Flex Disc options for the HP 9825

Paul Berger phb.hfx at gmail.com
Sat Aug 13 17:19:02 CDT 2016

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.


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

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