Skip navigation
Published Sunday, May 13, 2012 at 7:23 PM

PDP-11/35 Restoration: The Nasty Power Supply

Today's activities were spent taking the H750 power supply out of the BA11-B chassis. I haven't found a scrap of documentation on the H750 or the BA11-B, so this was a little harder than it sounds. Luckily for me, DEC engineers of 40 years ago put everything together into a package that was fairly easy to figure out, so it didn't prove to be too difficult. Actually, probably the hardest part was getting the BA11-B out of the rack and onto the table without destroying my back. This thing is flippin' heavy.

Once the BA11-B was on the table, I put it on its side, disconnected the bc05t power control, undid the obvious screws that held the H750 to the chassis, and slid it out.

h750_out-768x466.jpeg
Figure 1: H750 Removed from the BA11-B Chassis

Back when I picked up the PDP-11/35, before I had taken anything apart, I actually had high hopes for the power supply. My first peek inside didn't look too bad, actually. I thought it had been spared the worst of the damage because it was sealed up and protected from the elements. What I hadn't counted on was the damn mice. They must have found their way in through the fan, it was the only exposed opening. They stuffed fiberglass insulation everywhere, I assume making a nest.

xformer_mice-768x576.jpeg
Figure 2: Mounds of tightly packed fiberglass insulation.

The H744 +5V power supply module was especially hard hit. Take a look at the state it was in when I pulled it out:

h744_messy-768x574.jpeg
Figure 3: Insulation packed throughout the regulator module

They also peed on everything. I mean EVERYTHING. When I finally cleaned out the nesting, the damage was evident. Pretty much everything is corroded, rusted, and dirty.

bc05t_rust-768x576.jpeg
Figure 4: Rusted bc05t
rusted_parts-768x576.jpeg
Figure 5: Rusted fan, rusted capacitor cover and transformer core
h744_corrosion-768x576.jpeg
Figure 6: Corroded and filthy capacitor

I'm trying not to give up hope, here. I actually did find a spare H744 on eBay, and bought it for $65 (after shipping). I will try to restore this H744, but I don't know. I just don't know. This is a pretty bad sight. I have a feeling I'm going to have to tear the whole H750 apart, recap and rewire everything. It's going to be a whole lot of work, way more than I originally anticipated.

I sure wish I had some schematics right about now, I'll say that much. Wish me luck.