Thanks! Marc
> On Aug 13, 2016, at 1:19 PM, Rik Bos <hp-...@xs4all.nl> wrote: > > 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 >
