VT100 PSU smelling.

Mattis Lind mattislind at gmail.com
Fri Jan 22 17:56:20 CST 2016

2016-01-22 21:48 GMT+01:00 js at cimmeri.com <js at cimmeri.com>:

> On 1/22/2016 2:54 PM, Mattis Lind wrote:
>> 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.
> You don't use Variacs with switch mode power supplies... not unless you
> want to burn them up.   I'll sometimes use one, but only to provide a very
> fast soft start.  Even that's risky.

Well. It depends if you read the schematics before you do so. In this case
I powered the startup voltage from a 12 V bench supply. Normally the
startup voltage comes from a small mains transformer and a 7812. Then I
supplied the primary side voltage for the main switch transistor using a
variac. Works perfectly well if you know what you are doing. Have done this
practice with many SMPS supplies.

> - J.

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