PDP-5 Rescued!

From: Dwight Elvey <elvey_at_hal.com>
Date: Tue Apr 25 15:10:37 2000

Chuck McManis <cmcmanis_at_mcmanis.com> wrote:
> This didn't seem to make it out the first time, so here goes a second.
> I managed to rescue the PDP-5 that I've "owned" for some time now, but it
> was stuck in a far away city. Anyhoo, I've put up the obligatory pictures
> at <http://www.mcmanis.com/chuck/computers/pdp5.html> for anyone who cares
> to take a peek at it. This uses "DEC System Modules" the pre-cursor to the
> DEC Flip Chip technology. It hasn't been powered on in 15 years so I'll be
> a bit cautious getting the caps warmed up in the PSU. -15V transistor
> logic, wild stuff!
> --Chuck

Hi Chuck
 If I was worried about the capacitors, I'd disconnect them and
use a bench supply with a nice current limiting resistor. I
would think a step rate of 5% of working voltage every 1/2
hour would be safe. Put a volt meter across the resistor.
Any significant large voltage drop would indicate leakage.
These large caps are known to have some leakage. How much
is OK, I don't know. That is something you'll have to make
a judgment call on. I would think that for large caps like
I see in the pictures, 1/32 watt is on the large size for
DC power.
 Don't forget the smaller electrolytics in the circuits
of the supply. Do a value check on these. I usually lift one
lead and use an ohm meter. Comparable values should take the
same time to charge. You might want to connect these to
a setup like used on the large ones. If they respond
as having low capacitance on the ohm meter, you'll most likely
have to replace them, forming won't help.
 While you are fiddling with the supply, you might also add
some new heat sink grease under power transistors. The silicon
oil in this grease does eventually go away. It is easiest
if the transistors are socketed.
 I don't recommend the slow power on for the entire unit.
Many circuits will draw higher current until full up to
voltage. If you want to do a slow power up and the supply
is a linear, disconnect the load and just bring up the supply.
If it is a switcher, you may damage it with a brown out.
Most linears can handle it.
 Watch for any signs of smoke or popping sounds. These are
almost always bad news. At one of the meetings at Stan's,
one of the fellows, I don't remember who, had three tantalums
blow. These were all power line input filters on boards.
After we removed them, the unit worked fine. If we'd left
the power on, these would have burned the boards.
Received on Tue Apr 25 2000 - 15:10:37 BST

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