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Yet another PET story

Mike,

I just know that John (barythrin) won't think of it, but any guidance you could provide as to how to use the ROM expander would be greatly appreciated.

--T
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A bit of a misnomer I suppose; not really an 'expander' but a 'selector'. The problem was that there were all sorts of accessory ROMs (I've got a half dozen); some were actual software or components of software (like the Command-O ROM we're talking about here) and some were just copy-protection 'dongles'. Since there were only two sockets available (and most went into the 9000 socket anyway) you'd have to swap ROMs when you went from using WordPro to programming with Command-O (for example), so just like the C64 and VIC20 cartridge expanders presently being discussed on another thread you had these (EP)ROM expanders for the 24-pin PETs. Most of them (like the Quadra ROM pictured) would have a DIP or rotary switch to select the desired ROM; if you didn't feel like going 'under the hood' then you'd run a little cable to a switch somewhere on the side or back (but no serious user ever screwed down the top anyway). AFAIK there were no software-selectable versions; at least I never saw any...
 
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I did find the manual and it suggests that Command-O needs BASIC4, so you may be out of luck unless you can upgrade. Don't have a scanner available here at the moment, but I'll scan it anyway as soon as I do. I don't have a BASIC2 machine at the moment so I can't try your image right now.
Needing BASIC 4 would definitely explain it. Makes you wonder what the chip was doing in the machine in the first place, although I've come across much stranger things!

You did remember to move it to the correct socket? SYSing to an empty socket would indeed drop you into the monitor.
It's in UD3 right now which is meant to be 0x9000. I suppose I could always put it back into UD5 (where I originally found it) and see if that helps. It'd be a nice bonus to get it working, but I'd settle for bog-standard BASIC 2 at the moment!

Cosam: now that it's sort of working, did you read Tezza's story about transferring software to/from his PET? Time to get it doing something! Have you got a tape or disk drive for it?
I'm afraid it's all a bit temperamental at the moment. Even when it's running, the goofy display doesn't really allow for much tinkering. I'll definitely be trying out Tezza's techniques when it starts behaving itself. I even have a proper "PET model" 1530 which I picked up with a C64, although I'm not sure how easy it is to find even blank cassettes these days.
 
Since we're exchanging pictures, here's my B&C 1409 programmer, my two 2001 PETs (one with the keyboard upgrade), my PET EPROM programmer, a speech board and music interface, and a ROM expander. That particular 2001 PCB went down to the South pole BTW... ;-)

Nice PROM burners. I’m intrigued by the PET burner. One connector to the user port is for control but is the one to the cassette port used to obtain +5V power? Is the transformer on top of the unit used to generate a small AC voltage and then regulated to get the 25V programming voltage? You must have to be very careful with the PROM socket. It should have used a Textool zero insertion force (ZIF) socket.
 
I wish I had a local place that stocked old parts. Those kind of places are long gone around here. However I found a good mail order place in Texas that stocks old parts. He tells you if they are unused or refurblished from socket pulls. The refurbs I got from him had good straight pins and clean UV windows.
Yeah, I'm really spoilt here - I have that shop and another like it within walking distance! Between them they have pretty much anything I've needed up to now and, if they don't, they're knowledgeable enough to look out possible substitutes to get you going. A lot of it is quite obviously salvaged, but they also have plenty of old stock. The 4116s I picked up, for example, all have date codes from the early '80s and there are plenty of equally old 74 series chips right there on the shelves too. I get the feeling neither of them ever throw anything away, which is of course perfect when you need vintage parts! The best thing is that both shops are very well established and also have a healthy trade in more modern parts and gadgets, so hopefully they're here to stay.
 
Pity about the wavy screen. It seems to be common in older PETS (well...vintage computers generally) and was really pleased to find my own 3032 video rock solid.

I'm in awe of the gear you have there..not that I would know how to use it! :)

It's also great you can get parts so readily. Not the case here...often they need to come from overseas!

Tez
 
Nice PROM burners. I’m intrigued by the PET burner. One connector to the user port is for control but is the one to the cassette port used to obtain +5V power? Is the transformer on top of the unit used to generate a small AC voltage and then regulated to get the 25V programming voltage? You must have to be very careful with the PROM socket. It should have used a Textool zero insertion force (ZIF) socket.
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Yes, yes and yes; I did use ZIF sockets but instead of dedicating them I just plugged them into the 'real' sockets wherever and whenever I needed them.

Some more PET/AIM related pictures: The homebrew AIM65 EPROM burner, a couple of ROM emulators or RAM expanders (one commercial, one homebrew), a PET composite video interface, and a monitor extension cable to make working on the mainboard or monitor *much* more convenient; also a little beeper to let me know that the 8050 disk drive had an error (I always forgot to notice the error LED).

I also have an MTU graphic video board for KIM/SYM/AIM and the PET; now that you guys have reawakened my interest and got me digging through all those PET & AIM boxes I just may try to get it working again...
 

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This isn't really relevant to you guys with the 3032/4032s, but for Anders and any other owners of the 'old' 2001s:

I've got a couple of incomplete 6540 ROM sets where one or more has failed; I'm assuming that there must be others out there who have either replaced them with standard ROMS in an adapter or just scrapped the system, so, does anybody have some that they don't need (or, conversely, need any that I or someone else may have?) Maybe what's needed is a 6540 ROM/RAM registry somewhere (or is there already one somewhere?)
 
I don't own any chicklet 2001, but whenever I get the chance to go parts looking, I could keep an eye for 6540's. Since the oldest machine I've seen at the place I found all those PETs was a 2001N, I'm not sure there would be any parts predating it.
 
The PET seems to be in a good mood today. It's running quite reliably after replacing a couple of capacitors which were actually completely missing from the board (C25 and C64, IIRC) and resoldering the pins of those IC sockets from a previous repair. Still no idea what was wrong with the board as everything I've checked so far looks fine.

No joy on the display front, though. The vertical signal coming in from to motherboard checks out, but horizontal and video in look a bit odd. Then again, they look equally odd when the machine's just switched on (and the display is fine) so in all likelihood they are probably OK. I've hit a bit of a dead end troubleshooting the video board/monitor and the distortion/wobbliness are still present. I took some snaps of it today:
I had it running longer during testing and the BASIC banner and prompt had shrunk down to almost nothing and was flickering around, almost at the center of the screen. What's also weird is that column 23-or-so is about half the width of all the others, and the last few characters on each line are wrapped at the right.

What the photos don't capture is the waving/pulsating, so I shot a quick video of it (now on youtube). It's not too clear, but you can just about make out three "ghost" images rotating around each other. I'm not really at home in CRT stuff so maybe someone could tell me what I'm looking at ;-)
 
Looks like a classic case of "pincushion" distortion. Perhaps an adjustment pot is way off or the CRT has been magetized. If magnetized, somehow you will have to deguass it. Maybe someone here knows how to do this. Does anyone know what pots to tweek?
 
Looking at the utube clip, there is a lot more going on than pincushion and it may be thermally related. In the old days one would spray some 'instant cold' stuff from a can on the components until the problem part was isolated. This is a really tough problem.
 
Yep, pincushion it is, but "progressive pincushioning" is a new one for me ;-) There are a grand total of two pots on the board - one for brightness, the other for vertical size - and both of those do exactly what you'd expect and nothing more. I got the image re-centred, but that's done by twiddling two metal rings on the CRT's neck.

Magnetisation may be something, though. I know nothing about it, but the fact that the image corrects itself after cooling off doesn't really lend itself to any "permanent" cause. I'd love to be proven wrong!

After reading up a bit on CRTs, I'm wondering if it isn't just something simple like the power supply. The video board has its own rectifier, regulator and smoothing caps, after all. I reckon I'll take a bit of freeze spray to it and see if I can zero in on the culprit...
 
Ah - looks like we've got another case of simultaneous posting ;-)
In the old days one would spray some 'instant cold' stuff from a can on the components until the problem part was isolated. This is a really tough problem.
But hey - this machine is after all from the old days, so it's fitting to use contemporary techniques to fix the thing!
 
I'm no techie and can't provide anything except moral support.

However, stick with it! I've read somewhere that faulty power supplies can indeed cause displays to wave and bend.

Tez
 
Not much to those displays; the schematic and test point scope traces are all on the Zimmers site, so if you've got a scope it shouldn't be too hard to diagnose.
I thought it'd be pretty easy too - I found the video schematic with waveforms before I even picked the PET up and figured it'd be a quick diagnosis with all those test points. All the points on the vertical circuit correlate to the waveforms on the schematic. The horizontal and video in signals sort of resemble the illustrations, but look like they're missing cycles or have a couple of extra peaks here and there. As far as I can see those missing and/or extra signals are also present when the image is stable just after powering up, so I'm really none the wiser... The timescale on which the image deteriorates (and takes to recover) also makes it kinda hard to test.

As everything seems to point to a thermal problem, I think a few blasts of freeze spray will reveal the culprit quicker than the probing, waiting and probing again cycle.
 
check

check

It's not just pincushion, a classic pincushion fault will also have a vertical 'bowing in' component.
Can OP hook a volt meter to pin 3 of the flyback (which should be the collector of Q702, if I've got the right schematic),. Or even better, a scope. See if there's a 60-Hz sinusoidal component that increases in amplitude, and if the overall voltage decreases.
Have you checked if the output of the 7812 is stable over time, and if there is increasing ripple on it ?
patscc
 
Can OP hook a volt meter to pin 3 of the flyback (which should be the collector of Q702, if I've got the right schematic),. Or even better, a scope. See if there's a 60-Hz sinusoidal component that increases in amplitude, and if the overall voltage decreases.
Have you checked if the output of the 7812 is stable over time, and if there is increasing ripple on it ?

It's actually the earlier model video board (schematic here) but if I compare the two I think we're talking about test point 13. Unfortunately all that stuff is of course tucked away in a corner and a bit of a pain to get at...

However - I did leave the scope on the +12V line for a few minutes and you can literally watch it go wrong. Check this out:
I feel more than a little daft for not checking this before - I guess I'd convinced myself it was something more complicated!
 
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