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Mega PSU

Posted: Wed Jun 27, 2018 10:46 am
by atari030
I was just playing with my stuffed Mega4 and have something weird going on. I bought another PSU, I needed one anyway. It is doing the same as my other one which I swapped for a ST supply. Clearly I didn't investigate it enough. No load, fan spins and does its business. If I hook it up to the Mega4 then it acts like a short on 5v.

However, if I hook up the CDrom PSU I hotwired the molex plug up for the ST and it kicks the Mega4 board over. If I check continuity on the motherboard between 5v and Gnd it growls short.

I will gut the repaired Mega and check when I get a chance, at least one working unit is a good yardstick. Any thoughts?

Re: Mega PSU

Posted: Wed Jun 27, 2018 11:47 am
by exxos
Motherboards will measure temporary short due to capacitors on the 5V line. Try on resistance, value should go up.

PSU will not work correctly without a load. Fan may spin but likely at lower voltage then normal. PSU's need proper load on 5V to function correctly.

Re: Mega PSU

Posted: Wed Jun 27, 2018 10:40 pm
by atari030
That is what I don't get. Both Mega PSU's come to a grinding halt when plugged into the mainboard. Both spin up the fans without an additional load. The CDRom supply just works, albeit with sod all current delivery. But it is enough to jumpstart the boards.

I figured it was caps playing funny muppets with the 5v rail. I have done a partial recap, time to investigate a bit further. Until I can get the boards firing properly, I won't worry about getting the second board repaired just yet.