IBM 5150 Power Supply Repair

[Tempest]

I have an IBM 5150 that has a weak power supply.  When I put too many cards in my system there's noise on the 5v line which comes out the speaker as a 'whistle'.  I cracked open my power supply today, but I can't see any obviously bad caps.  Does anyone have any experience with fixing these?  I think this may be beyond my abilities.

 

 

[wierd_w]

I would check for hot voltage regulators.

 

 

[Tempest]

41 minutes ago, wierd_w said:

I would check for hot voltage regulators.

 

 

Where are they?

[ChildOfCv]

Apparently IBM never made schematics for their PSUs.  Probably because they have upwards of 300V in places and don't want the average joe poking around in there.  Sams Computerfacts has a reverse-engineered schematic for one of them though.

 

If you see what looks like liquid residue around any of the large capacitors, that's a good sign of leakage.

 

There does not seem to be an actual regulator on the 5V output line, only the -5V line.  Instead, the early stage uses an oscillator to send square waves through a small transformer, which drives a larger transistor that outputs square waves through the large multi-tap transformer.  One of those taps is the 5V output.  It uses opto-isolators to decide when to cut back on oscillation.

 

The most likely failure point does seem to be the capacitors though.

[ChildOfCv]

Just another note though:  The PSU can output up to 7A on its +5V line.  If there are leaky capacitors on the motherboard, they could be consuming more than their share of that amperage.