More parts I need to replace on the VT100

Tony Duell ard at
Wed Nov 30 18:44:02 CST 2005

> Okay, I pulled the power supply on the VT100 after replacing the two video
> board components, only to find 3 resistors on the PSU burned out.  I know I
> should probably get a book on how to read a schematic (I will soon) but in
> the meantime can someone tell me what these resistors are?

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 

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.


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