My second PDP-8/M is just about back in the land of the living. I followed largely the same process as with the restoration of my first 8/M. Luckily this examples case wasn’t corroded and so I didn’t bother respraying and left it in original condition.

The main sticking point with getting this machine back was a failed +5V and +15V rail on the PSU. Being no expert on PSUs and having been mildly ‘warning’ zapped by it when I decided to see if anything obvious was failed, I had a hunt for a company to perform the repair.

I opted for Electronic Repair Centre Limited after some internet searches. I was very impressed with their service, which was fast and very well priced. I thoroughly recommend them to anyone looking to get a power supply repaired.

After getting the PSU back, I reassembled the machine and with a bit of CPU board swapping (3 of the originals turned out to be failed), I now have a mostly working machine. The one gotcha I currently have is that on entering the DEC accumulator counting test, the accumulator counts at full bore and cannot be seen neatly increasing on the console. My suspicion is that the line time clock might be the cause of this.

The investigation continues…

I’ve just taken possession of a great haul which was literally sat on a scrap heap ready to go to PDP heaven. Highlights of the find include most of a PDP-8/M, a PC04 paper tape drive, a TD8E simple DECtape controller, an ASR-33 and a H960 rack. Whilst pretty rusty having lived in a barn for a while, most of the bits look to be restorable.

The sad news is that the TU56 that was with the machine got scrapped (brings tears to my eyes!) a few weeks ago although I did manage to recover a pair of motors and the chassis.

Unfortunately the core memory sets have gone from the 8-M, although there is a slight chance I might be able to regain one of them. If not, I will have to build up a solution using SRAM/NVRAM, which whilst not exactly authentic, will get the machine back into the land of the living.

Also, amoungst the haul were a PDP-11/40 chassis, a PRIME rack and disk and a TI-990 mini computer. You can read about these on my other blog

Here is a review of recent known PDP-8 sales. As ever, prices appear to be heading upwards.

PDP-8/M

Ebay US sale, item #260385453584. Sold for £1023.23 / $1500.00

Original Description:

Winning Bidder will receive one (1) Complete 100% Working Digital pdp8M Computer with 8k of Iron core memory, the computer comes with the below listed boards, unit was removed from a 100% operational cnc control cabinet. Listed shipping fee for lower 48 only. (The top cover is included)

M8330 Timing Generator

M8300 Majo Registers

M8310 Major Registers Control

M837 Memory Extension Control

M847 ROM

M848 Power Fail / Auto Restart

G111 Sense Inhibit (1 of 3 8k stack)

H212 Planar Stack (2 of 3 8k stack)

G233 XY Driver (3 of 3 8k stack)

W968 I/O Interface

M849 Shield

M8320 Bus Load Module

Commentary

A very nice looking 8-M. The 8-M is a nice compact machine and this one looks like it should make its new owner very happy. The price seemed to be about spot on for a working 8-M. Interestingly its RF shield appears to be in the wrong location, I would normally expect it to sit between the CPU/options and core memory.

PDP-8/A

Ebay US sale, item #230334635912. Sold for £525.25 / $769.99

Original Description

Dec PDP-8/a computer removed from a working machine ( Wire EDM ) seems clean and in good condition but due to it’s age I cannot guarantee its operation.
Includes boards M8315 M8317 M8316 H219B G649 Power supply and an AXIS Board.

Computer has been in storage for 10+ years – it IS dusty, and needs cleaning – Collectors please note! the computer did run when removed but there is no guarantee to it’s current operation, sold AS IS because of it’s age .

Commentary

A very clean looking machine. Price was in my opinion on the high side given that the machine is the less desirable 8/A and not in known working condition. The machine has (for an 8/A) a fairly modest 8KW of core memory.

Having reinstalled the power supply into the chassis, the console was put back into slot 1 and carefully reassembled.

The minimum board set was then installed consisting of:

• Slot2: M8330 KK8-E timings
• Slot3: M8300 KK8-E major registers
• Slot4: M8310 KK8-E major register control
• Slot 20: M8320 KK8-E bus loads

The machine was then powered on and LEDs on the console sprung into life. Some basic operation from the console was possible (I was able to set the PC) although bit 5 of the PC appears to be stuck off. I will have to investigate whether this is a fault on the console or not.

I also installed an 8KW core memory set and retested with this in place. With these boards in place, the PC was all bits high and could not be changed.

So still some way to go! The next step will be to establish if the KK8-E is fully working and track down the issue that is causing the bit 4 of the address bus to appear stuck low on the console.

The PDP-8/M uses a H740 switch mode power supply that provides +15V @ 1A, +5V @ 17A, -15V @ 5A together with LTC-L, PWR OK L, PWK OK EN, AC LO L, and DC LO L signals (it should be noted that not all of these signals are used within the PDP-8M). This power supply is also used within the 5.25″ PDP-11/05 and PDP-11/10.

To rebuild, I stripped the supply down and gave it a gentle clean with electrical cleaning fluid and a clean cloth. A paint brush was used to dislodge any dust in hard to reach places. The electrolytic capacitors were then removed and reformed.

The power supply was then reassembled and plugged back into its mains connector in the chassis. Power was then applied for short periods of 1 second, 5 seconds, 10 seconds, 30 seconds, 1 minute, 5 minutes, 10 minutes.

After each application of power, I carefully touched the cases of the capacitors to confirm there was no build up of heat. Care should be taken as these beasts hold a lot of charge for a long time, believe me you really don’t want to find out how much it hurts to have one discharge through you.

Once I reached 1 minute, I also tested the +5V, +15V and -15V DC ouptuts to confirm all was well. Luckily for me it was.

The final step was to hook up an oscilloscope and to check each of the power rails to check that they were stable and to confirm all of the control signals were working correctly. The line time clock (LTC-L) is the only signal of note when checking with an scope as this should give a 50HZ (or 60HZ in US) square wave.

Having passed all checks, the power supply was reinstalled into the chassis and connected back upto the OMNIBUS backplane. The consists of 4 connections, -15V, +5V, +15V and the PWR OK L signal (spade marked DV2)

Useful Links:

• H740 Schematics (PDF)
• Article on reforming capacitors

The chassis had all internal components removed (after carefully photographing everything to aid in reassembly) before being sanded down. Any areas of rust were sanded back to bare metal.

Unfortunately I could not persuade the build label to come off of the chassis cleanly so this had to be sacraficed. Before I removed it I carefully took a scan of it by placing the chassis on a flat bed scanner. This will allow me to replace the label with a replica at the end of the rebuild.

The chassis was resprayed in 3 coats of grey primer and then 4 coats of matt black paint. I’m really happy with the results, although in some areas I should have spent longer sanding to get a really perfect finish.

I’ve finally decided to bite the bullet and start the rebuilt of one of my OMNIBUS PDP-8s. I decided to start with the more challenging of the two using the theory that by tackling the worst I would be much better prepared for a simple rebuild of better condition machine.

An appraisel of the machine

Before commencing, I will carry out a thorough appraisel of the machine so we can get a measure of the job in hand:

Front View

The Backplane

Inside, the box is mostly clean, a few rust blisters but nothing too worrying. The backplane looks in good shape.

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The build label

The build label reveals that the machine was once fitted with:

• KK8E Processor
• KL8E Serial Console
• 3 x MM8E core sets (giving 12KW of memory)

When the machine arrived it had no cards. I’ve since aquirred a KK8E, KL8E and RX8E. For memory I plan to use one of the MM8E sets from my other 8/M

Power Supply

The power supply arrived loose in the box. It isn’t too bad, although has received some somewhat rough patches during its life. One area of concern is the two missing power transistors at the front of the supply.

1. Power supply strip and rebuild
2. Clean and respray of chassis
3. Clean and repair of console
4. Reassembly of chassis/console/PSU and backplane
5. Commissioning of power supply
6. Installation and test of KK8E and MM8E
7. Installation of RX8E and connection to RX02 drive

Its always good to see a PDP-8 in action, if nothing else to inspire us to get on with our own rebuilds. During my web travels, I came across this nice little video here of a PDP8/M running a demo program. (more…)