Low 5V line from (recapped) SE Power Supply

[joethezombie]

I had low voltages (as measured on the Floppy port) on the 5V line in my SE/30.  The SE power supply is the Sony branded version.  Anyway, I found leaky caps inside the PS, and replaced all of the electrolytics.   But, after the recap, I still have low voltage on the 5V line.  I turned the adjustment pot all the way (post recap) and only get 4.6V.  It's enough to boot the machine OK, but any PDS cards draw the voltage down to 4.2V and the machine will not boot.  This is on a proven recapped motherboard.

I can't find any info on troubleshooting these guys post capacitor replacement.  Anyone know any next steps?  Could the Analog Board be pulling the voltage down?  I haven't yet replaced the capacitors on that.

 

[joethezombie]

I had a little more time to devote to this and have discovered that it is an issue with a particular logic board rather than the power supply.  The recapped PS works exceeding great on different logic boards, even with multiple PDS cards.  But on this one logic board, down the +5V goes.  Logic board in question has received a full recap.  Any hints?

 

[tomlee59]

If the logic board is drawing enough excess current to pull the 5V line low, something on it should be getting decently warm. With care, you should be able to find it. The area between your upper lip and nose is particularly sensitive, so if you don't own a thermal imager, that's not a bad substitute.

 

[joethezombie]

Thanks so much tomlee! I went sniffing around the board and found UE8 wasn't just warm, but fairly hot! Replaced that guy, and my voltage jumped up to 4.92! I wasn't experiencing any video problems, so I never would have suspected our old friend UE8!

 

[techknight]

Ohh... I suspect UE8 on everything these days.