Repair methods (was Cromemco 3101/Beehive B150 score)

For modern machines, that is the way the manufacturer wants it done, when doing warranty work, they will not pay for true diagnostic work. My company is not even required to have an oscilloscope, though we were until recently required to have an implosion booth with a mirror for when calibrating a CRT monitor or changing a CRT. Now all the CRT monitors are out of warranty and they only sell LCD displays we have taken it apart. Yes, and of course maybe the temperature had changed slightly or the voltage had drifted. Four of the power rails in the 1301 are fitted with marginal check facilities, it is possible to switch them off of the standard power stabilisers and wind them up and down via knobs on the machine's front panel. There is also a facility for varying the clock rate up and down. In service, engineers were issued with special hammers to hit the printed circuit boards with to check that vibration did not cause a failure. In all there are about 100 power supplies in the machine, and nearly all have potentiometers hidden away under the covers which adjust the voltages. There is a big difference between fault finding digital electronics and designing digital electronics. Where there is an analogue signal, like in the Drum Write amplifiers I can cope with that, but there is some weird stuff, like there is a piece of what looks like 4 inches of coaxial cable - which is apparently a delay line and the signal bounces off the far end and comes back a bit later. There are 250 of these in the core store, they never go wrong, but the boards they are on tend to burn out one of their resistors frequently, and the core store itself is a current device, so fault finding with voltage based instruments can be tricky. What I find difficult in an analogue circuit is finding the order of what drives what. Sometimes its easy, but often the circuit diagrams are drawn in an illogical order, and sometimes there really is deliberate feed back, the flip flop is an obvious example. One time one of the three phase transformers was frying, it was in a very awkward place. I pulled the output fuses and it continued to fry. Eventually I changed it and the new one did the same, it was the three phase bridge rectifier (six big diodes) which was shorting out, I had not realised it came before the three fuses (feeding three circuits) rather being on the three secondaries of the transformer. Yes I should have checked the circuit diagram, but that meant searching through several hundred circuit diagrams, and I thought I knew what was doing. Wrong! Anyway, no harm done. I have thought about it, its one of those things I will do when I get a round tuit. But I am hoping a museum will take it sometime as a working exhibit, it is the last working one if its kind, and at one time its type was about 25% of the computers in Britain. Its biggest selling point was pounds shillings and pence in hardware. Then the IBM 360 came along and wiped the floor with all the other computer makers. What museums hate to see is modern components, so where there is a choice I stay with original parts, and always keep the original faulty parts when I have to use modern - such as when I put an old Apple switch mode power supply in place of an old supply which used a transformer which is no longer available. Actually there were several identical power supplies in that Paper Tape Reader, and they all plugged into a big connector which had links to configure the power supply for the required voltage needed in that position. I put the duff one in a position where the required voltage was nearest to the Apple's voltage (I think it was -17.1 volts, so I used the -12 and +5 volt rails and kept the other voltages, including ground and the case insulated). Roger.