21 Apr
2009
21 Apr
'09
11:13 p.m.
Tony Duell wrote:
You are free to wonder.
I suspect it is as Dwight was suggesting, the cap probably isn't doing much of
anything and the lack of C just doesn't matter in the scenario.
I was repairing an HP 9815 (desktop programmable
calc) a month ago. While
tracing it out to make the schematic, I noticed that a 680 uF, 25V electrolytic
I am wondering why you traced out the schematic of a 9815, when you can download both the official service manual
(boardswapper guide) and
unofficial full schematics (covering both main versions of the CPU board)
from http://www.hpmuseum.net/
The service manual I found previously, and contains the schematic only for
the power supply (which I had already RE'd by the time I found the manual).
IIRC, during a web search I saw something about a schematic or full service
manual available on a CD.
Anyway, it (my RE'd schematic) is all done.
Reading the ROM contents might be a future project.
Anyway... the 9815 is a somewhat odd machine. It's
the only HP desktop
What was the fault?
Symptoms were that sometimes certain (groups of) keys would not work. As might
be expected the groups corresponded to row/columns in the matrix, but the ones
that were dropping out didn't make sense in terms of the scan sequence. It
turned out merely that the +5/GND to the logic boards was a little low. Some
cleaning of an inter-board power supply connector and it was fine. One of the
keyboard scanning ICs had simply been the first to suffer as the supply dropped.
I made note
of it in a repair log for the unit, but left it as is. Someday it
night be interesting to take it out to measure it's characteristics.
Alas I didn't keep the one I replaced in the HP120, so I can't check that.