[Info-vax] VAXStation 3100

[David Wade]

On 16/01/2019 20:41, Bill Gunshannon wrote:
> 
> OK, before I go screwing with it any more than I need too....
> 
> I fired  up the first 3100.  It has 7.1 (but I don't remember
> any of the passwords.  Not a problem, I know how to break into
> the box and reset the system password.) and 5.5 on it.  That
> must go back quite a ways.
> 
> But here's the new problem.  I plugged an Expansion box in
> the same outlet strip and connected them together. Kicks
> the GFCI breaker!!  Disconnect the SCSI between the two
> boxes.  As soon as I turn the second one on it kicks the
> GFCI.  Either one is OK by itself, but the two together
> kicks the GFCI.  Other than a wiring problem in the power
> supply (I assume it would take one of them having the white
> and black wires reversed in side the box) what else could
> cause them to kick a GFCI as opposed to a circuit breaker?
> 
> bill
> 

Its almost certainly not a faulty GFCI. Thats expected behaviour with 
older equipment that has a slight leakage to ground. GFCI's are set to 
trip at certain leakage current, in the UK its 30Ma but may be different 
in USA.That is if the current on the live is more than 30ma different to 
the current on the neutral it will trip.

So if each box leaks 20Ma to ground they will work fine when connected 
separately. Connect them both to the same strip. Leakage goes above 30Ma 
the GFCI will trip.

You always get Earth Leakage in computer equipment, especially in older 
equipment because they often have filter capacitors that connect the 
live and neutral to ground, to filter out spikes.

If these capacitors become leaky then they conduct more than the design 
spec.

Putting the two devices on separate GFCI's may not help, because when 
you connect the SCSI between them you connect the two "grounds" 
together. In practice the two will not be at the same potential and may 
increase the un-balance and increase ground leakage.

A UPS may help if it has floating sockets, as will an isolating 
transformer, but you then have no ground, and you may cause RFI or other 
problems.

The real fix is probably to replace the RFI filters on the PSUs...

Dave