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PDP-8/m power supply test, bringup?

Crawford

Experienced Member
Joined
Oct 21, 2008
Messages
275
Location
Maryland
Folks,

I recently acquired an empty PDP-8/m case that was empty of boards. The rest of the case is intact(ish), backplane, power supply, fans. There is no front panel, only a red plastic insert, and the keylock is just sitting inside the case (still wired).

I am familiar with linear supply bringup, but this is my first really old switching supply.

After I get this all cleaned, are there any particular do’s or don’ts for bringing up and testing the supply? I popped open the back access panel to the supply, and it looks surprisingly clean.


-Crawford
 
I recommend Jerry Walker's series on restoring a PDP-8/M You can probably skip to the "Part 2" video, and begin watching at about 9:30 from the beginning.

I don't recall that Jerry mentions reforming the electrolytic caps, but I did that first. Reformed C1 and C2, the big input caps and also C7 (on the +5V output and C14 (on the -15V output. There was a noticeable decrease in the leakage current of C1 and C2 after being biased a 50V for a few days. I have some data that I can post.

After electrolytic reforming, I pretty much followed Jerry's process of verifying each of the outputs and confirming the OVP circuits. For testing each supply output I used an input voltage of 29.5V because the three bench supplies that I have output a max of 30V. Would have used 39V like the real circuit if my lab supplies were capable. I also purchased two Kunkin KP184 electronic loads on ebay, which were quite helpful. (Jerry uses three.) After testing the +15V output using the electronic load I connected a 16 ohm 100W load (two 8 ohm 50W resistors in series) and used the KP184's on the +5 and -15 outputs.

The fan in the power supply housing was quite dirty and wouldn't spin up by itself. Took the fan completely apart and cleaned everything and lubed with 3M 08897 silicone lubricant. It works great now, but kind of loud.

A little about the process I went through is in this thread: https://forum.vcfed.org/index.php?threads/fun-stuff-just-got-a-new-old-pdp-8-m-chassis.1238187/
 
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Thanks! I am still in the disassembly / cleanup phase of this.
This phase of the project can take longer than it would seem it could. One thing leads to another. I took many photos during the disassembly and cleaning so I'd know how everything should go back together.
I started the electrolytic reforming in parallel with the cleaning, mainly because I wanted a fun electronic project activity.
 
Indeed, this is taking more time than anticipated. Two words: disintegrating foam! Ugh, I had partially forgotten about this stuff. I got the mat on the slide-off top, and the mat under the omnibus mainboard. Now I am into the back side of the power supply. That foam is extra-sticky. The foam has held some moisture, and caused some corrosion also, but fortunately only to the steel parts, not electronics. Oh, well. Back to scraping and cursing.
 
In my 8/m the rubber underneath the capacitors was also eating the copper traces... So I removed it, and tinned the copper again. I did not have too many problems with these power supplies. Just slowly powered them with a variac and a 150w lightbulb in series for current limiting. When it gives the right voltages I remove the lightbulb and start to test them with a load.
 

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Back to scraping and cursing.
I've used packing tape (masking tape if you are worried about finishes), sometimes wrapped around my hand sitcky side out, to get the particles under control. Also, a couple of applications works a treat for the stubborn leftovers after the scraping.

Vince
 
When you've gotten the big chunks off, generous applications of GooGone and patience (let it sit for 15-20 minutes before scraping) seem to do the trick without as much cursing.
 
Jack,

Yes, much Goo Gone has been used in this exercise. I vacuum as much as I can, then scrape any loose stuff, then use Goo Gone. I may also clean up with WD40 as needed. Cursing is mostly therapeutic.
 
Indeed, this is taking more time than anticipated. Two words: disintegrating foam! Ugh, I had partially forgotten about this stuff.
hahaha.... I feel your pain... very short video attached so others can fully appreciate the disintegratedness...
(it's a .mov file captured with my iPhone)
 
Folks,

I’ve found some minor rust on the steel of the case and its components like the p/s cover. Not enough to warrant a full strip and re-paint, but I do hate rust. I’ve been scraping out some of the deeper spots, and I used naval jelly on the p/s cover. I only have gloss black paint at hand, so I need to find some flat black to proceed with spot touch-up.

-Crawford
 
The Rustoleum professional products, the primer and semi gloss black are excellent at these kinds of repairs.

When painting non gloss black you don't need a perfect surface as the black will hide a lot of issues. On the flip side of this, if you want a perfect surface you can hide a lot with sandable primers.

There are also products that can convert rust into a primed surface. You knock off the scale with a wire brush and then apply the converter.


A black so black your eyes slide right off it.
 
I’ve found some minor rust on the steel of the case and its components like the p/s cover. Not enough to warrant a full strip and re-paint, but I do hate rust. I’ve been scraping out some of the deeper spots, and I used naval jelly on the p/s cover. I only have gloss black paint at hand, so I need to find some flat black to proceed with spot touch-up.
In my recent work on a PDP-8/M chassis I used Rust-Oleum Flat Protective Enamel, Black, product code 20066-77768, available at Home Depot. Not sure if this is the best product available but I've used that brand before and had good results, and it was available.

First, rust was removed using a fine grit sandpaper, cleaned and dried the surface and then applied the paint from maybe 18" away from the surface. The longer you let it sit outside in the sun the better so it dries well and there's less smell when you reassemble the chassis. I noticed that large painted areas were still sensitive to getting scuff marks, such as the outside of the H740 power supply housing where the power cables exit the housing.

I was careful to mask all of the original stick-on labels so the unit would still look as authentic as possible.

Also, didn't repaint the entire chassis. Just did localized removal and spot repainting, except that I did repaint the entire H740 supply housing piece, both sides. The flat black matched the original black well enough so localized spot touch-up was not noticeable. Just take care to blend it well.
 
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