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…

Here are a list of my spare OMNIBUS cards:

G104 MM8E 4K Memory sense/inhibit
G227 MM8E Memory X/Y drivers
H212 MM8EJ 8K Memory stack
M837 KM8E Extended Memory and Time Share Control
M8300 KK8E Major registers (x4)
M8310 KK8E Major register control (x4)
M8319 KL8A PDP-8/A 4 channel serial I/O
M8320 KK8E Bus loads (x6)
M8321 TM8E Magtape output control module
M8322 TM8E Magtape control and data break module
M8323 TM8E Magtape transport status and control module
M8327 TM8E Magtape register module
M8330 KK8E Timing board (x6)
M8335 VT8E
M8336 VT8E Video Display Clock and Frequency Divider
M8337 VT8E Video Display Line Buffer Module
M8340 KE8E Extended arithmetic element
M8341 KE8E Extended arithmetic element
M8342 LP8E Centronics printer control (replaces M8329) (x2)
M8357 RX8E RX01/RX02 interface
M840 PC8E High speed paper tape reader/punch (x2)
M8416 KT8AA PDP8A Memory Management (hex wide)
M8417 MSC8DJ PDP8A 128K MOS Memory (hex wide)
M842 XY8E X/Y Plotter Control (x2)
M8433 RL8A Disk controller for RL01/02 (hex wide)
M847 MI8EP Hardware Bootstrap Loader for RX01/RX02
M848 KP8E Power fail and auto-restart (x3)
M849 KK8E RFI shield (x4)
M860 DK8EP Programmable Real Time Clock
M865 . Console Teletype Control (current loop only)
M8653 KL8M Modem Control, used with KL8* to provide data ready stuff
M8655 KL8JA Terminal Control (UART based substitute for M8650) (x3)
M868 TD8E DECtape Control (x2)
M880 MR8E H241 daughter board. Labelled MAINDEC-08-DHMRA-A
M935 OMNIBUS bridge (joins two adjacent backplane segments) (x4)

A couple of new machines have joined my stable, a very clean PDP-8/L and another PDP-8/E.

The /L is complete with the (somewhat important) exception of its core stack which has been lost at some point. If anyone has or knows the whereabouts of a spare stack (I believe they were common to the /I and PDP-12), please let me know. If I can’t find one I will have to resort to fabricating an alternative using NVRAM chips,although I doubt this will be trivial.

The /E is generally clean although its plexy is pretty scrappy. I quite like the dog eared look and so I will leave it this way. The console itself is missing a couple of keys although I have replacements for these. The machine came with no cards, but I have spare CPU sets and so I’m only left needing to find memory which should be achievable.

My PDP-8s have finally got a new home in the form of a very rusty H960 that I recovered in my recent haul.  Before fitting my -8s into it I decided to strip and respray to bring it back to its former glory.

Here are the steps I went through to achieve this:

1. Thoroughly wire brushed all rusty areas to remove the scaly rust.
2. Further removed the rust using a flap disk mounted on a powerdrill.
3. Any remaining rust was neutralized with a rust inhibiter
4. All original labels were carefully masked
5. The rack was resprayed in 2 costs of grey primer followed by 4 coats of satin black paint.

I’m very pleased with the end results, huge improvement. Not perfect as the rust has left the metal work a little pock marked but still very acceptable.

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