Having taken a look at and a brush to, the first few boards, I have come to the memory boards. This machine has 32kwords of 13 bit core memory in two memory modules.
There is a memory bus controller board but I didn’t take a photo of that. I did get photos of the memory modules themselves though.
The memory modules were made by Ampex and have a customer part number containing the initials CCI. This must mean the boards were made specifically for CCI and were not off-the-shelf-items just bought in.
Looking from either side of the module it is not overly obvious that there is any core memory at all. It’s well protected between the two PCBs that make up the module.
Identical in every way?
When I was cleaning the first of the modules I noticed a broken resistor. I struggled to read the value bands so I took a look at the other module, one I hadn’t cleaned at all, and it had the same broken resistor!
I had a word on the VCFED forums and the view of the team was that the resistors were 510 Ohms. A quick raid of the parts bags and two new resistors were fitted.
Having to part the two boards that make up each memory module gave me the opportunity to take a look at the core memory itself.
The core array is behind a steel plate and a sticker warning me that I was about the void the warranty.
I was very careful when removing the metal plate and I’m glad that I was. This is the smallest core I have see I think. Sadly I don’t have a standard banana for scale but trust me, the cores are tiny.
Having worked my way through the power supplies, it’s time to take a look at the circuit boards in the card cage.
The original PDP-8 had a number of “Flip-chips” that held basic logic circuitry made from transistors. These boards were plugged into a backplane that routed the signals between the appropriate gates on the flip-chips. Later PDP-8s had the processor built up of 3 large PCBs and the later PDP-8 compatibles from DEC, such as the DECMate had a single chip with the processor.
The CCI machine is none of those. Instead it has six boards that make up its CPU. Each is clearly marked with its function, “Major registers”, “Accumulator” etc.
Here, I’m going to describe working my way through these boards, visually inspecting them and giving them a clean.
Please keep in mind the size of these boards. They are huge by Today’s standard. 40cm x 25.5cm.
Front panel / operator console.
Standing on the front of the machine, the front panel contains the switches and LEDs needed to load simple programs into the computer. PDP-8s of this vontage didn’t have any ROM code so do nothing when first powered up. By using the toggle switches and push buttons, it is possible to load and run simple programs.
This machine is a bit unusual in that it uses 7-segment LEDs to show the register contents rather than the more usual one-LED-per-bit arrangement of genuine PDPs. It also uses push buttons instead of spring loaded toggle switches.
On the right you will see the board after I have removed the red tinted acrylic sheet. There is nothing much wrong there, it just needs a good dust.
Com Seq Gen board
The second board in the cage is labelled “Com Seq Gen”. I’m assuming this mean command sequence generator and I’ll continue to believe that until other evidence comes along. It is clearly labelled Sep 01 1976. Most of the other boards are either 1977 or 1978 with chips with dates anywhere in between.
Interestingly, this is the only board in the first six to have any lacquer on it and it’s only on one side.
Just needs a good clean.
Sequence Input Generator
Another sequence generator. This on is labelled “7807” so I’m thinking July 1977. By now CCI must have given up on lacquering their board as this one is raw fibreglass.
I love the idea that all of the parts that make up the CPU are spread around a number of boards rather than under a plastic or ceramic lid.
Extended Memory Control
The basic PDP-8 architecture can only address 4k words of memory. This machine has 32k as we’ll see later. The original PDP-8 got around this limitation with a K8ME board and I’m assuming that this povides the same functionality.
A small about of patching on this board in red wire (centre left).
Here is the brains of the outfit. The accumulator. Nice isn’t it.
Memory bus terminator
I think this is the last board of the CPU. Alternatively, it’s the first board of the not the CPU.
I wonder if there are enough cards on this page to prove that the CPU is alive?
It’s time to take a look at the PSUs and see what state they are in. There are three PSUs in this unit, two are the same, the third is different. All are big and heavy.
From the photographs you can see a big transformer, two massive capacitors, two lesser capacitors and sundry bits and bobs. So what we have here is a pretty standard, if somewhat large, linear power supply.
This PSU has been sitting for possibly decades and so it the capacitors may need reforming as they can degrade with time. In the next photo, I’ve popped them out and given them a clean. There is no signs of leaking or bulging so it’s so far so good.
Out with the trusty capacitance meter… All four are giving plausible values so the signs are good. Time to get some power onto them. At this point I connected the capacitors, one at a time, to a variable voltage power supply that had an adjustable current limit. I set that limit to about 20mA and the voltage to about 3v and gave the capacitor some power. At first the current limit came on. This is to be expected. Then the voltage rose to 3.5v and the current limit went off. Next I increased the voltage by a volt or two. The same thing happened. I steadily, over the course of a few minutes, increased the voltage to the full 15v these capacitors are rated at. There were no surprises and no incidents. Few!
Next, I removed the two PCBs, on at a time and cleaned up the terminals and fuse holders with a brass wire brush. These had a layer of corrosion on them and it’s a good idea to get rid of that.
Having cleaned up the terminals and given the whole thing a clean, it was time to put it all back together.
With it all back together it’s time to see if we’ve got a good power supply of a machine for making smoke.
As this PSU is supposed to kick out about 10v (unregulated) I got a couple of 12v car bulbs and added spade connectors so I could use them to put a bit of a load on the PSU. With a linear supply, this is not stricktly needed but it’s a good idea.
At this point I admit I was a little scared that I might let the smoke out and so the first test was done in my workshop with the PSU on the end of an extension cable and with me in another room with my finger on the mains switch.
Flick… All is well. A cheery glow through the crack in the door showing that the bulds were lit. I left it like that for 20 minutes before switching it of and moving it back inside for the photos you see below.
I have measured the voltages and both sides of the PSU are giving a smidge over 10V and according to my ‘scope, there is very little ripple.
Well done our side, just the other two to go now 🙂
In the previous post I described getting my CCI PDP-8 clone. As I write this, I don’t know for sure that it’s a PDP-8. I was told it was and looking at the style of the machine it look perfectly reasonable but looking around the web, you won’t find many PDP-8 clones.
Looking at the front panel above those rocker switches have PDP-8 written all over them. So, what clues does the inside give us?
Those capacitors look a bit scary. I would be suprised is they are OK. I will need to be careful with that PSU.
The first slot has the front panel. The next 6 slots seem to contain the processor. I think the PDP8-a has three for the processor so it’s not one of those in a different box.
Slot 6 is labelled “ACCUMULATOR”. The photos below show what it looks like.
The logo on the second photo is that of CCI and so It’s safe to say that this isn’t a re-homed PDP-8l, m or similar. It looks like CCI made their own machine. Curious.