Flex Disc options for the HP 9825

[Rik Bos]

Marc,

Building a crowbar is easy, just take a Zener a little higher than the psu
voltage eg 5.2V for 5V rail put a resistor of 1k in series take a Thyristor
big enough to take about 150% of the schort current and connect it to the
zener through a small resistor. I suppose you can do the math  ;) 
Be sure to put a crowbar on both the +12V and +5V and you could consider one
on the -12V rail but that voltage doesn't have a pass transistor but a 320K
voltage regulator which has a reasonable protection for over voltage.
The -5V is connected to the -12V through a zener.
Tony reverse engineered the schematics, you can download them a the HP
Museum website or become a member of the HPCC and order the CD with all
Tony's diagrams (a lot of eexcellent work).

-Rik

> -----Oorspronkelijk bericht-----
> Van: cctech [mailto:cctech-bounces at classiccmp.org] Namens
> curiousmarc3 at gmail.com
> Verzonden: zaterdag 13 augustus 2016 12:15
> Aan: General Discussion: On-Topic Posts
> Onderwerp: Re: Flex Disc options for the HP 9825
> 
> Thanks for the info. Any schematics of the modification?
> Marc
> 
> 
> > On Aug 13, 2016, at 2:37 AM, Rik Bos <hp-fix at xs4all.nl> 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
> >