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
10 Mar
10 Mar
4:07 p.m.
New subject: replacing failed components question causal or coincidental
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.
I think that risk is very small. Certainly I've demonstated classic
computer equipment to otehrs (for example at HPCC meetings) and I
_always_ take it apart to explain the internals. I don't worry about
damaging it.
I use my clasiscs. They are not there to look nice on the shelf (or in my
case 'look nice in a large pile on the floor).
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.
But it's a pretty certain test. If the unit works cosnistently with the
new CPU and fails consistently with the old one, it's a good bet the old
CPU is defective.
I've had LSI chips fail in very obscure ways, to the extent that they'd
pass a lot of simple tests, but fail in actual use. Of coruse there's a
test they'd fail too, but unless you know exactly what your tester is
doing, you can't be sure it's going to exercise the faulty bit of the chip.
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.
Odd. I guess I have a lot more confidence, but I am always pulling
machine apart -- right apart. I've never done irreprrale damage.
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.
This is one of the arguaments I use against module-swapping. That a
faulty module could damage the new replacement (especially if the faulty
part is, in fact, the power supply!).
But I think the risk is miniml with a CPU chip. Moreover, the machine has
been used with the (suspected) fault CPU in it (to the extent, IIRC, that
it would boot on OS,) with no other damage occuring. I'd not worry about
damage from that source.
And if the machien is so stouchy that removing and replacing a socketted
chip causes problems, then IMHO it's not fully repaired. I'd certainly
not trust a machine like that.
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.
And be very careful with those testers. Some of them will pass chips that
don't work correctly in the circuit. THe main problem is timing-related
(I've had chips 'go slow' and have excessivley large propagation delays,
most testers will not pick that up). But some of the cheaper testers fail
to distinguish between a 'high' output and a floating one, and thuse
don't fial chips where the upper transistor of the output stage has
failed. And yes, I've had that.
-tony
4:41 p.m.
New subject: replacing failed components question causal or coincidental
Odd. I guess I have a lot more confidence, but I am
always pulling
machine apart -- right apart. I've never done irreprrale damage.
Of course, your idea of "reparable" is a bit more..inclusive..than
most. :-)
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9:12 p.m.
New subject: replacing failed components question causal or coincidental
From: ard at p850ug1.demon.co.uk
---snip---
And be very careful with those testers. Some of them will pass chips that
don't work correctly in the circuit. THe main problem is timing-related
(I've had chips 'go slow' and have excessivley large propagation delays,
most testers will not pick that up). But some of the cheaper testers fail
to distinguish between a 'high' output and a floating one, and thuse
don't fial chips where the upper transistor of the output stage has
failed. And yes, I've had that.
Hi
It must have been a Fairchild part. When at Intel they were
notorious for that.
Dwight
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11 Mar
11 Mar
2:15 p.m.
New subject: replacing failed components question causal or coincidental
to distinguish
between a 'high' output and a floating one, and thuse
don't fial chips where the upper transistor of the output stage has
failed. And yes, I've had that.
Hi
It must have been a Fairchild part. When at Intel they were
notorious for that. 
12 Mar
12 Mar
4:53 p.m.
New subject: Data I/O 29A vs. 29B
In order to support my classic computers I need a GOOD EPROM Programmer (
that puts it on topic ... ).
I've used and like the Data I/O 29B but don't know squat about the 29A. Are
they about the same or is the 29A a POS ??
What would be a FAIR price for either ? Is it worth more to opt for the 29B
?? Thanks.
Best regards, Steven
P.S. I bought one of those Ebay special ( German ? ) single-board "
Universal " programmers and it IS a POS !!
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ard＠p850ug1.demon.co.uk -
dkelvey＠hotmail.com -
lynchaj＠yahoo.com -
mouse＠Rodents.Montreal.QC.CA -
steven.alan.canning＠verizon.net