Hi,
I am trying to repair the PSU of an HP 8594E.
So far I replaced a couple of resistors, one diode and the two pulse generators for the mosfets.
I replaced the big fat capacitors, as well as the noise filtering capacitors, as those are probe do explode (Dave made a video of it).
The result is that the PSU turns on and the status leds on the PSU show that all voltages are OK. The spectrum analyzer starts, but after 2-3 seconds the PSU switches off. If I disconnect the power cable and wait 3-5 minutes, I can turn the device on again and it will run for another 2-3 seconds, before the PSU shuts off again.
On the PCB of the PSU there is no visual sign of any damage.
I am now kind of stuck: what could be causing the PSU to shut off?
My next steps would be:
- replace the MOSFET's
- replace the big fat capacitors back again (I could not get premium brand replacements and I wonder if I should put the original ones back - they measure OKis outside the PCB - around 430uF instead of the specified 470uF, but so do the new ones I bought)
-

Please help, as I am kind of struck! Also, I already resoldered all solder points, in case there was a micro-fissure.
I actually have three (!) of these broken PSU's, so I could swap parts around. But doing that randomly seems futile due to the sheer amount of components.
Measuring with live mains is not easy, as the PSU has two daughter-boards (these are OK for sure) and the PSU only turns on when there is a load.
I have been trying to repair this for the last two months and I am frustrated, but I won't give up. So far I learned quite a bit about switching power supplies, but these f***ers are quite complicated.
Another question:
The MOSFETs are Motorolla (I guess) P5N40E and P4N50E.
From what I gathered so far, The P5 means 5A, P4 of course means 4A.
the 40 means 400V and the 50 means 500V.
Can I replace a P4N50E with a P4NK80Z (supposed to be 4A 800V)?
How risky is it to replace the P5N40E with this P4NK80Z? I couldn't get any closer match in the local supplier...
Thanks for all the help, suggestions and the ultimate tip to get this PSU back to work.
Regards,
Vitor