I wonder if the crowbars might have been a later revision of of the
power supply. After the accident with my 9835A I had to completely
rebuild the +12V regulator both the 723 and the pass transistor as well
as a few passive components. I did not see any evidence of crowbars,
and the power supply in my 9835A seems to match the diagram in the 9835
service guide. If I was to add crowbars to this power supply I would be
temped to monitor the voltages and crowbar the raw DC when an
overvoltage is detected, in my case it was the -20V raw that did all of
the damage.
Paul.
On 2016-08-13 6:37 AM, Rik Bos wrote:
For what it's worth a small warning about the HP 9825 series computers.
The power supply doesn't have a crowbar(over voltage protection), so a
transistor failure in the Psu can be catastrophic.
On the other hand the two 9835's I have, which uses the same form factor and
almost the same power supply layout are HP modified with crowbars added.
It seems to be good practice to add some ov-protection to the HP 9825 supply
because the switching transistor and 723 voltage regulators don't have the
eternal life.
And there no certain prediction in how they fail, short or open circuit, I
found out the hard way several years ago.
-Rik
