The power supply of my vax 3900 is making a weak squeaking sound

[Tony Duell]

> 
> It is on the CPU side. It sounds like there is a high voltage leakage
> problem. Before I open it for an investigation, could somebody with
> experience give me directions about what is the usual cause of this problem,
> which component to check, and how to fix the problem? I have experience with
> analog circuits. Thank you!

A squeaking SMPSU normally means it's detecting an error (overcurrent, 
which may, in turn be due to overvoltage tripping a crowbar circuit which 
then shorts out the suppyl), shutting down (the oscillation frequency may 
then go trhough the audible rangs), then starting up and trying again, etc.

If the problem is certainly inside the supply (i.e. it contines doing it 
on summy load), I'd suspect either a failure of the regulation feedback 
loop (thus casuing the overvoltage problem), or a shorted capacitor or 
rectifier diode on the scondary side of the supply.

-tony