VAX 4000-500 PSU Overload?

[Glen Slick]

On Thu, Dec 10, 2015 at 2:30 PM, Robert Jarratt
<robert.jarratt at ntlworld.com> wrote:
> My VAX4000-500 will no longer power up, with the PSU starting up and then
> immediately shutting down. I suspect a possible short somewhere. I have
> measured the resistance of the load presented to the PSU by connecting
> probes to the backplane sockets used to power the machine. The odd one is
> the 5V load. With all the boards in and drives inserted I measure a
> resistance of about 4R. As I pulled out boards, drives and fans, it
> gradually crept up to 6R. So with nothing connected to the backplane I get a
> 6R load across the 5V supply.
>
> Additionally, the 12V side seems to be charging a capacitor as the
> resistance slowly climbs to about 130K. Is that reasonable? Again, nothing
> but the backplane.

In the VAX 4000-200 in a BA430 chassis the M9715 board contains two
+5.3V supplies based on LT-1086 regulators and powered from the +12V
supply to provide two separate DSSI and SCSI bus termination voltages.
Each supply has a 100uF caps across the regulator inputs and outputs.

In addition the M9715 board contains a DS8641 bus transceiver powered
from the +5V supply. I'm not sure exactly what that does. I believe it
is monitoring the SCSI bus termination voltage and driving a bus
signal back to the H7874 supply, possibly to shut down the supply if
too much current on the SCSI bus termination supply causes it to drop
out.

I haven't pulled a BA440 chassis apart enough to take a close look at
the equivalent backplane circuitry there. In one photo I took a while
ago I can see two TO-220 packages and at least a couple of caps in the
middle of the storage backplane. Possibly the BA440 backplane contains
the same DSSI and SCSI bus termination supplies and monitoring
circuitry that is contained on the M9715 module in the BA430 chassis.

So with all boards, drives, and fans removed from a BA440 chassis
there will still be some load on the +5V and +12V lines.