Category Archives: Retro Computing

Consolidated Computer Inc. PDP-8 Clone – Part 9. the regulator boards.

On this machine, the PSUs rectify the power but don’t regulate it. The regulation is done by a number of boards at the right hand end of the card cage.

As before, I’ll just show the photos here and perhaps give more details, if needed in later posts.

-5v regulator board. Top view.
-5v regulator board. Top view.
5v 20A regulator board. Top view.
5v 20A regulator board. Top view.
Regulator board 3. Top view.
Regulator board 3. Bottom view.

Consolidated Computer Inc. PDP-8 Clone – Part 7. the memory boards.

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.

Top view of the 16kw x 13 bit core memory module.

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!

Two view of the upper memory board showing the 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.

Top view of the upper memory board showing the replacement resistor.

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.

View of the core memory array.

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.

Close up of the core memory array.

Consolidated Computer Inc. PDP-8 Clone – Part 6. Working through the PCBs.

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.

Major Registers

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).


Accumulator board top view

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?

Consolidated Computer Inc. PDP-8 Clone – Part 5 3rd PSU refurbishment.

So we are two power supplies down, one to go. This one is a bit of a monster with three main voltages and two extras.

Top down view of PSU 3 before cleaning.
Top down view of PSU 3 before cleaning.

As with the previous PSUs, this one transforms the mains into lower voltages, rectifies them into DC and smooths them out but like the others it does not regulate the voltages. That’s done by a number of regulator boards at the end of the main card cage.

As with the other PSUs, this one is also filthy and with questionable capacitors.

PSU 3 chassis split into three pieces before being cleaned.
PSU 3 chassis split into three pieces before being cleaned.

My first job was to split the two halves and see what we are dealing with. As you can see it’s a mucky mess with dirt everywhere and a lot of rust on the capacitor clips.

The capacitors were removed and tested and just like the other supplies, they were fine. They don’t make them like that any more.

The rear of PSU 3. There are 3 PCBs with fuses.
The rear of PSU 3 showing the PCBs and fuses.

The three PCBs were removed and cleaned up. I took a wire brush to the fuse holders and blade connectors, cleaned the fuses and gave the boards a general spruce up.

One or two of the spade connectors had perished and failed when I was removing them so I made some new cables with new spades and replaced some of the other, dodgy connectors.

I replaces a lot of the nuts and washers as they had corroded quite badly. I didn’t replace the capacitor clips as I want to keep as many original bits as possible but I gave them a good sanding to get the rust off before re-assembling the whole thing.

It all went back together and is looking good. The voltages seem OK too!.

Top down photo of PSU 3 after refurbishment.
Here is the finished unit. You can see the capacitors have been cleaned and refitted. As have the capacitor clips and the cables.

Consolidated Computer Inc. PDP-8 Clone – Part 4 2nd PSU refurbishment.

The second PSU is the same model as the first and I went about its refurbishment in the same way.

Firstly, I removed the capacitors. Here was the first problem. One of the smaller capacitors was leaking. Straight over to RS components to see if I could find a replacement. Bingo, next day delivery! Amazingly I found a capacitor of the same diameter with the screw terminals on the same centres with the same screw thread. Marvellous.

The faulty capacitor from the second CCI power supply. The new capacitor is on the right.

I tested the other caps with my capacitance meter and they all looked feasible. I then brought the up, one by one on my bench suply with the current fixed at < 20ma. As each one stabilised at it correct working voltage, I noted the leakage current and this was typically around 0.6-0.8ma. That’s not bad at all for 40 year old caps.

Next the whole unit got a good dust, the terminals and fuses were cleaned with a wire brush and everything was reassembled prior to testing.

The second CCI power supply after refurbishment. The new capacitor is on the left.

On of the wires running from a rectifyer was showing signes of bad corrosion so I made a new cable up with new connectors and fitted that.

I went through the same tests with car tail-light bulbs as I had for PSU1 and they lit up beautifully.

Two down, one to go.

Consolidated Computer Inc. PDP-8 Clone – Part 3 PSU refurbishment.

A photo of the back of the rack showing three power supplys, one on top of the other.
Here are the three PSUs in situ.

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.

One of the PSUs removed from the machine showing the dirt.
A grubby PSU

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.

One of the larger caps connected to the capacitance meter which is showing 211,000 MFD.
The meter shows there is a lot of capacitance there.

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!

One of the two PCBS from the power supply.

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.

Ready for testing.

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 🙂

To give a clue of the size. A drinks can next to one of the caps

Consolidated Computer Inc. PDP-8 Clone – Part 2

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.

The front panel of the machine showing the 12 rocker switches.
The front panel.

Looking at the front panel above those rocker switches have PDP-8 written all over them. So, what clues does the inside give us?

A view of the bottom of the cab showing the power supply.

Those capacitors look a bit scary. I would be suprised is they are OK. I will need to be careful with that PSU.

The circuit boards stand vertically in a large card cage.
A view into the top half of the right hand side of the cabinet.

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.

Consolidated Computer Inc. PDP-8 Clone

CCi PDP-8 clone in 19 inch rack along side is a 9 track tape drive in a  similar 19 inch cabinet.

Sometimes friends with machines they no longer need will get in touch to pass them on to me. Sometime they have friends who have something they want to get rid of but can’t bear to throw away and so again, I get a call.

I got a message from an old friend of mine, John, to say that a friend of his had a PDP-8 clone that he no longer had space for and was loathed to scrap so asked if I would like it. Of course I said yes please.

At this point all I knew was that it was a PDP-8 clone and it was Canadian. I searched the web for clues but didn’t find anything.

John sent me some photos over and my jaw dropped. I was expecting something like a PDP-8a; A 19″ rack about a foot high. I didn’t expect two chest high 19″ racks, one with the computer, the other with a 9 track tape drive.

Have I been a bit hasty in saying yes?

I did have second thoughts but it’s not everyday someone offers you a PDP and so I couldn’t really turn it down. Luckily I have more than one friend and another, rather generous one, said I could store it in his storage unit for a bit.

The identity plate inside the cabinet. It shows the CCI logo and text.
The identity plate inside the cabinet. It shows the CCI logo and text.

From the photos I could see that it was badged as a Consolidated Computer Inc, machine but a lot of searching on the web turned up very little. I found that CCI had been a computer company in Canada and that they were well know. I found that they did indeed make their own computers but I haven’t been able to find anything about the computers themselves. Not even the name of one let alone the specifications.

A close up of the 9 track tape drive.
A close up of the 9 track tape drive.

The important thing was to get the machine first. I’d worry about the rest later.

After a brief visit to see the machine in the flesh, I hired a van and got some friends together and we got both cabinets into the back of a van and into the storage unit.

No one was harmed in the making of this journey.

Nascom-1. Back on the workbench – Part 3.

For a while now I’ve been trying, on and off, to get some progress on the Nascom.

The Nascom board on the bench

Thing have not been going particularly well since I last wrote a post about this nice but frustrating machine. I decided to remake some of the solder joints in case a dry joint had got worse but that didn’t help. Now I am working my way through the circuit diagram checking each joint to see if that gives me any clues.

The circuit print below shows that I have not got very far. I think the Nascom is going back on the “Shelf of good intentions” for a while.

Nascom-1. Back on the workbench – Part 2.

In the last post about the Nascom-1 I showed a photo of the screen whilst it was running some test software. At first glance it looks good (don’t be put off by the fact that the top line is very different to the bottom, that’s due to the way the screen is mapped).
If you look a little more closely you will see that about half way across and two thirds down there sits a spurious ‘2’. I blame my excited state for not spotting it myself and for having to have it pointed out to me.
This raises a little doubt as to how well things are actually working. A few more tests are called for.


Before I start some more tests let me explain a little about the power on reset circuitry on the Nascom-1. There isn’t any power on reset circuitry on the Nascom-1.
The first image is the reset circuitry of the Nascom.

The second is that of the Sharp MZ-80.
You can see that there is just a pull-up resistor on the Nascom but a more elaborate scheme on the Sharp. The upshot on this is that you get no sense out of the Nascom until you press the reset button.
When I talk about power cycling or powering on, there will always be a manual reset involved. This has become evident 🙂

and more tests.

I did a few test cycles. Each cycle comprised of powering on the machine, pressing reset 10 times and looking for any unexpected characters.

  • Cycle 1, no abnormalities.
  • Cycle 2, fine.
  • Cycle 3, perfect.


A variation on a theme.

Neal suggested a variation on his VDU_test program which was to build the image in ordinary user RAM and copy it across to video RAM.
Over to the editor. crank over the assembler. Fire up the Stag.
Here are two photos showing the results.

This is what I did…

  • Powered up the Nascom.
  • Pressed reset.
  • Took the photo.
  • Pressed reset.
  • Took the photo.

You can see the distorted pattern. As the difference between this and the previous screen dumps is that the image is built in user RAM and copied to video RAM using  LDIR, I’m thinking there seems to be a problem copying from user RAM to video RAM. Perhaps some of the buffers are a bit iffy?!?

Have another think.