iso p500-spp 400 watt power supply cpu issue

[Boyxba]

i have a power supply with issue in -12v rail. after 15seconds of power the diagnostic begins beep and shows LL ON -12V which means undelvoltage .. i have replaced the most of the capacitors due to missing capacitance (checked with capacitometer)..but i have stil the same problem . can anyone help me to find the solution to fix it....the diodes are ok...i have checked..

[stj]

Re: iso p500-spp 400 watt power supply cpu issue

maybe the diode.
diodes can look o.k. on a meter and then fail as they warm up in circuit.

only 2 ways to detect that.
1: blast it with compressed air or freezer spray while running to see if it fixes the problem.
2: meter both the ac input and the dc output to see if the input drops when the output does.

[Boyxba]

Re: iso p500-spp 400 watt power supply cpu issue

maybe the diode.
diodes can look o.k. on a meter and then fail as they warm up in circuit.

only 2 ways to detect that.
1: blast it with compressed air or freezer spray while running to see if it fixes the problem.
2: meter both the ac input and the dc output to see if the input drops when the output does.

why does it on -12v rail all the other voltages are ok

[stj]

Re: iso p500-spp 400 watt power supply cpu issue

negative rail has it's own transformer tap and diode

[momaka]

Re: iso p500-spp 400 watt power supply cpu issue

i have a power supply with issue in -12v rail. after 15seconds of power the diagnostic begins beep and shows LL ON -12V which means undelvoltage ..

Are you using one of those ATX PSU "testers" that just plugs into the ATX connector and turns on the PSU? If yes, ditch that thing where no one will find it. I've seen those "testers" mis-report a lot of power supplies as being bad. The problem is they don't put a significant load on the output of the PSU. Therefore, the -12V rail will almost always show low voltage. On group-regulated PSUs, other rails may appear low too. For more accurate results, add a load to the PSU - preferably at least 1-2 Amps of current draw from the 3.3V, 5V, and 12V rails each. That should bring the -12V rail to a more normal value.

So for the above, I suggest connecting the PSU to a crappy motherboard (something like an old useless Pentium 4 or similar board), turn on the motherboard, and then measure the voltages with a multimeter at the ATX connector. I bet you the -12V will be OK then.

Also, since you mention this is an "ISO" -branded power supply, you may want to post some pictures here. Some of them are built well and worth recapping, like this one:
https://www.badcaps.net/forum/showthread.php?t=46636

But some cheap variants of the ISO line can be total crap and very gutless. Here's is a gutless one that probably will need a ton of modifying if meant to be used with a PC again:
https://www.badcaps.net/forum/showthread.php?t=39370