Power Supply Rebuild #1

Well, the replacement capacitors arrived from Mouser this week, so what better way to spend a Friday night than fixing up a rusted old power supply?  I cleaned off some space on the workbench, put on some MST3K ("The Screaming Skull") and got to work.

The 18V Memory Power Supply

I thought I'd start with the 18V supply for the core memory.  It's mounted to the outside of the chassis, it's easy to disconnect, and it looks like it needs the most actual work -- why not start with a challenge.

Do you know the Muffin fan?

As you can see from the above photo, we've got a big ol' transformer on the left, a Muffin fan in the middle, and the rest of the power supply on the right.  You might also notice some extra rust on a few power supply components...

Uh oh.

Can't say I was happy to see the heavy rust on the two potentiometers here.  These are labeled "LIM ADJ" and "OVP ADJ," which I would wager stand for "Limit Adjustment" and "Over Voltage Protection Adjustment."  I don't have schematics or documentation for this power supply, so I have no idea what values these pots have, nor do I know how to properly adjust them.  Given that, my plan was to clean them as well as possible and hope I could either find identifying marks or at least be able to measure the range of values and the current settings myself.   You can also see a bit of rust on some of the output transistors, but this shouldn't affect operation.

So, let's start with these, shall we?  I desoldered the pots one at a time and gave them a nice long (15 min) bath in "Blaster PB," a strong smelling rust remover that I used extensively on a rusty Nash Metropolitan a few years back.  This stuff is nasty, so I cracked a few windows...

This loosened the rust quite well and the potentiometer wipers now turned freely.  They still looked pretty ugly.  I scoured them a bit with some Scotchbrite and that got a lot of the surface rust off, but they're never going to look all that nice again.

One potentiometer, now slightly less awful.

I gave them a bath in soap and water to remove the Blaster residue, dried them out and gave them a nice go over with some contact cleaner.  Amazingly, they now read nice clean resistance values from one end of the wiper to the other -- no dead zones or otherwise.  The insides are nice and clean.  So, I guess for now I'll continue using them.  I returned the wipers to their original positions so they wouldn't be too far off their original values.

There are four capacitors in this power supply that I'm going to preemptively replace.  I tend to go for a "shotgun" policy on old capacitors since capacitors this old are basically ticking time bombs.  Given the condition of this machine, I don't trust these things at all even if they do test good.

The capacitors to replace are:

• 11,000mfd 50V (large can, screw leads)
• 1,000mfd 50V (axial leads)
• Two 470mfd, 35V (radial leads)

Some quick desoldering and the job's complete.

All cleaned up.

Now we wire up an AC cord, cross our fingers, and plug 'er in...

The fan spun up (and sounded good, which is nice... I don't have to replace the bearings or anything fun like that) but I only got about 0.5V from the supply.  I powered it down and checked a few things for continuity.  The fuse tested good earlier, but I never checked that it was making good contact with the holder.  It wasn't.  Some contact cleaner and Scotchbrite rectified that.  On the next powerup I got 18.8V.  Not bad (sometimes the simplest things are the problem -- I prefer it that way...)  I adjusted this down to 18V with the voltage adjustment pot (the one on the far right in the photo above), though this will need to be re-adjusted once it's running with an actual load.

Lookin' good!

I'll need to do some more testing under an actual load tomorrow, but the initial checkout looks promising.  A nice steady 18V with no appreciable ripple.  I'll take that for tonight.