Yet another PET story

[cosam]

With all the talk about PETs lately, I've been keeping an eye out for one. I managed to pick up a non-runner last night for a pretty tidy price, from a collector who was having a clear-out. It has a 2001 case and 9" screen, but the main board appears to be from a 3/4000 series machine with a full complement of RAM.

It doesn't do much just yet, though. It has the common "garbage screen" when switched on, but only for a second or so, then... total darkness. Reseating everything that's socketed didn't help any, so I whipped the board out for closer inspection. There were some pretty gruesome-looking repairs on the back which I'd like to fix properly before continuing the diagnosis. Looks like someone had set to with a pretty beefy soldering iron and half a bucket full of flux! Most of that is cleaned up now and I'm making a start on repairing some cut and lifted (or even completely absent!) PCB traces.

Should at least keep me busy for a while ;-) I've whipped up a quick web page with some pictures, on which I'll be documenting my progress.

[pavery]

Hey, congrats on your PET acquisition, cosam.

As I was involved with faultfinding Tezza's PET - I'll be interested to see what you find.

It doesn't do much just yet, though. It has the common "garbage screen" when switched on, but only for a second or so, then... total darkness.

As far as I know, it is normal for the garbage screen to be displayed for the first second or so while initialisation occurs, then the screen clears and Basic is presented.

Two possibles come to mind: Perhaps you have a ROM problem (like we did) and Basic isn't coming up successfully. Or perhaps the machine is just shutting down after a second or so - perhaps Power Supply is failing.

I'll certainly be following your progress with interest - please keep us posted.

Oh, what is the exact Model Number stated on the rear of the case?

Philip

[tezza]

Cool! Seems to be exactly the same model I've got, the one Philip (pavery) helped me fix. I see what you mean about the soldering..ewww..

Anyway, I'll follow with interest. At least is doesn't seem to be hanging at the initial boot up.

Tez

[tezza]

P.S. from the number of ROM chips it looks like you've got BASIC 4.0 in there. (assuming all the ROMs are for BASIC of course).

Tez

[cosam]

I finished up the PCB repair last night and reconnected the board to see if it'd made any difference. Garbage... blank... BASIC! Not what I was expecting, but something I did has apparently breathed life into the old girl. Most annoyingly, however, I've no idea what the original problem was! So the logic is working, although it's revealed a problem with the display (more later).

As far as I know, it is normal for the garbage screen to be displayed for the first second or so while initialisation occurs, then the screen clears and Basic is presented.

Yep, you're right. I vaguely remember reading that elsewhere too, so I wasn't too worried about the garbage part - more so about the blank screen afterwards!

Two possibles come to mind: Perhaps you have a ROM problem (like we did) and Basic isn't coming up successfully. Or perhaps the machine is just shutting down after a second or so - perhaps Power Supply is failing.

The power supply is always the first thing I check ;-) After reading about Tezza's machine, I suspected a bad ROM too. Whilst waiting for some supplies for the PCB repair I dumped them all and checked them against some known-good images. My el-cheapo EPROM programmer didn't take them directly, but their pinout is similar to a 27xx so I made a quick-and-dirty adapter out of a couple of IC sockets. It's a BASIC 2 ROM set, BTW. That extra chip is an expansion ROM called "Command-O" which AFAICT implements a few extra commands after initialising with some SYS-number or other.

Oh, what is the exact Model Number stated on the rear of the case?

I'll have another look for you, but I've not found a model number anywhere yet. I'm also running out of places to look ;-) Best I have right now is 2001-N, and I can confirm it has 32K of RAM.

As for the display problem: right now it works fine just after powering up, but soon after that the image starts to wobble. The shaking gets worse the longer it's left on and stays bad after a power cycle, so I'm guessing something is playing up when it gets warm. Analogue electronics aren't exactly my forte, but I'll have a look at it tonight.

[carlsson]

Apparently what it says on the rear and on the front can be two quite different things. Then if the motherboard was updated or replaced you may find it act like a third model. 2001N-32 sounds as good as anything else. As Nils on the now disabled PET Alive forum mentioned, CBM boards were designed so a newer board can (always?) replace an older board, at least as long as jumpers are set correctly.

[tezza]

As for the display problem: right now it works fine just after powering up, but soon after that the image starts to wobble. The shaking gets worse the longer it's left on and stays bad after a power cycle, so I'm guessing something is playing up when it gets warm. Analogue electronics aren't exactly my forte, but I'll have a look at it tonight.

That's a shame. At least it goes! Hopefully it will be an easy fix.

Tez

[cosam]

Well... I finally got set up to take some measurements as described in the service manual. The video board is a bit of a pain to get due to all the wires going to the CRT. Unfortunately I only got to the third test point before my oscilloscope (an old Tek 2465) started to fizzle and let out a big puff of exceedingly aromatic smoke. Looks like the PSU is fried, so I'll need to get that fixed before I can get back to the PET. So much for my weekend project! :-\

[patscc]

Hey, hey, hey....a 2465 ain't that old. Aromatic smoke, did a cap in the PSU blow, by any chance ?
patscc

[cosam]

Hey, hey, hey....a 2465 ain't that old.

Heh, I guess it's not - the PET itself predates it by a few years, after all!

Aromatic smoke, did a cap in the PSU blow, by any chance ?

That'd be my guess too. Nothing quite like that sickly smell of boiling electrolyte ;-) In any case I don't think it's too serious as I still had a nice trace even when it started doing its chimney impersonation.