Testing VT101 Power Supply

Robert Jarratt robert.jarratt at ntlworld.com
Sat Jan 17 12:09:42 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of tony
> Sent: 17 January 2015 16:54
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: RE: Testing VT101 Power Supply
> > In my case of vt100 powersupply test (i hope that i remember corretly) i
> to load 5V with a small load.
> > With nothing connected, some voltages fail.
> From what I remember, the VT100 and VT101 PSUs are very different. The
> former is a fairly conventional SMPSU, albeit one which (as was typical
for DEC
> at the time) has the control circuitry on the output side of the isolation
> and a little linear startup PSU. The VT101 has a large mains transformer
and a
> regulator board, I think some of those regulators are switchers, but they
> from a lowish input voltage (not mains) and the isolation is provided by
the big
> transformer.

Yes, that is right. There is a big transformer and the outputs are 10V and
18V (the service guide for latter shows 20V on the diagram and says 16V in
the text!). 
> I don't know about running them with no load, although I could well
believe the
> VT101 PSU will work correctly like that.

In the end I created a dummy load. The outputs all look fine and there is no
ripple. So I am very pleased with the state of the PSU. I am replacing a
couple of caps on the monitor control board, one because it has high ESR and
one (C439) because this page (https://trmm.net/VT100) says it is underrated.
Then I am going to have to bite the bullet and connect it up, hoping the
monitor control board is OK and isn't going to damage the flyback or
something else.



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