Introduction - Terry Stewart (tezza)

I've never had to remove any sizeable creatures, fortuantely... And I tend to work on machines that are large enough to contain them. Ouch!. It depends on the power supply. You want to load it at a reasonable current, but not enough to damage it (obviously). So for the sort of SMPSU found in older desktop machines, I might load the 5V line (main output) at about 1-2A. So perhaps a 6V 12W car bulb. For some of the PSUs in my larger machines, I've used the 2 filamanets of a 6V headlamp bulb in parallel (say drawing 10A between them). In genreal, the other ouptus 'tag along' and don't need to be loaded, but there are exceptions to this. If an output has its own _swtiching_ regulator, then it probably does need a load. How do you know if it's has its own switching regualtor? Well, a scheamtic of the PSU will tell you, but they are hard to find for many machines, even when there is a service/technical manual. Often the PSU is a bought-in module, and no schematic is given. Even when it is, it may be misleading (the one in the TRS-80 M3/M4 manual is definitely incorrect!) If in doubt, it doesn't hurt to load all outputs. Perhaps about 0.5A for the +12/-12V outputs (A 12V 5W tailalamp bulb :-)). Of course if there's a label on the PSU giving the output current ratings, you cna use that as a guid. Use a load around half the maximum load given there. In amny machines the PSU is a separate model with a cable carring the outputs to the logic boards. This is the easiest type to test, just unplug that cable and conenct up the resistors. Then tun on the PSU and check the voltages across them. Some machines (I am thinking of some of the HPs I've worked on) have much of the PSU on its own PCB(s) which plugs into the machine's backplane. The problem comes when that backplane also carries a significat amount of logic in that you can't easily isolate the PSU outputs from that logic. In some cases I've made up test boxes with suitable connectors to plug the PSU board into, wirtes to resistors as a dummy load. That's probably only worth doing if you have several such amchines to work on. Probably the worst are machiens where the PSU regualtors are on the same PCB as the logic. better-grade stuff (again, some HP instruments are like this) have jumper links you cna remove to disconnct the PSU and thus test it withoug risking the logic. Others don't. I am not sure what yuou could do with a VIC20, say, other htan power it up and hope. Incidentally, my general procedure for bring up a newly acquired machine goes something like this (it can vary, of course, depending on the machine) Dismantle it as far as is sensible. I don't open HDAs, but just about everything else comes apart Visually inspect everythign. Look for bad connections, burnt parts, missing components, modfications, etc. If necessary, trace out full schematics (alas this often _is_ necessary for the machines I collect)., Dismantle the keyboard, clean keycaps and switch contacts. Dismantle, clean and oil fans and other mechancial bits that might need it. Obviously this depends a lot on the type of device. Assemble enough of the PSU/machine to test the former. Sart by doing an earth continuity test (at a suitably high current, I don't have a PAT tester, but testing at twice the fuse current should be OK for home use :-)), and then do an insulation test (at 1000V). Then apply mains and check the PSU outputs on dummy load. Check and align floppy drives on the exerciser (did I mention I have some odd test gear here :-)). A hint here. If you are goign to dismantle a (working-ish) floppy drive, connect it to the exerciser first and seek to cylinder 0. Then record the head position, e.g. by measuring the gap between the head carriage and the end stop with feeler gauges. When you put it back together, again seek to cylinder 0 and adjust so the gap is the same. This is not accurate enough to replace a proper alignment, but it will get the heads near enough that you can at least see the CE pattern on the alignment disk. Put the machine together, starting with a minimal configuration, even one that will fail power-on diagnostics in a known way. An example of that is that an HP9836 will give an error if it can't find a floppy controller board. But powering up without the floppy controller and getting that error is basically saying that part of the machine is working. Power up and see what happens. Now for the fun part. Track down and cure all the faults... A couple more tips : Get one (or more) of those divided plastic boxes (The Raaco ones are about the best we get over here). When you take the machine apart, use a compartment for each set of mounting hardware (casing screws; PSU fixings; mainboard fixings; etc). If necesary put a slip of paper in the compartment indentifying the hardware. When yuu come to put it back you'll know that the PSU screws are longer than the motherboard screws (or whatever). Don;t be afraid to make notes. I am old-fashioned, so use a paper notebook and pen to record things like which way round a connector goes (if it's not obvious). Or which way the fan points. Or... Some people take photographs for this. Not having a digital camera, I've not tried that method, but it sounds OK. ------------- Incidentally, I've been poking around your web site. I don't agree with all your views on the machines you mention, but why should I :-). A couple of points... Actually the serial and video connectors on the BBC micro _are_ standard DIN plugs. They're not hard to get in the UK. My moan is that the serial connector is stupidly wired. The point is that the qunicuncial (Form C 6 pin) DIN plub will fit either way up. But turning it over doesn't produce a sensible swap of signals. On the Tatung Einstein, turning the plug over swaps TxD with RxD and RTS with CTS, which is useful. I see you have dismantled an IBM colour monitor.... Been there, done that too.. You are lucky it wasn't the 5151 MDA monitor. On that one, you flip off the little coves on top and take out the screws under them (as on the colour one). Then put it face-down and undo the 2 screws going through the flange on the rear case into the front case (again, as on the colour one). There arre 6 more screws on the bottom, you undo _only_ the 4 on the rear case section. Then the rear case comes off. If you undo all 6, the heave mains transformer unit comes loose and hits the back of the CRT. fortuantely I did not discover that the hard way. -tony