More parts I need to replace on the VT100

Julian Wolfe fireflyst at
Wed Nov 30 20:08:40 CST 2005

Thanks for all the advice, and no, I didn't find any offense whatsoever in
your post.  I think you for the advice and I checked that book out from the
library here at work (I work at a community college)


> -----Original Message-----
> From: cctalk-bounces at [mailto:cctalk-bounces at]
> On Behalf Of Tony Duell
> Sent: Wednesday, November 30, 2005 6:44 PM
> To: cctalk at
> Subject: Re: More parts I need to replace on the VT100
> >
> > 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
> 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

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