Reforming caps and CRTs

Tony Duell ard at
Wed Apr 22 15:12:58 CDT 2009

> Tony Duell wrote:
> >
> > >  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
> You are free to wonder.
> > download both the official service manual (boardswapper guide) and
> > unofficial full schematics (covering both main versions of the CPU board)
> > from
> 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).

I don;t know where you found that manual, but it is certainly available 
from, along with the schematics of the 9815 (and many other 

> IIRC, during a web search I saw something about a schematic or full service
> manual available on a CD.

Ah yes,, the HPCC schematics CD. That contains the same schematics as on along with a few others and ones for most of the LED HP 
handhelds. FWIW, foe all I spent _many_ hours producing those, I don't 
get a penny (cent, whatever) from the sale of the CD-ROM.

> Anyway, it (my RE'd schematic) is all done.

Well, if people would rather produce their own, I'll stop doing them 
unless I also need them....

> > Alas I didn't keep the one I replaced in the HP120, so I can't check that.
> 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.

Almost certainly. It was just a decoupler on the 12V rail (and not a 
particularly good one, the HF stuff being taken care of by several 
ceramic capacitors).. I didn't other to test it, when I found it was in 
backwards, I dug out a replacement and fitted it. On the grounds that 
correcting a thing like this prevents 'interesting' problems later.


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