VT100 PSU smelling.

Mattis Lind mattislind at gmail.com
Fri Jan 22 13:54:21 CST 2016

Short: R27 in my VT100 PSU is hot and smelling. Why?

Long: I think it has been 20 years since I powered up this VT100 so I did
it carefully. Used a Variac and a bench supply. It switched just fine and
delivered the steady 5V out when the input was at approx 50V (115V input).
All the other voltages looked fine at full AC input. But there was this
little smell from R27.

Then I plugged everything in and fired it up. Now there were considerable
more heat generated in R27 which is a 13W / 1kOhm power resistor. But I
still had 4.99V over a random TTL gate on the logic board so the PSU seemed
to operate just fine.

R27 is part of the snubber network on the primary side of this forward-type
SMPS PSU. But why it it getting so hot. Is it normal? I have completely
forgotten how a VT100 smell when running...

Anyone out there with a VT100 that can put his or her nose above R27 and
tell me the temperature?

One interesting thing is that DEC apparently did some kind of ECO since my
resistor is 1k while the schematic tell me 500 ohm. It doesn't seems to
have been replaced.

I checked D27 and that one looked fine. Could there be some marginal
problem in the circuit somewhere that cause excessive hearing of R27? The
primary side looks fairly simple so there are not many components that
could fail completely which wouldn't cause complete death of the PSU.

C19 is part of the snubber network and is a 0.0033uF 1600VDC film
capacitor. Can these go marginal somehow? I never heard of that but maybe?


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