Tag Archives: pdp

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 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.

Tadaa!

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.