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Yet another PET story

Hey, well done cosam on your PET fix. I've been following this thread since the first post and really appreciate you detailing your fault-finding work and the technical input by others (as no doubt you did too), as it gives good insight to this aspect of vintage computing - hardware repair to component level.

Your website covering the fault & fix process is great too. Certainly a worthy resource for future PET repairers.

cosam said:
That flipflop is on one of a row of three replacement ICs on the main board. Pins 4 and 6 are meant to be connected, but they weren't, leaving one of the inputs floating.
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How come pins 4 & 6 weren't connected? Were the PCB traces gone?

(An observation of mine: An increasing number of the faults we are encountering today in this vintage hardware is the result of a *previous* repair, or rather an *attempted* repair). IE, it's not always old components that are to blame, but increasingly so the action of some humanoid!

Philip
 
pavery said:
Hey, well done cosam on your PET fix. I've been following this thread since the first post and really appreciate you detailing your fault-finding work and the technical input by others (as no doubt you did too), as it gives good insight to this aspect of vintage computing - hardware repair to component level.

Your website covering the fault & fix process is great too. Certainly a worthy resource for future PET repairers.
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Thanks, Philip. I try to document as much as I can about what I do with any of these old machines, especially I don't find a remedy just from documentation and a bit of Googling. If it can help even one person get their classic computer(s) running again, it's time well spent as far as I'm concerned. And indeed: top marks to those who contributed with their knowledge, time and experience. I'd probably have got there alone eventually, but it would have definitely been a long way down the line!

pavery said:
How come pins 4 & 6 weren't connected? Were the PCB traces gone?
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It's not too clear why the pins weren't connected. That area is a bit of a mess, even after tidying it up. I suspect removing the original IC probably broke a track - something I've done myself by being a bit heavy-handed with the desoldering pump. Going by the number of "solder bridges" on the board, I wouldn't be surprised if it was in fact my reflowing that uncovered the problem.

pavery said:
(An observation of mine: An increasing number of the faults we are encountering today in this vintage hardware is the result of a *previous* repair, or rather an *attempted* repair). IE, it's not always old components that are to blame, but increasingly so the action of some humanoid!
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You're not wrong... I'm doing my best to avoid being one of "them" myself!

Anyway, today I had the nice simple job of cleaning out (read: completely dismantling) the keyboard. Just when you can finally actually use the thing, sticking or otherwise temperamental keys seem to appear from nowhere. Now the keyboard puts the rest of the machine to shame, so I suppose I'll have to give the case a scrub up too! As if I haven't revealed enough problems by solving others on this project already! ;-)
 
I've not quite figured it out yet, but I found an interesting problem whilst trying to load programs from tape today. I did get the PET to save a program to tape and read it back properly, which didn't work before. But then a couple of times, I'd issue a load command, press play and, whilst searching for the program, the PET hung and the tape drive stopped. After power cycling all seemed fine, so I carried on. However, after one hard reset, the PET started writing characters to the screen on its own and after that I ended up with that all-too-familiar blank screen.

Seeing those characters appear from nowhere, I thought there might be a problem not far from the keyboard interface. Sure enough, whilst poking around with the 'scope, I found that /IRQ was being held low, apparently by the PIA responsible for the keyboard and tape interface. Popping this out solved the blank screen and got my BASIC prompt back, although I of course couldn't use the keyboard. After moving the other PIA (for the IEEE-488 interface, which I'm not using right now anyway) to UC7 (the keyboard/tape PIA's socket) everything seemed to work again.

Now I'm wondering if a dud PIA was the original cause of the occasional blank screen I was getting, or whether it's just now gone bad due to to some (possibly tape drive-related) problem. I'm rather reluctant to start experimenting with my one remaining PIA if the latter is the case, especially seeing as I just had the tape stop again during a load.

Is it under any circumstances normal for the tape to stop like it did, or should I be looking for a potentially PIA-frying problem on the main board?
 
The cassette interface was always very touchy. It would often not find a file especially if written from another PET but those were alignment problems in the reader. I do not remember it 'hanging up' and certainly the screen never went blank. Hopefully it is just a bad PIA. There is still a source of PIA chips from Western Design Center in Arizona. But the bad news is that while the cost of a chip is only $7.50 US, they ship overseas using US Global Priority at a flat rate of $25 and a special handling fee of $15. Quite a surcharge for a little chip. Here is the website info on the 65C21. They claim it is totally compatible with the MOS Technology or Rockwell N-Mos chip.

http://www.wdesignc.com/wdc/w65c21s-chip.cfm
 
dave_m said:
The cassette interface was always very touchy. It would often not find a file especially if written from another PET but those were alignment problems in the reader. I do not remember it 'hanging up' and certainly the screen never went blank.
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Yes, I should probably point out that the "problem" tape was one I made myself on a different tape deck. I have however made working tapes in exactly the same way on this deck before. I can understand a bad tape (whether it be down to alignment or anything else) not being readable, but crashing the machine?

Probably also good to clarify that the blank screen did not occur directly after the stopped tape episode. When the tape stops, the screen is as you'd expect, but RUN/STOP does nothing. Only after a power cycle did I get the stream of characters on the screen, and it took yet another hard rest to get to the blank screen.

I'm still wondering whether the PIA was the cause of my original blank screens, but the funny thing with those was that if you left the machine off for long enough, it'd start right back up as if nothing was ever wrong. I guess it could've just been on its last legs and getting it to read and write tapes finally finished it off.

dave_m said:
Hopefully it is just a bad PIA. There is still a source of PIA chips from Western Design Center in Arizona. But the bad news is that while the cost of a chip is only $7.50 US, they ship overseas using US Global Priority at a flat rate of $25 and a special handling fee of $15. Quite a surcharge for a little chip. Here is the website info on the 65C21. They claim it is totally compatible with the MOS Technology or Rockwell N-Mos chip.

http://www.wdesignc.com/wdc/w65c21s-chip.cfm
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Thanks for that - at least I know if all else fails I can get a replacement, even if it does end up costing an arm and a leg...
 
Surely someone will have one that's closer and cheaper; the 6821 is also compatible.

Time to get acquainted with some of your fellow Dutch PET enthusiasts; there are a few. And there's Anders; he's got everything and knows everyone;-)

Good luck!
 
Thanks all for the leads on getting a new chip - looks like I'll be able to get my hands on one without too much trouble.

What I'm more concerned about right now is whether my main board (or tape unit) is some kind of PIA assassin. Looking at the tape interface, I see that the motor is switched by the PIA. Supposedly any number of hang/crash situations (including /IRQ being forced low by the PIA) could cause that signal to drop and therefore the motor to stop. I almost daren't test the thing for fear I may be slowly roasting my last remaining PIA... Any inspired theories or am I just being overly cautious?
 
Well, 30 year old transistors will lose some 'beta' (gain) so the PIA outs may not source or sink current as well as they used too, but perhaps the problem has more to do with old motors/capacitors and dirty read/write heads in the tape transport. I would start with that and not worry too much about blowing the PIA. To be safe, monitor the PIA outputs to the tape drive while they are active to see if there are large inductive spikes being induced on them somehow. I would hope that the PIA signals are buffered in the tape transport, but I do not know.
 
I suppressed my irrational fear of blowing stuff up and to had another go at experimenting with the tape drive. Not surprisingly, the PET has a preference for it own tapes (i.e. those written from the machine itself) and reads them flawlessly. I did get a "foreign" tape to load, although by no means reliably. Had a couple of hangs, but the tape kept rolling this time.

I noticed a slight "step" in the spool of tape after it came out of the deck I used to write it, so it could just be down to an alignment issue. It's is a vertical affair, so there's more possibility of the tape wandering over the heads without gravity's assistance to keep it in place, as is the case with the C2N. I don't know if that fraction of a millimeter would make any difference - maybe I'm just on the threshold. In any case, with the vertically oriented drive laid down on its back, I managed to write a working tape. I'm sure I'll be able to reproduce the hang and troubleshoot that properly some other time.

On the parts front: I think I might have to add a replacement VIA to the wish list, too. Anyone else notice the PET was stuck on lower case? I wouldn't mind, but it meant I couldn't get any of those lovely PETSCII graphics. I checked the character ROM and it's fine - even went so far as to write a script to print out the characters at each location. It would appear the VIA is pulling GRAPHIC high, regardless of POKEs to 59468. PEEKing that same address shows that the POKE has "taken" so, as there's nothing between the VIA and A10 of the character ROM, I reckon the VIAs had it, partially at least. I read a small resistance between VCC and CA2 - not sure if that's normal. For now I've tied GRAPHIC to ground as a workaround, but it'd be good to fix it properly.
 
Cosam,

Been reading your contributions with much interest.

I have a spare PET without keyboard lying around, probably a 4032 or a 4016 with the early metal housing (it's boxed and stored at the moment). Perhaps that will help?
 
Vlim said:
I have a spare PET without keyboard lying around, probably a 4032 or a 4016 with the early metal housing (it's boxed and stored at the moment). Perhaps that will help?
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Hmm... what did you have in mind? I'd definitely be interested in some parts if they're up for grabs. Or are you looking to get rid of the whole machine? OTOH: Maybe you should just try to source a keyboard for it ;-)

PS. This is probably getting a bit borderline re. public/private posts. Drop me a PM if you see fit (Dutch is fine).
 
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Well, I picked up Vlim's spare 4016 earlier this week. Unfortunately all the bits I need appear to be soldered in, so it'll take a bit of work to transplant them.

Even better though: he's also set me up with a 8050 dual disk drive. No cable to test it with yet (hence the post in the Wanted section) but it has quite literally given me the green light, so it's at least passing its POST. If it wasn't for it needing a good clean-up, it would've almost been boring ;-) More about that here.
 
Congratulations and good luck! They're much more fun with a disk drive.

That's an excellent article by the way, as are your others; is English a second language for you (if you don't mind the question)?

(Loved the 'scuzzy drive' pun BTW! ;-)
 
MikeS said:
Congratulations and good luck! They're much more fun with a disk drive.
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Yeah, the tape drive is fine if you just want to run a particular program, but I'm looking forward to the extra speed and convenience of disks. I really don't know how I managed to put up with such long loading times back in the "old" days!

MikeS said:
That's an excellent article by the way, as are your others; is English a second language for you (if you don't mind the question)?
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Thanks :) English is in fact my mother tongue although, after ten years living over here, it's starting to deteriorate!
 
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Woohoo!

Woohoo!

Had a very productive hour or so this evening, desoldering the VIA and a PIA from the 4016 board. At 40 pins a pop, they weren't the easiest ICs I've ever removed. Good news is they emerged unscathed and after cleaning up and straightening the pins, they're a good fit in the PET's sockets.

The PIA went in first. No blank screen, so I think it's OK. Won't know for sure until I try out that disk drive. For a moment I thought I'd replaced the original VIA with one even worse, but the blank screen I got had a much simpler cause this time: apparently these things work better with the power cable attached to the main board... ;-)

So I now have upper case as default, without my little grounding wire. A choice POKE to 59468 switches between the two character sets, so it looks like we're finally there. Again though, the VIA has to prove its real worth by working with the disk drive...
 
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Good to see that it worked out.

Any idea what the piggyback board in the donor-PET was yet?
 
Vlim said:
Good to see that it worked out.
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Yep - thanks again for the supplies!

Vlim said:
Any idea what the piggyback board in the donor-PET was yet?
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Had a look yesterday when taking it apart. It appears to be a 64K memory board. Looks like it connects to the main board where the original CPU would be, the CPU then being inserted in the expansion board.
 
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