VAXmate PSU Failure

Rob Jarratt robert.jarratt at
Sat Feb 15 17:02:33 CST 2020

> The UC3842 datasheet does show you can draw up to 20 mA from the Vref pin,
> which should certainly power up a 555.  But, this is a 5 V regulator, and
> not be working properly.  it may be that driving several mA to the 555, E2
> other components has cooked it over time.  With the 555 removed, apply
> resistors between pins 1 and 8 of the 555, and see how much load the
> can supply.  I'm guessing that at 1 - 2 mA the Vref output is going to
> indicating the UC3842 is "weak". If the Vref does droop, make sure the Vcc
> still well above 5 V.  If so, it has to be the UC3842. If the Vcc is also
> then follow the chain back to the power input to find the defective
> It could be Q1, R27 R28,
> R32 or for some reason the crowbar D19 is being fired.

I just tried this with a 1K resistor I had to hand, so 5ma not 1-2ma, and
the Vref droops. Vcc does not droop though, so I think the UC3842 needs to
be replaced. I will try that.

Incidentally, the UC3842 datasheet says that Vcc must have a short-path
bypass capacitor to ground. If I socket the UC3842 is that going to be an



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