13 Aug
2016
13 Aug
'16
1:19 p.m.
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
>