In article , mike wrote:
Virtually every TM500 mainframe I've bought used had at least one bad
pass transistor.
Checking them with an ohm meter is definitely better than nothing.
But...they're often overheated for long periods of time.
On a curve tracer at high current, they can look more like resistors
than transistors.
Breakdown voltage can also come way down. Beta can go to hell, then the
driver in the plugin overheats and shorts out blowing up the plugin when
the voltage goes to the rail.

Thanks for all the info!

The transistor junctions all measure about .54V drop one way and open the
other, on diode test. The high-power NPN is a little lower than the others.
I guess I'll still look into rigging up a curve tracer. How much
current/voltage do you usually have to put into them to make the problem
show?

There's not a cheap sub for these transistors if I just wanted to replace
them all preemptively, is there?

I wouldn't sweat the ESR of the caps.

I think you're right because on the two I checked it was too low to measure.
(No, the caps aren't shorted.) I still haven't gotten at the board to check
the others and reflow the solder. It looks like I'll have to remove half the
fasteners and panels in the unit?!

Just put on a load and look at the
ripple. IIRC, full load minimum line should have a trough voltage of at
least 22V on the 25V supplies. Don't remember the number for the 11.5V,
but think it was 7V trough. But it's been three decades since I worried
about the numbers.

I didn't see those numbers in the TM504 manual. Are they given in the
custom plugin kit manual or something?

Take a good look at the connectors. Replace the plastic key. You
wouldn't think it important, but it helps guide the plugin so you don't
blow it up when you plug it in hot...at least not as often. Same thing
can happen if the top and bottom of the connector wears on the
fiberglass and lets the plugin slop up and down.

The barriers appear to be in decent shape. Are replacements readily
available, anyway?

And use your expendable plugin to test the hole for the first time.

Uh-oh. I guess the DC504 will have to be the guinea pig.

BTW, my unit is too old to have the two 600V/1A anti polarity reversal
diodes shown in the manual addendum, so I'm going to put in 1N4007s
where they're shown in the schematic.

Thanks again for sharing your experience with me.

-- Adam