Looks like a few months have passed since my last post, as seems to be typical.
Never you mind, let's just ask the question: How'd that whole "Restore a rusty soot-covered PDP-11/70" thing turn out? Well, I don't want to spoil anything. Let's pick up where we left off.
November 2020:Make it Look Good
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| Looking good! |
Well, first I installed the replacement front panel assembly. That'll get you 90% of the way there, as anyone who restores old computers can tell you.
December 2020, January 2021: Cleaning, Capacitor Reforming, and Fan Replacement
Despite a pretty new face, the computer was still extremely dirty. When I first brought the system home I'd given the rusty parts a rough sanding to get rid of the grit and loose paint, but the inside of the chassis was still amazingly filthy.
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| Dirty Little Fingers |
The boards themselves cleaned up quite well: they were all covered in a fine grit of soot and who knows what else, but soaking them in warm soapy water for 10-15 minutes then scrubbing with an old toothbrush eliminated most of the detritus. After drying in front of a box fan, the gold fingers were cleaned up with liberal use of Scotch Brite(tm) cleaning pads.
None of the 17 boards that comprise the Processor, Cache, and Memory of the system appeared to be seriously damaged. That's good!
My next major concern was whether the backplane itself was hiding some corrosion -- corroded pins make poor contact with the boards and might never work reliably. And these pins aren't exactly trivial to replace -- while it might theoretically be possible to undo the wire wrap to a bad pin, desolder it from the backplane assembly PCB and remove it... no, you know what: it's impossible for all intents and purposes. If you have a dead pin on a backplane like this, the backplane (and it follows, the computer) is toast.
This weighed fairly heavily on my mind, as soot and moisture could easily have destroyed this computer.
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| Empty backplane, mostly. |
Repeat this for all 6 sections of all 44 slots. I did this over the course of about three weeks, a few slots every night to prevent my fingers from falling off. Most of the slots were already pretty clean, but on a few the cleaning card came out quite dirty and required a few extra passes. The last few slots toward the rear of the chassis took the most time to clean.
Had I to do this again, I might have tried removing the entire backplane from the chassis and rinsing it out with water or isopropyl first, but I got pretty good results with this approach.
Reforming Capacitors
In the midst of the above cleaning process, I started in on the power supplies.
The PDP-11/70 gets its power from two H7420a power supplies. Each H7420 is a large bulky unit with an extremely heavy transformer up front; this transformer provides 30VAC to up to five modular power supply units which take the AC and provide regulated DC. Different modules provide different voltages; the H745, for example, provides -15V at 10A; the H744 provides +5V at 25A, the 754 gives you +20V and -5V.
Depending on what system you have and what options it has fitted, the H7420 might have a variety of these modules installed. For the PDP-11/70 system I have, it's entirely H744s -- seven of them, providing a whole lot of +5. The H7420 itself provides + or -15VDC, as well as 8V and the ACLO and DCLO signals used to let the computer know if it's about to lose power.
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| A really dirty H744, prior to cleaning. |
Over the course of a month, each of the H744s was removed, disassembled and cleaned. The capacitors were removed and reformed. Normally, I like to replace capacitors, rather than reforming just for the sake of reliability and peace of mind. However, I thought I'd give reforming a try this time around, mostly due to cost considerations: Each H744 contains three large capacitors, and with seven of them to restore (plus two extra for spares that I happened to have lying about), it was looking like I'd be investing about $750 to replace them all.
I won't go into details on capacitor reforming here -- it's well documented all over on the 'net (David Gesswein has a nice write-up here) and it's not all that exciting. The upshot of my reforming experience was that of the 28 capacitors in the supplies, four of them ended up being marginal, and two were completely dead. Not too shabby.
Testing of the supplies (and the capacitors) was done using an electronic load that I bought for the occasion. It's a lot more convenient than using banks of resistors, and it looks cool too:
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| Burning in an H744 (middle) on the bench. Electronic load on the left, H7420 on right. |
The electronic load gives me the ability to vary the load while testing, starting with a small amount of load for initial smoke testing, then ramping it up to really soak test the thing. I let them run for a couple of hours each. While this is going on, the output of the supply is monitored on an oscilloscope, to check that ripple is within tolerances (about 200mV, max). This is also a good time to do an initial adjustment of the voltage level (each H744 has a small potentiometer exposed on the front that is used for this purpose).
Of the 9 H744s, all but one tested out fine and required no additional repairs. The one that failed would occasionally make an interesting short squeaking noise, with an associated drop in voltage. I put that one back on the shelf for a future investigation (which as of this time has not yet occurred).
Fan Replacement
I have neglected to mention the state of the fans in this system: they were bad. Very bad. There are 18 fans in the PDP-11/70: 8 in the two H7420s, and 10 in the processor chassis. Of these 18 fans, only three actually spun freely and even those sounded pretty bad when doing so.
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| OBEY THE PAPST FAN |
I opted to replace these entirely. While some of them were designed to be disassembled and cleaned, they were all rusty to the point where I just did not want to bother. I found a decent supply of Papst fans on eBay for a reasonable price. These are nice fans, well built with metal blades rather than the more common plastic ones. Heavy. Elegant. Subtle. Hungarian. I like them.
I installed 8 of them in the H7420s and reinstalled six of the H744s (and one H7441 that snuck in there while I wasn't looking. See if you can find it, it's really exciting).
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| The supplies: all cleaned up and put back together! |
At this point, all the wiring was double-checked, both for continuity and also for shorts and breaks in the insulation. Everything checked out OK, so I fired the system up with an empty processor chassis, while holding my breath:
Hey, not bad. No smoke or fire or bad smells, and the front panel lit up (all the lights are on by default since it's disconnected from the logic that normally drives it). All voltages at the backplane were tested, per the service manual:
This is actually kind of a pain in the neck because you're finding tiny pins in a rat's nest of
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| The wire-wrap side of the 11/70 backplane: Where's Waldo? |
and trying not to accidently brush against another pin while doing so. I used a set of small jumper wires that clip over the ends of the wire-wrap pins to help keep things isolated. Even so it was kind of nerve-wracking. Long story short: all voltages were present in the right places on the backplane, and the ACLO and DCLO signals were both high, as they should be.
Conclusion:
As January drew to a close, I had gotten the PDP-11/70 to a point where it was clean and safely powering up. What would the following months bring? STAY TUNED TO FIND OUT!
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