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Add another PET to the emergency room

Congrats Tezza!! I was just posting a reply but my pc timed out and I lost my message!!

I was going to post "dog with a bone Tezza don't let it go!!"

When I sold the pet I didn't fix I have always wondered what the fault was!
It still makes me wonder even now.:mad:

I don't normally give up easilly but that pet just did not give me any glimmer of hope,
just flickering garbage that never altered.

I will always have that stink of failure!! but It was costing me a lot in time and parts.
I have fixed quite a few pets since that one but damn it still bugs me!!

I am glad you got to the bottom of the fault and praise you on your perseverance! ;)
You would have kicked yourself if you didn't get to the bottom of it!!
 
tezza said:
Mystery solved. Now I'm content. :)
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Tez,
Yes, you earned this sweet feeling of victory using knowledge, logic and common sense over Murphy and the other electronic demons. They will return on some other computer project, but you are now better prepared for their onslaught. Congratulations.
-Dave
 
Thanks guys,

I think I'm lucky that neither of my two boards had been hacked around with and were in pretty good shape as far as traces being where they should be etc. Sockets are an obvious weakness in these models though.

Eudimorphodon said:
+1 on congratulations as well. I'd ask to buy your now repaired board off you if shipping wouldn't be a problem. ;^)
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;) I'd love to help but I like to have double-up working boards with all my really old computers if I can. I find it makes life so much easier when diagnosing problems if you can have a working circuit to compare things with. They are also a source of spare parts if necessary. I put a high value on having working models as oppose to cases which you can only look at.

I have every confidence you'll get that 4032 going in the end. :) With all the PETS so far discussed on the forum I can see a creeping progress towards repair.

Tez
 
I wonder what happened with all the other SF Bay (?) PETs from the same repair center. A few of you became regulars on this forum, but what about the rest that didn't end up at the recycling station? I assume a number of those wouldn't work fully, so maybe you can post on some local forum, Craigslist or whatever about offering to take care of any broken PETs that remain at bummed collectors.
 
I'm assuming the bulk of them will find their way to eBay sooner or later. I was amused to notice someone trying to get $400 for a *broken* 8032 on the local Craigslist a week or two later, that almost certainly came from the same haul. Uhm... yeah.
 
Yup, boundless optimism. There was somebody in santa clara that was asking $295 for a Mac Plus on craigslist recently.. good luck with that
 
Thanks for writing it all up.

It's been bothering me a little how the second board's problem was the borked A15 address line given the PETTESTER ROM ran, but... it occurred to me why that's the case. (I double-checked my code to make sure.) The non-intuitive thing you have to remember is when a line is broken in these things it usually causes that line to be considered as "stuck on" downstream, not "stuck off".

(If the reverse was true the PET wouldn't of been able to see its ROMs at all, as the entire upper 32k of memory space would of been cut off.)

What was actually happening was that any access to the lower 32k was being offset upwards 32k. So when you had the PETTESTER ROM plugged in and it ran the memory test it was *actually* testing the video RAM. It wouldn't of shown as bad because the test pattern is written in one loop, while another loop does the test read followed by the status write to the screen. That would of course immediately clobber the test value, but the program wouldn't care. And of course the video character set display wouldn't care either.

With the BASIC ROMs plugged in you'd end up with this bizarre situation where VRAM would essentially be mapped as the CPU's zero page and stack. I'm sure what was going on when it crashed was it set up a few vital things, called the "clear screen" routine, and then wet its pants shortly thereafter as it blew away all its low memory variables. :^)

PETTESTER definitely needs an extended-test mode where it does things to look for stuck address lines.
 
Eudimorphodon said:
Thanks for writing it all up.
Click to expand...
angel_grig said:
Nice repair job Tez!Thanks for sharing your experience with us!
Click to expand...

Thanks for that. Knowing that people actually get something out of these things gives me motivation to write them up.

Eudimorphodon said:
What was actually happening was that any access to the lower 32k was being offset upwards 32k. So when you had the PETTESTER ROM plugged in and it ran the memory test it was *actually* testing the video RAM. It wouldn't of shown as bad because the test pattern is written in one loop, while another loop does the test read followed by the status write to the screen. That would of course immediately clobber the test value, but the program wouldn't care. And of course the video character set display wouldn't care either.
Click to expand...

Ahh, yes I see what you mean. I'll alter the article to reflect that (or maybe just point to your post for those that want an explanation).

If the PET TESTER was extended it could indeed be a very powerful tool.

Hopefully you'll be able to make progress on that 4032.

Tez
 
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