PDP-11/35 Restoration, Part 8

As promised, I received my replacement H744 +5V regulator in the mail last Tuesday. I haven’t tested it yet, but it looks brand new. Other than that I don’t have any more progress to report on the PDP-11, but a big project at work is just winding down, and I’m taking several days off next week. That will give me some time to really start tackling the power supply. I’ll post pictures and updates here as I work on it.

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 Removed from the BA11-B Chassis
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.

Mounds of tightly packed fiberglass insulation.
Mounds of tightly packed fiberglass insulation. No way it could have gotten in there except for mice.

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

Insulation packed throughout the H744 +5V regulator module
Insulation packed throughout the H744 +5V 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.

Rusted bc05t
Rusted bc05t
Rusted fan, rusted capacitor cover and transformer core.
Rusted fan, rusted capacitor cover and transformer core.
Corroded and filthy capacitor in the H744 +5V Regulator
Corroded and filthy capacitor in the H744 +5V Regulator

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.

PDP-11/35 Restoration, Part 5: Investigating the Diablo 30 Drive

I got some guidance today on the classiccmp mailing list, and finally figured out how the AED 2200 Disk Controller and the Diablo 30 connect to the Unibus. So, with that fresh in mind, tonight I decided to de-rack the Diablo 30 and inspect it.

It came out easily enough, and I wiped it down with a moist cloth. It was startlingly filthy, but after a few passes with the cloth, it started to look like a disk drive again. And with the dust and dirt off the front panel, I was finally able to see inside. Yes, there’s a disk pack in there. I don’t know how bad that is – I’m pretty sure you’re not supposed to move these drives with a disk pack inside, but I’m not sure. The head was obviously locked (there’s a big “LOCKED” tab that is very clearly visible), so maybe it will be OK?

The screws that hold the top cover to the chassis were all badly rusted, but still came loose easily. Once inside I saw that things are mostly good looking. There’s rust on some of the parts, but once again the PCBs and electronics look good, and the disk head looks OK. For now, I just buttoned everything back up and left it de-racked. I will get around to further inspection when I get to the point of trying to actually get it powered on. That will be a while.

PDP-11/35 Restoration, Part 4: The System Modules Come Out

Big day today, restoration wise. I extended the 11/35 out on its rails until they locked, and flipped it up so I could get access to the underside. Like the top cover, the bottom cover of the 11/35 is missing – I assume ATARI got rid of it years before they scrapped the system. But the system modules (as the backplanes are called) seem to be relatively unscathed. The pins show signs of oxidation, but no severe corrosion. Electrical connections are probably OK, but I’ll need to verify by tracing, once I’ve washed out the backplane slots thoroughly. Not tonight, however. For now, all I’ve done is remove the system modules and set them aside for future cleaning and testing.

PDP-11/35 Bottom
All three system modules. From left to right: DD11-A, MM11-S, and KD11-A.
Closeup of the DD11-A System Module
Closeup of the DD11-A. So much wire wrap!

The other task was to completely disassemble and remove the front panel, starting with the metal (aluminum?) bezel, then the plastic overlay, then the KY11 board. The switches are in excellent shape, both mechanically and cosmetically, and the KY11 PCB looks OK. I have high hopes that this machine may yet ride again.

KY11 Front Panel with Cover Removed
KY11 Front Panel without face
KY11 removed from system
KY11, Removed from the chassis

I also took my first peek inside the power supply. It’s spotty with oxidation here and there, but otherwise looks in very good shape. I will pull it out just as soon as I figure out how, so I can inspect it and test it on the bench. Time to buy a Variac!

PDP-11/35 Restoration, Part 3

More vacuuming, and started using distilled water and clean rags to wipe up surface dirt on the chassis, front panel, and cables.

To switch gears a little and pulled the DSD-440 dual 8″ floppy drive from the rack and and cleaned the chassis with a soft damp rag. Upon opening it up I discovered that the interior is almost pristene! Very little dirt inside. This bodes well.

PDP-11/35 Restoration, Part 1

My first night of restoration! I started by getting hundreds of photos of everything from every angle, so I could document the original state as best I could. I also made note of the system configuration before I started to touch anything.

Next, I did a little gentle vacuuming of the surface dirt. There was plenty of it. Fiberglass insulation had become packed into some areas of the card cage, along with the remains of insects, some live spiders, and a few acorns. I sucked this out as best I could.

The original state, as seen here, is just disgusting, really.

PDP-11/35 Card Cage
Mouse-nest infested and very dirty BA11-D chassis

After some light vacuuming, I began to remove cards. I started with the four farthest to the right, the M7232, M7231, M7233, and M7235. Each card I pulled was photographed so I could know its original state, gently washed in warm soapy water, and rinsed with distilled water before being allowed to air-dry in front of a fan. Remarkably, the cards so far are in great condition! There’s some corrosion and spotting here and there, but nothing so bad that I think the electronics won’t work. Mainly it’s bits of surface rust and discoloration on chip leads, nothing worse.