Peter Chou USP-11-150X recap and +5 volt issue

[DuffCon]

Sanity check. I noticed that on the +5v rail, the positive connection of C25 is on the same pad as the output wires that go to the PCB. On the -5 Circuit the Negative lead of C29 is on the pad with the -5 output wire.

I'm not an electrician; what messes me up is the positive side of C29 sits on the same trace as the negative side of C25. Can someone tell me i'm not crazy?

 

[mecha]

I colored the lines for emphasis. the orange and green lines aren't connected at all.

 

[DuffCon]

I guess what I'm asking is I see the negative lead of c29 is on the same pad as the -5 output (upper green line) is that correct?

 

[mecha]

the - sides of the caps traditionally will be connected to ground.

 

[DuffCon]

Right, but on my board, this point on the red arrow is the negative lead of C29. The blue arrow is the positive lead of C25, so the symbols are kind of opposite.



I guess i'm confused by the down arrow symbol used on both caps.

 

[DuffCon]

So I just went with the silkscreen and the previous cap position. Put it back together and i'm having the same issue that prompted the re-cap. It will startup and do +5vDC without issue, If I adjust the +5v pot up or down, it makes a buzzing sound and the voltage drops to 1 or 2 volt. If I unplug it for about 15 seconds and plug it in again, it's happy and the voltage changes to whatever the pot was set at. Looking at the diagram, the pot sits in between R22 & 39, in between the +5v output and leg 1 of IC1:


I guess my next steps are checking R22&39 (no values on the diagram, of course) and possibly IC1? Any ideas? I know @SillyB has done a bunch of these...
Note that the original fan was completely toast, right now there's no fan connected at all, but I doubt that has anything to do with it (12v fan)

 

[SillyB]

In my experience this behavior is just the pot itself. The PSU is very sensitive and when the pot is dirty and has intermittent contact it goes into protective shutdown. I do a bunch of Deoxit or contact cleaner on the pot and wipe it back and forth 4 or so times, let it dry out and 90% of the time it's good to go. These pots aren't usually failing since they are rarely adjusted, they get oxidization from not being adjusted.

I'm not sure on your schematic question from an electrical theory, but I can say from experience if it was in backwards it would have popped and would not be putting out 5V!

My standard procedure on these PSUs is clean out the blankets of dust build up, replace the dead fan and clean the pot. Most of the time this is all they need - they are actually very tough units. Then I test for voltage under load and if there is an issue I replace the caps and check the high failure resistors.

 

[DuffCon]

@SillyB You nailed it! I remember reading about your Deoxit cleaning of the +5v Pot in another thread but didn't do it on this one because the pot was pretty clean and turned freely. So I gave it a dab and wiped it back and forth a few times. Worked like a charm! Thanks very much for your help!

 

[mecha]

if the +5V adjustment gets dirty and the power supply can't sense where it's set, it shuts down.

I've had those cheap junk power supplies lose the adjustment knob and go full blast. which is great if you want to fix game boards afterwards.

the Peter Chous are finnicky compared to the Happ ones. also inspect the exhaust fan, the Peter Chou fans are prone to seizing. spin it with your fingers. if it doesn't spin freely for several revolutions, replace it. it's not supposed to stop like a brake. dirty adjustment pots and bad fans aside, Peter Chous are quality power supplies.

 

[DuffCon]

Oh yeah, 100% needed a new fan. The old one had a shot bearing and fell apart!

 

[19kfox]

The main fuse blows immediately after power is applied to the board. Any suggestions on where to start?

 

[DuffCon]

The main fuse blows immediately after power is applied to the board. Any suggestions on where to start?

I would start with all the transistors; pull them and check them for shorts.