9 Mar
2008
9 Mar
'08
5:14 p.m.
On Sun, 9 Mar 2008, Andrew Lynch wrote:
I think repairing old computers is just like any human
endeavor - it is
flawed and sometimes I follow incorrect logic and/or misdiagnose before
finding and fixing the real broken part. Sometimes good parts get
mistakenly identified as bad. That's life and if a part falls under
suspicion I am likely to just replace it just to test the theory, right or
wrong it gives information and vital clues. Anything labeled "suspect" is
pulled aside and labeled as bad regardless.
These parts are cheap and plentiful enough that if a few good ones get
trashed in the process of fixing an old machine I consider it a worthy
investment. Probably I am tossing some good parts occasionally but it is
worth it time wise to take the chance and maybe find the root cause. Were
I
more skilled technician maybe I could fault isolate
with more precision
and
just fix exactly what is broken but this is just a
hobby and I do not have
the time/experience to get more precise.
Never discard the old parts, nor charge the customer, until the actual
defect is found.
Any reason to NOT plug the suspect Z80 back in and see whether the
problems follow the chip v coming and going from other "random" acts?
-----REPLY-----
Hi Fred,
Yes, I agree. I always "tag and bag" discarded parts from vintage computers
I repair. "Tossing" was a poor choice of words. It means tossing them into
the box of tagged baggies of suspect/broken parts.
Since I am the owner I don't need to worry about charging anyone but I agree
with the sentiment.
Especially if this were a business I would not feel comfortable releasing
the system unless I had firm evidence of root cause and not some
coincidental fix.
However, that is the dilemma. How to establish what exactly is the root
cause without endangering the original piece of equipment? I would like
some independent verification if possible. Everytime you touch or make a
repair to an old computer there is a risk of unintended damage.
Yes, I could swap the questionable components back into the system but I am
very reluctant to "screw around with it" once it is working. I dislike
using the restored piece as test equipment.
Generally speaking, once it works reliably, I leave it alone until there is
reason to attempt more repairs. I have found it is very easy to
accidentally break things. That is why I rarely ever clean the boards or do
anything to them other than the bare minimum to make them work again.
One concern about swapping possible bad parts into a working system is that
they could induce another failure and start the repair cycle all over again.
What would be ideal is some sort of test equipment I could plug a bad part
into and check to see if it is good or not. I can do that with EPROMs and
apparently there are TTL chip testers like the TOP2049 which provide 74LSxxx
testing.
How do you test a LSI component like a Z80 CPU, PIO, DMA, or DART? Short of
plugging it back in and hoping it doesn't cause more problems.
I am an engineer by education and practice but self taught as a technician.
Sometimes I wish I had more experience in these matters or at least took
some coursework on how to diagnose and repair equipment.
Thanks for your advice!
Andrew Lynch