PDP-8 Restoration

[SierraFoxtrot]

For me, one of the real jewels of the ACMS collection is the original model PDP-8 (aka the straight-8). There is so much to love from the whopping 4k words of core memory to the wood grain paneling she is a thing of beauty.

At the end of September, @daveeverett and I took the perspex side panels off the cabinet and started a full inspection. Apart from dust and grime, everything looked to be in pretty good shape. So we set to work restoring it to working order with the first objective being the model 708A power supply.

Goal #0 for this project: To restore the PDP-8 in as original condition as possible choosing repair over replacement every time.

The power supply is tucked behind the glorious front panel and is somewhat accessible when you extend the computer itself out on the slides. Peering down in the supply, you get impression DEC got a pretty good deal on massive electrolytic capacitors back in the day.

Looking down from the top, you can see a few highlights:

1. The ferroresonant transformer and it’s matched capacitor (Blue)
2. Some of the 7 high current diodes bolted through the chassis (Gold)
3. The “power OK” relay to prevent erratic supplies corrupting core memory (Red)
4. The memory power supply regulators (Green)

SLOWLY SLOWLY…
The 708A features 15 silver coloured big boy electrolytic capacitors which have been sitting powered down for (likely) decades. If we were to throw 240VAC across the input at this stage, they would fail where the aluminium oxide has become thin over the years as it was eaten away by the electrolyte. The likely result being several large bangs. That means it’s time to reform them.

The basic procedure is to isolate each capacitor. That bit is made easier by the spade lugs! Then start by applying 10% of the working voltage. It’s extremely important to limit the to something small e.g. 10-20mA. I did this with a power supply that supports a controllable max current but a series resistor will also do the trick.

The setup for two at a time:

Assuming we don’t already have a dead short-circuit… the capacitor will SLOWLY charge up
to the supply voltage. If you look with a digital ammeter it will appear that current has ceased flowing. However a decent analogue ammeter like the AVO pictured will show that a small amount of current is still being drawn. This is actively reforming the oxide layer in the capacitor. After some time (minutes or even an hour), the current will flatten out and then eventually decrease towards zero. Bump the supply up another 10%, leave the current limited, rinse and repeat until you get up to the full working voltage.

What I’ve discovered: If you think you are rushing, you are. If you have to ask “how long”, the answer is longer.

A short video of the process

After a large number of Saturdays, we now have 60 year old caps operating pretty close to spec. Zero bangs, all original parts!

More to come…

[SierraFoxtrot]

I’m playing catch-up on journalling this so here is another entry to take us up to mid December 2025

Part 2: Turning it on…

With the caps refreshed, and hopefully no longer threatening to act as a dead-short, it was time to check out the rest of the supply and work towards powering it up.

Diodes

Most of the diodes could be tested either in situ or by pulling a spade lug to isolate them. Some however, required pulling out the socket set to remove them from the chassis. Nothing says power electronics like pulling out the real tools.

Impressively, these 1n3208 diodes are still manufactured and sold through DigiKey and the like. Given the maximum of 2A from any of the channels, their rated max of 50A does seem like overkill.

Regardless, testing ahead of time was going to avoid unpleasantness down the road so we took the new high “Multifunctional Tester” out for a spin. Thank you Michael for the crash course!

NOTE: While the basic diode setting on your Fluke (or Fluke knock-off) will check forward and backward bias with a Volt or two, this unassuming little gadget will generate a few hundred volts and let you know the reverse breakdown voltage of the diode under test. This is very handy for aging silicon which may be under-performing in terms of specification but will fly under the radar on a typical DMM.

I was almost disappointed when they all held off a couple of hundred volts. Almost.

At this point, we are running with all original diodes now too!

Transformer

A brief diversion i.e. a furphy led to pulling out the transformer to check for shorts. It was either that or keep tripping the main breaker for the whole building a few more times (sorry Sean & Tim!).

The transformer turned out to be fine, as was its companion capacitor. But doing a thorough inspection on these was definitely worth the time.

Pro-tip: Unlike the electrolytics we reformed above, these oil-filled capacitors of the era are chock full of Polychlorinated Biphenyl, the nasty kind of PCB. PPE is recommended!

Fortunately, both checked out so back in it went.

Firing it up

At the (correct) insistence of @DigitalRampage we just flicked the switch which rewarded us with the prettiest sight of all:

With the gremlins banished, the usual tripping breaker was replaced with the buzz of the transformer and the warm glow of neon. Absolutely delightful. The +10V and -15V channels were rock solid with a 1A load.

Given an allowable ripple of 700mV and that we can expect a bit of a voltage drop over the massive cable loom up to the computer, 40mV or ripple at 10.4V will do quite nicely for now.

This is fortunate as the maintenance manual states that if it is above 700mV of ripple, it’s faulty so just fix it.

Just before we go celebrating too soon, a quick check of the margin check voltages showed them way off spec and not responding to adjustments. But that had to carry over to a following Saturday just like a Star Trek cliffhanger back in the day.

[SierraFoxtrot]

Part 3 Fixing the missing channels

Margin Check Supply

In the PDP 8 power supply, the margin check channels exist to deliberately run the logic from an adjustable substitute supply so you can “margin” the machine and expose borderline faults. Basically, running with the voltages a little high or low, stress tests borderline components ideally to catch faults during periodic maintenance rather than normally running. A nice idea from a time when components were less reliable, computers failed as a regular event and hardware maintenance was something you planned.

In practice, you select the margin check supply in place of (either) the main +10 V rail or the main −15 V rail via switches on the computer’s backplane. Next you vary the voltage output up and down while watching for errors, crashes, or specific symptoms. A switch, dial and convenient meter on the front allows you to adjust and measure.

If the system only fails when you push the supply slightly high or slightly low, you have found a marginal condition (weak gain, leakage, poor connections, drifting reference points, etc). This is useful both for preventative maintenance (confidence that the system still has noise and tolerance headroom) and for troubleshooting intermittent failures that do not reproduce at nominal voltages.

Fixing our margin supplies

Last time, we observed that the margin supplies were both very low and not responding to adjustments. A quick look at the schematic (the relevant section is in green) shows some likely suspects.

Firstly, the variable transformer on the front (Blue on schematic) was patchy and indeed open circuit in many spots after decades of sitting idle. Fortunately with some liberal use of contact cleaner (we really should hit up WD-40 or 3M for sponsorship…) and repeated turning, it came good giving a nice smooth adjustable DC voltage on the front panel meter.

After premature self-congratulations (are there any other kind?), I spotted that the output on the terminals was still shoddy. That left literally one component, switch S1 (Red on schematic).

Switch Surgery

After contact cleaner failed, I was tempted to pop down to Jaycar and fetch a simple toggle. But remember that we are aiming for repair over replace. Figuring that I couldn’t make things worse, the switch was disassembled.

What happens to grease over 60 years? Apparently it turns to something resembling wax, coats everything and helps to oxidise all the terminals. So after some cleaning and buffing of the contact points, it all went back together without too much fuss beeping appropriately when tested with a DMM.

A further quick check with the multi-meter confirmed that the margin check channels were now switchable, adjustable and present at the output terminals.

SUCCESS!

A trial separation…

One thing I failed to mention in the previous update was that working on the supply had become nightmarish. Extending the computer out the front of the cabinet to get access to the underlying supply threatened to topple the whole thing over!

Because of poor cable management and a rather difficult mounting arrangement, removing the supply from the cabinet threatened to be irreversible. Interestingly however, the computer is fairly self contained. So with Murray and @DigitalRampage’s help, the computer and supply were separated from each other for the first time this century.

That brings us up to date. Work has now started on the power control panel responsible for routing AC power throughout the cabinet.

To be continued…

[mdtapp]

So cool, wood paneling! Hope you live-stream the first power-up !

[SierraFoxtrot]

Thanks Mike! The wood paneling is the most cooperative subsystem so far. First power-up will definitely be documented. Live-streaming may depend on how brave I’m feeling that day, but it’s certainly tempting.

[mdtapp]

@SierraFoxtrot

at

Any computer with an Hour-Run counter and built-in Voltmeter has my automatic respect.
I have a modest-by-comparison Cromemco Z2D restoration project in the wings for 2026 and will be leaning on your Cap reforming and transformer testing hints there for starters. Nice work documenting that.

[SierraFoxtrot]

I’ve only ever seen the exterior of those but I’ve read that they also have a nice linear supply with big ol’ electrolytics. What those supplies lack in efficiency, they more than make up for in charm! I’d love to read a write-up if you get the chance!