More parts I need to replace on the VT100

-----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Tony Duell Sent: Wednesday, November 30, 2005 6:44 PM To: cctalk at classiccmp.org Subject: Re: More parts I need to replace on the VT100 video know I in I have not tried to seriously look at your diagram, but I am told it's part of the PCB layout with 3 resistors marked on it. What you need to do first is to find the component layout diagram (it should be in the printset) and find out the references for those components. It will be something like 'R34'. 'R' for resistor, of course -- Other common ones are : C : Capacitor D : Diode T : Transformer (sometimes transistor) U or E : IC CR, MR : Diode (Crystal Rectifier, Metal Rectifier) Q : Transistor L : Inductor TR : Transformer The, you find those resistors on the schematic and work out what part of the circuit they're associated with. Please do not take the nest part as any sort of flame, it's not intended as such... I notice you've been offered a complete replacement PSU for $75. That seems like a reasonable price _but_ I think the old one can be repaired for less. I am going to asusme that you do not depend on this VT100 for income, health, or anything like that, and that while you want to get it working again, time is not too important. The think is that resistors do not burn out for no good reason, particularly not in SMPSUs (Switch Mode Power Supply Unit). Most often that 'good reason' is a shorted power transistor, which may, in turn, have failed due to a problem elsewhere (perhaps a faulty capacitor). You need to find the real cause, then (and only then) replace the faulty parts. SMPSUs are notoriously nasty to work on. They have live mains where you may not expect it (although the VT100 supply is relatively nice in this respect, much of the control cirucitry is isolated from the mains so you can make measurements there in relative safety. There is live mains on the PCB of course). A relatively minor fault can ruin a lot of expensive transistors at switch-on. Yes, I do think it can be repaired, and I think you will manage to do it. What I don't think is that just replacing these 3 resistors will help. You are going to have to learn to read and understand a schematic, and learn some of the theory of the SMPSU. If you don't already have it, get a second edition of 'The Art of Electronics' by Horrowitz and Hill. The second edition has a section on SMPSUs (with the full schematic of the Tandy 2000 sypply...). This, IMHO, is the single best book on electronics around. Yes, there are plenty of other books you may need or want later on. But this one is very clear, I've found nothing to really disagree with in it, and it goes way beyond the 'this is a resistor' level. When you've identified which the resistors are, I'll look at the prints and try to explain that particular bit of circuit. -tony