Power Supply Rebuild #2

Decided to bite the bullet and finish off the main power supply for the Imlac this evening.  Put on some MST3K ("The Mole People," man is John Agar insufferable) and got to work.  I'd replaced the big can capacitors last week, but I had yet to take care of sundry details like installing a new power cord.  (The old one had deteriorated into an extremely dangerous fire hazard.)

The newly re-capacitored power supply.  Shiny.

Armed with experiences from the last supply rebuild, I also cleaned all the fuse holders and cleaned the regulator PCB's edge connector.

The wiring harness for the main power supply

For testing, I opted to pull the cards from the backplane rather than undo the backplane power wiring.  There are a lot of wires and it'll be easier to check voltages with them screwed down.  Pulling the cards is simple and also ensures that if I miss a wire somewhere (or if there's an unfortunate short I didn't find in the backplane, for example) I don't destroy the CPU if the supply's way out of tolerances.

Having done all of this, I plugged the new power cable in and flipped the On/Off switch.  The power indicator lit up, the fans spun up and there were no loud bangs, smoke, sparks, or fire.  That's a good start!  Checking the outputs from the regulated supply, the -5V, -15V and 15V supplies were good, but the 12.5V was at 2.5V and the 5V was at 1.25V.  Hm.  Voltages at the unregulated end (+20V, -20V and +10V) were all fine.

Partial schematic for the regulated supply

Powered down and double-checked connections but it made no difference.  Time to do some deeper checking. I figured I'd start with testing the power transistors, these are TO-3 units (the four transistors on the upper-left side of the schematic).  As luck would have it, Q4 was bad!  For good measure I tested the other three, and they all tested out fine.

Bad transistor! Bad!

These are 2N3055s which are still in abundant supply so it should be an easy part to find.  Looks like I'll be making a stop at Vetco tomorrow to pick up a replacement.

So, that's the progress for today.  Perhaps tomorrow we'll have a working supply, and if so I'll be seeing how the CPU runs (or doesn't).  Stay tuned!