VAXstation 4000/VLC with pumping powersupply - any repair experiences / Tips here ?

[Wildfire]

Hello folks,

during testing of some systems one VLC who hadn't seen power for 1-2 years was also in my testing queue and when powered on it started pumping.
Circa 1 sec on, one sec off, repeat...

I had an DEC AUI-BNC transceiver on the AUI port and the LED's on the backside also did the same disco as the ps was pumping.

So, there may be the usual error sources: Mainboard / harddrive defective or the ps itself has issues like resistors / capacitors or eventually switch mode transistors.
I will try to look after the system the next weeks.

What i wanted to ask here - are there eventually some members who had the same problem and have any tips / hints for me ?

TIA,
Kai

 

[MattisLind]

This type of ticking in the power supply is usually due to a short-circuit on the secondary side of the PSU or a short circuit in the load.

Check output rectifiers and capacitors of the PSU and use a dummy load for testing it.

 

[g4ugm]

I feel its a PSU issue. Has the PSU been re-capped? Is the fan working (not sure if the VLC has a fan, I think so) Like Mattis says check the PSU with a dummy load.

 

[Wildfire]

Ok, i will check for shorts and add a dummy load...
Up to now the PS is not re-capped.
It is still in the condition it was when i got the system. I don't think the previous owner did some maintenance, and also there were no repair stickers on it.

 

[Wildfire]

A small update,

i was able to figure out that the mainboard and harddrive are ok.
The problem is in the PS, secondary side.
There is an 3A or 5A 1N5400 style rectifier diode with a shortage, besides it a zener diode also with shortage, a 7912 voltage regulator behind those 2 also seems to have bad measurement values - this is all measured still soldered in. Caps are optical looking good, no leakage...
Next steps are desoldering defective diodes and regulator out and new in, and then check caps and the other few diodes and regulators
i was not able to measure before further disassembling...

 

[MattisLind]

An 1N540x on th secondary side? Normally those are too slow to work well at the secondary side of a PSU. You are confident it is the original rectifier diode? Not something that has been replaced by some clueless person? If the diode is directly after transformer I would replace it with a fast recovery diode.