Processor card for a HP 9835.

Paul Berger phb.hfx at
Fri Oct 16 14:46:42 CDT 2015

Hi Rik,

As near as I can tell the connections are good, but I am pretty sure it 
is the CPU, the key had fallen out of the monitor cable and I 
accidentally plugged it in backwards, and when I turned it I almost 
immediately noticed the smell of magic smoke escaping.  The +12V 
regulator was wrecked, both the 723 and its pass transistor plus a 
couple burned foil traces on the power supply board.  I have repaired 
the power supply and all the voltages are good now,  the clock input to 
the CPU is good, however the clock outputs are dead, and I do not see 
any evidence of life from the CPU.  From checking after disconnecting 
everything it appears that the +12V output went up to more than twice 
its rated level.   I also fear for the ROMs in the machine but I can fix 
that as I had already designed a ROM card for option ROMs that I could 
easily modify for the system ROMs. I am still evaluating the full extent 
of the damage.


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