Need contact information for dkdkk

Tony Duell ard at p850ug1.demon.co.uk
Sun Jul 17 18:24:21 CDT 2005


> > Suppose you have a non-working HP9810 calculator (well, it's almost a 
> > computer :-)). You have a choice of 2 repairers.
> > 
> > A) Worked ofr HP as a service engineer for many years. He's got the
> > official service manual (which is a boardswapper guide), and can follow
> > the instructions which are basically to replace all the plug-in PCBs until
> > it starts working again. The fact that you can't get replacement boards
> > any more is another matter... 
> > 
> > B) Has never worked for HP or any other computer company. But he was 
> > given a broken 9810 a few years back and spent a couple of months working 
> > out how to repair it. This guy can connect a logic analyser to an 
> > undocumented internal connector and trace the CPU microcode. He's got his 
> > own microcode listings and can thus tell just what the darn thing is 
> > doing. 
> > 
> > Now, who do you pick?
> 
> B. Component level debugging and repair is a rare and valuable skill. Chances

Of course... 

Perhaps I should have re-phrased that question. Who do you pick if all 
you've asked about is what experience A and B have of the machine. _On 
paper_, A seems to be the better 'qualified' -- he worked for the 
company, he's got the official manual. In practice, you'd want B.

> are, B could probably debug the thing down to a dead logic gate, whereas A

As indeed I have, several times (not all on the same 9810, I hasten to 
add :-))

> would probably go down to board level and go looking for a new CPU microcode
> board or whatever.

Odd you should pick that one. The CPU control board -- 09810-66513 -- 
which contains the microcode, is the only logic board I've not had to 
repair in a 98x0 machine.... Still, I am sure they can fail.

> 
> There's also the point of actually knowing what your tools are telling you
> and how to make the best use of the tools. It's no use knowing in how to use
> an oscilloscope if you don't know how to interpret the information it's
> giving you.

Oh, absolutely. Good test equipment is useful, but it's not essential, 
and it's not a substitute for knowing what you are doing. Many people 
(not on this list I hasten to add) seem to think there's a magic box you 
can connect to a brokent <foo> that will tell you exactly what's wrong 
and how to fix it. Of course there isn't.

I've said this many times, but it's worth repeating. In order to repair 
something, you should : Make measurements (and otherwise gather evidence 
as to the problem); Think about what the measuements are telling you; 
Then, and only then, replace anything.

It is often possible to get the evidence you need without fancy test 
equipment. The person who moans 'I could fix this if only I had a logic 
analyser (or storage 'scope, or...)' is probably wrong. Not because such 
instruments are not useful, they are. But becasue even if he had them, 
he'd not be able to interpret the display. If he had the skill to do 
that, he could probably manage without the instruments.

-tony




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