctant to attempt any repairs because I do not have any spare boards. I do have spare components from old failed boards but I am worried I will risk damaging our only unit.

Does this mean you have more than one of these machines, and you identified that this board has a failure by swapping it with a board in another machine? Just asking to clarify what the situation is.

 

[Little Bill]

That's exactly what I was about to say.

Me too! Except I didn't see the other POT!

 

[tthh]

Near the POTs are what appear to be two semiconductor devices in TO-8 packages. Their higher lead count indicates they contain integrated circuits. They also appear to be inserted into white colored IC sockets. That notion is confirmed by the lack of a yellowish conformal coating on the device and within small circular area surrounding the socket.

And so if they are available, these semiconductors might be replaced without any soldering involved. The location of the metal tab on the package should be marked on the socket to insure that a replacement is properly oriented.

Can you read the labeling on these semiconductors, and/or provide a picture that's zoomed into their location so that we might read any markings?

Honesty, the first thing I'd do it take those socketed semiconductors out of their sockets and push them back in. You might get lucky and it's just some oxide preventing a good connection.

 

[synchro]

Honesty, the first thing I'd do it take those socketed semiconductors out of their sockets and push them back in. You might get lucky and it's just some oxide preventing a good connection.

I thought the same thing as well. You'd have to be careful that the individual leads are all aligned with the socket contacts so they don't get bent.
It would be interesting to know if they have industry standard part numbers on them or if they used custom ones.

 

[zbang]

Might get lucky and find those are just 741 op amps (looks like a Toshiba logo but the numbering isn't clear enough).

 

[nuckythompson]

Near the POTs are what appear to be two semiconductor devices in TO-8 packages. Their higher lead count indicates they contain integrated circuits. They also appear to be inserted into white colored IC sockets. That notion is confirmed by the lack of a yellowish conformal coating on the device and within small circular area surrounding the socket.

And so if they are available, these semiconductors might be replaced without any soldering involved. The location of the metal tab on the package should be marked on the socket to insure that a replacement is properly oriented.

Can you read the labeling on these semiconductors, and/or provide a picture that's zoomed into their location so that we might read any markings?

Zoomed in picture attached. And yes, there are 4 machines, I swapped in a card from another unit and everything functioned properly.

Toshiba 7A7504M 7G

 

[nuckythompson]

Picture of the back... do you see any damaged circuit traces? Looks like maybe the middle right side?

 

[gadfly56]

Picture of the back... do you see any damaged circuit traces? Looks like maybe the middle right side?

There might be some corrosion on the traces under the conformal coating. You could take a soft eraser and see whether it's just dirt on the coating. Just take care not to remove the coating.

 

[synchro]

Zoomed in picture attached. And yes, there are 4 machines, I swapped in a card from another unit and everything functioned properly.

Toshiba 7A7504M 7G

Looking at their datasheets, the Toshiba TA7504M appears to be equivalent to a 741 op-amp such as the following one (as zbang suspected):
https://www.mouser.com/ProductDetail/Texas-Instruments/LM741CH-NOPB?qs=QbsRYf82W3F%2BGG/P9hHMMw==

A datasheet for the TA7504 in plastic packages is at the link below.
https://tinyurl.com/2jyerey7


The 2SH13 devices soldered to the board appear to be unijunction transistors, which are very unlikely to be used in any modern designs:

 

[zbang]

Toshiba 7A7504M 7G

That's a TA 7504, not 7A (makes a world of difference), it's a bipolar linear op-amp.

 

[tthh]

Picture of the back... do you see any damaged circuit traces? Looks like maybe the middle right side?

Typically on a circuit board like that, you can easily see damaged traces. That looks like just a two layer board -- that is, traces on the front and back. It doesn't get much easier than that. A trace on a board like this really only gets damaged from a burned up component, a cold solder joint or some sort of mechanical stress. Putting it under magnification you should be able to pass judgement. Also, fixing anything should be easy. A cold solder joint can just be redone, a damaged trace can just be replaced with a piece of discrete wire.

 

[synchro]

The pinout of the OP-amps from the datasheet is:


Reorienting it so that the pin numbers are correct from the back of the board (although a little harder to read) we get:


Then by looking at the board traces, we see that the single turn POTs at the top of the board are for adjusting the DC offsets on each OP-amp.


The pinout of the blue colored ten turn POTs is:


As a result, the pin 1 on each of these these POTs is connected to its corresponding OP-amp output.
Pins 2 and 3 of the POTs are tied together, and connected to the inverting input of the corresponding OP-amp. These POTS are therefore configured as rheostats which provide a variable resistance that adjusts the closed loop gain of each OP-amp circuit. The resistance from the wiper contact to the resistive element within the POT is in series with the intended resistance. Therefore, I think it could be worth rotating the screw ajustment of the two POTs 1 or 2 turns in each direction, being careful to come back to the original setting. This could help clear any stray material, tarnish, etc. that might be on the wiper contact and causing a problem.

 

[hbiss]

This would be a simple matter for an experienced technician to at least determine if these boards can be fixed. They are vintage and may require some modifications and engineering to use current components. But I don't think the OP has much experience with electronics or component level repair from what he's asking. Likely he will do more harm than good.

If I were him, I would send them out to that board repair place instead of playing with it myself and winding up with a brick.

-Hal

 

[MTW]

If you have one board that is working and some not, that is the time to take them to the bench repair technician, so that he can compare readings and functions between boards. Get all of them repaired while you have a working model.