View Full Version : Lesson Learned! Power Supply Woes...

April 25th, 2014, 08:06 PM
I have recently had a run of bad luck dealing with bad power supply issues. Out of 4 computers, I actually had three power supplies take a dive on me, and go DOA. Luckily, I have been able to connect with local friends and have resolves two of my PSU adventures. My IBM 5151 expansion unit was the worst that we were able to fix though. When the fan motor went out, it caused some other electronic problems on the circuit board requiring the replacement of several of the components. I didn't do the repair though. A friend who knows a lot more repairing these units came to my rescue. Now, my 5150/5161 system is running silky smooth, and very quiet. In fact, it's like a brand new system. My other 5150, which just recently arrived is working perfectly though. Although I have yet to install a new video board, and a 284Kb memory upgrade to max out the computer memory to 640 Kb.

There is still a problem with the XT power supply though. It is only providing 80% of what it should be on all +/- voltages. The problem is that the schematics and technical guides I've been able to locate don't indicate the values of the coil and capacitors needed in order to repair the unit. Schematics just don't have that info. So, if someone out there in Vintage Computer land knows what I need to buy to repair the power supply, please point me in the right direction.

As always; thanks much.

April 27th, 2014, 12:36 PM
I'm not an expert, and buggered if I know the correct terminology but here is my two cents anyway. Maybe it'll inspire someone with more knowledge to post up their thoughts.

I had the same thing on a Commodore PSU (PC clone) - all voltages below spec by the same percentage. Commodore was nice enough to put variable resistor between the sense line and the PWM chip - which let me fix it with a simple turn of a jewellers screwdriver.

Point of the story is - if all the voltage outputs are lower than they should by an even-ish amount - look upstream. One cause (it was in my case) is the sense pin on the PWM is getting too high a voltage reading. That little chip feeds a transistor which switches the transformer(s) on and off - and feeds the entire PSU - if it's switching off too much then it'll make ALL voltages go too low.

Edit: oh and I know you were looking for info on transformers/coils etc, but the transformers only pulse on when it tells them to, so I'd start there. If they were worn, the PWM would just run them harder, you'd still get +12V. The fact you get less on all voltage levels makes me think it's the PWM circuit. For resistors/caps - just replace with the same value you removed.

Edit 2: Looking at the 5160 PSU circuit diagram, I can see it's not a simple 'chip', but the SAMS schematic does show expected waveforms and voltages. I wouldn't know how to adjust that one or what to replace.