Testing H7864 (MicroVAX II) PSU With No Load

tony duell ard at p850ug1.demon.co.uk
Sun Nov 1 13:54:27 CST 2015

> You mentioned DEC PSUs, this one is actually made by ASTEC. I will use a

Firstly, every Astec PSU I've seen has been a swticher. They probably did make 
linear PSUs, but not common...

Secodnly, it may have been made to a DEC design (rather than just a specification). 

I believe you replaced the chopper transistor. Astec were, alas, notorious for using
'selected' transistors in some of their supplies. Often marked with a house number
which I guess yours wasn't, but it is one to watch for.

> dummy load and put it on an isolating transformer when using the scope.

Good idea. Don't do what is suggested in the Philips P854 manual which is that if
an isolating transformer is not available to disconnect the earth lead of the 'scope
inside the plug and have the 'scope floating. The idea of a metal cased 'scope being
a couple of hundred volts from local earth does worry me!


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