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-boun...@classiccmp.org] Namens > curiousma...@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-...@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 > >
