Flex Disc options for the HP 9825

[curiousmarc3 at gmail.com]

Thanks!
Marc

> On Aug 13, 2016, at 1:19 PM, Rik Bos <hp-fix at 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-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
>