D&R 8000 PSU

[smilan]

Hi,
I'm helping a friend of mine to repair an old D&R 8000 console.
The output voltage on the -24V rail is -36V and the 1k trimer doesn't making any effect on the output voltage.
I've replaced the 723 regulator and tested both transistors with a Chinese LCR meter and both seems to be good.
What else I should look for to find the problem?

 

[Khron]

Broken solder joints or pcb tracks? Is the trimmer working (wiper still making contact)?

 

[smilan]

Yes, the trimmer is working and and connected to the 8k2 and 2k7 resistors at it's ends and to pin 4 of the 723.

 

[Khron]

So you can measure less-than-1-k between each end of the trimmer, and the wiper?

Is it a sealed multiturn one, or an "open frame" one? If the latter, some contact-cleaner might be worth trying.

What voltage can you measure between pins 10 and 7 of each 723?

 

[smilan]

So you can measure less-than-1-k between each end of the trimmer, and the wiper?

Is it a sealed multiturn one, or an "open frame" one? If the latter, some contact-cleaner might be worth trying.

What voltage can you measure between pins 10 and 7 of each 723?

There are 240 Ohms and 680 Ohms between the wiper and each end of the trimmer.
There are 25.4V between pins 7 and 10 of the 723.

 

[Khron]

25.4V on the "bad" supply rail? What about on the other 723's? (That's why i said "each 723")

And/or also measure the voltages between pins 10 and 11 on each 723 (on the bad rail AND the good ones).

 

[smilan]

Thanks, there was a faulty trace on the -24V rail.
After fixing it the PSU seems to be OK.

 

[Khron]

I had called it early on, but great to know you tracked the issue down!