Powering up a 20-year old MicroVAX II

[Tony Duell]

> The harness I have is not the same as either of the two that someone sent me
> pictures of. It is not a ribbon cable, it has discrete wires, of different
> lengths, its part number is 70-20450-01, rev C1 and there is a date of 9th
> Dec 1985 on it. The actual connectors though look like the connectors on the
> bad ribbon cable harness, I think they are MTA-156.

AFAIK this is a side issue. While the older pawer harness can overheat 
_in use_, it will not cause the PSU to fail at switch-on. In other words, 
if you have the older harness, you should reuild/replace it, but this is 
not the cause of your problem/

> 
> Some have suggested it blew because I had insufficient load. I had a TK50
> and an RD53 attached, would that no be sufficient. Furthermore I did not

As I said earlier, I'd not use thos 2 devices as dummy loads, they are simply
too valuale for that. Get soem 6V and 12V car bulbs...

While I beleive there are SMPSUs that fail if turned on with no/insufficient
load, I've yet to meet one. I've seen supplies that trip repeatedly if 
given no load. I've seen supplies (including ones from DEC) where some 
of the outputs give far too _low_ a voltage if the main output is not 
suffiiently loaded. But not one that fails.

> actually switch the PSU on, just connected it to the mains, would the load
> connected to the PSU matter when the PSU is not actually switched on?

Hang on Are you saying the rocker switch on the front was not turned on? 
In which case most of the PSU electronis was not powered. About the only 
thing that could have failed in the way you descrie are the mains input 
filter capacitors. 

Have you opened up the PSU case yet? Can you see anything obviously 
burnet or exploded? If so, what?

> 
> There was also a suggestion that the PSU would have needed switching
> separately for 50Hz operation as well. The hardware manual I have for the
> machine tells me how to switch between 110 and 240, but does not say
> anything about switching it for frequency, so I suspect this was not an
> issue here.

It isn;t. There is no frequency adjsutment on the BA23 PSU.

> 
> >From the various responses I think it would be unwise for me to attempt a
> repair. Is there anyone on this list who is in the UK who would be willing
> and able to fix this PSU, or who knows someone who might be?

Where are you in the UK?

-tony