H780 power supply - test-drb@ccmp.vtda.org

10 Apr 2013

...
  Date: Tue, 9 Apr 2013 17:04:30 -0700 (PDT)
 From: Brian Roth <abacos_98 at yahoo.com>
 To: ClassicCmp <cctalk at classiccmp.org>
 Subject: H780 power supply
 Message-ID:
 	<1365552270.75693.YahooMailNeo at web141403.mail.bf1.yahoo.com>
 Content-Type: text/plain; charset=us-ascii

 Started tearing into the 11/780 and pulled the 11/03 and the RX01 out to clean and test.
Looks like the PS is bad in the 11/03. It powers on and the fans run because they are AC
but I am getting sometimes 4.5 volts on the 5v line and 0 volts on the 12v. I have 34
volts coming out of the diode rectifier. I suspect switching transistors.
    First, if it has Molex/AMP Mate-n-Lock connectors, check them for bad 
contact.
DEC loved these during the PDP-11 era, and I had a constant battle with our
used 11/44 to keep it running.  The contacts would develop an oxide 
film, run
hot and eventually burn up the contacts, connectors, wire, etc.  Especially
with fluctuating output, that soulds very much like poor contacts.
I never tore the LSI-11 apart on a 780, so I don't know what the PS
looked like, but I would bet there are some of those AMP connectors in
it with the crummy tin plating.

Jon