Big Iron (was VLSI of classic architectures)

Roger Holmes roger.holmes at
Thu Jan 17 13:51:27 CST 2008

>> Current custodian of the University of London machine, having  
>> bought the
>> 200,000 pound machine for 150 pounds in 1976.
> Still a 200,000 pound machine, regardless of the price.
> That was big iron back then. :)  How hard is to keep and get spares
> for that system?

I bought a machine which I stripped for spares and also the previous  
owners had got most of the parts from the factory when they had  
cleared their parts store.

Some SES light bulbs are still available from classic car parts  
stores, and wedge lamps were used in old telephone exchanges which  
sometimes come up on eBay, I even got some 17volt ones a while back,  
and some are still available new from RS components and they even  
make LED equivalents at a price, though they're the wrong shade of  

The rarer transistors sometimes come up on eBay too. The commonest  
(Mullard GET871/GET872/A/B/C) are difficult to get, though I passed  
on a half dozen on eBay where the blue plastic had turned brown -  
looked like they had blown up or got hot when de-soldered.  
Fortunately I have enough for the foreseeable future.

Diodes (mainly OA5 and OA10) did have sky high prices and now are  
unavailable but I've got thousands.

Of course resistors, capacitors and inductors are still available,  
though they do not look the same, and I still have to find out why  
the resistors have double green bands where there should be a gold  
band for the tolerance. I have been lucky so far with the  
electrolytics, not one has blown up, despite being 46 years old. They  
bought the best components available back then, and hand built them  
and tested every soldered joint and marked it with lacquer to show it  
had been tested. The biggest electrolytics are bigger than a hand  
grenade, so I hope they never fail!

There two types of valves in the tape drives. I got about a hundred  
of one type with the machine, and not used one, the other type I've  
picked up in the States on eBay, they are used in old CNC spark  
eroders apparently.

I am rapidly getting though my stock of 'rubber' pinch rollers for  
the mag tape drives, which go hard and crack up. The same thing  
happens with the rubber-like roller in the punch card reader, but  
that is about 4 inch diameter, so it is currently wrapped in four  
layers of motorcycle inner tube, though its not very effective and I  
may have to mould a new one in synthetic rubber.

Most of the mechanical parts are simply unavailable, but I have a  
lathe and a milling machine.

I bought some flat belts for driving the tape capstan rollers, they  
are commonly used as knitting machine drive belts apparently. V belts  
for the drums and line printer are fortunately standard car parts.

Line printer ribbons will be a problem (when I get the printer  
working again). I have a box of a dozen or more but if they have now  
dried up by now I will be amazed. There used to be re-inking firms  
around, or maybe I could just buy the ink and get my hands dirty.  
Should not be unsurmountable, even if the fabric has failed, its just  
(nylon?) cloth, should be able to get something to do the job.

I've a good stock of blank punched cards and paper tape (5 and 8  
track) and enough mag tapes to keep me going for a long time. Fan  
fold paper is probably still available new and when I see any cheap  
locally on eBay I buy it up.

The logic indicator tubes (DM170 or something like that) never seem  
to fail. The microswitches (particularly the start switch which takes  
a hammering when single stepping a program) are in short supply,  
though I don't see too much of a problem in fitting modern sub-micro  
versions and extending the actuating arms. Multi-segment rotary  
switches just need a squirt of switch cleaner/lubricant from time to  

One problem, and it sounds stupid, is the rotary switch knobs. They  
are made of plastic and they fit on a 1/4 inch shaft with a flat  
section, nothing unusual there. There is a boss on the back which  
fits into a recess in the formica covered blockboard (yes the  
computer has wood in it) front panel. These bosses break off. The  
shafts are not long enough for most modern knobs, and anyway I want  
to keep it looking original. Because of the recess, it is not  
possible to use knobs with a screw. I am thinking about making some  
aluminium knobs which I can glue or screw into the original front  
parts of the knobs. The problem is making the flat part which fits  
the flat of the shaft. I suppose I could make them of brass and then  
put sufficient soft solder in the hole so that I can shove it onto  
the shaft so that it won't come off easily. I guess there must be a  
tool for making D-shaped holes somewhere, or maybe there should be  
some spring steel in there somehow.

The relay contacts need cleaning sometimes, though they are platinum  
so are pretty reliable.

There are no edge connectors in the computer itself, every one of the  
4000 PCBs is wire wrapped in, which means there are about 50,000 test  
points. The racks in the computer are also wire wrapped together, no  
plugs and sockets. This all improves reliability and I think accounts  
for the machine's survival (well that and my eccentricity). The tape  
drives have some plugs and sockets and edge connectors, but have so  
far been reasonably reliable as well.

Castors were a nightmare. The tyres on the ones on the (800 lb) tape  
drives crack up. The ones on the computer have a flat top with a stud  
poking out. The stud has a washer and a nut and fit into an open  
ended slot with another nut on top. Any jolt and the stud bends. If I  
have to move anything heavy now, I remove the washer and first nut  
and make up a three inch piece of wood the equivalent thickness to  
take their place, which spreads the load over a larger area instead  
of concentrating it on the stud.

Fuses: small ones still available from Maplin for peanuts, large ones  
are NATO standard and cost silly money, so I drill the ends, empty  
out the sand and solder 10,15 or 20 amp domestic fuse wire into the  
old cartridges.

Meters, so far very reliable apart from one where the needle would  
not go above a certain point, so I had to dismantle it and glue the  
glass back in which was what the needle was jamming on.

I think that just about covers all the parts which go wrong.

Roger Holmes

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