H7140 PDP11 PSU - test-drb@ccmp.vtda.org

30 Apr 2011

...
  I have extracted the leftmost board, which is a
5413605, according to
 printset that is a "bias and interface" board. Looking at where it is
 mounted in the PSU, it *appears* to be connected to the fan spade
 connectors, so would be involved in powering the fans (the motherboard
 printset would seem to confirm that). There is a 74LS00 chip on it to which
...
  I will attach wires to Vcc and Ground so I can test if
for power. I followed
...
  the Vcc track on the back of the board and it does not
appear to go to the
 boards "backplane" connectors. I tried to read the printset to see which
 pin(s) on the connector the power comes in on but I don't understand the
 conventions about which pin is pin 1 on the connectors, so I can't at the
 moment add wires for that. I will go ahead and add wires to the 74LS00 in
 the meantime. 
See my other messages :-)

The 5V line you are interested in comes from a 7805 regulator (TO220
package,
looks like a power transistor) on that PCB. I suspect it does power the 
74LS00 you mention too... 

The ground for this supply is the same as the machine's logic ground (this
will
not be the case if we have to sort out the control circuitry of that little
SMPSU). 

Yes, the fan full-H driver circuit is on this PCB, so it's not suprising
it's
connected to the fan terminals.

-tony

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