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]

Scope

Scope

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.

 

[cosam]

Got the scope fixed today - it was indeed a bad cap which had left a nice trail of what could easily have passed for burnt Marmite on the PCB. At least it was easy to replace, once I finally got to the thing...

So I got back to the video board and it looks like at least one of the three signals coming from the main board isn't quite right. I put the monitor back together and started probing under the hood instead. Unfortunately, while I was tinkering away something went "pop" and I'm back to the blank screen again! Not knowing what was wrong last time, I'm not really any the wiser than I was on day one. Looks like I've got work to do after all ;-)

I've started working my way through the schematics again. CPU and its busses seem fine, but video RAM seems decidedly inactive. All I've found so far is that the video data bus is all zeros, apart from SD5 being stuck high, which seems a little odd. A10 on the character ROM (labeled GRAPHIC) is also stuck high. I think I'd better check the "regular" RAM before drawing any conclusions, though.

 

[MikeS]

It won't affect the PET working, but if that's a Command-O ROM (CO-40N) I think it belongs in UD3 (9000H), the right-hand socket and SYS 36864 to enable (unless that's some non-standard version).

Does anyone know if there's a binary image and/or manual for the Command-O ROM on the Web somewhere? If not and you haven't already, could you dump it for us while you're at it?

Thanks,

mike

 

[cosam]

It won't affect the PET working, but if that's a Command-O ROM (CO-40N) I think it belongs in UD3 (9000H), the right-hand socket and SYS 36864 to enable (unless that's some non-standard version).

It's actually labeled COM40N, but putting it in UD3 certainly makes sense as I did indeed read somewhere that SYS 36864 would enable it. Good catch ;-)

Does anyone know if there's a binary image and/or manual for the Command-O ROM on the Web somewhere? If not and you haven't already, could you dump it for us while you're at it?

I've just had all the ROMs out again to check they're still OK. Only trouble is I'm not getting consistent reads. Some parts are being repeated as if some address bits are getting stuck, but the addresses repeated are different from read to read. I'm guessing it's down to how I'm reading them, whether that be due to the adapter I made or that I'm trying to read them too fast. When I get that nailed and start getting consistent dumps, I'll upload an image.

As for a manual, so far I've only found some photos of a few pages which were taken for an eBay auction. If there's a known-good image or a full manual out there, I'd definitely like to see them!

 

[MikeS]

I do have the Command-O manual somewhere (such as it is) so if it isn't already out there somewhere I could look for it and scan it when I find the time (which is even harder to find ;-).

As to the ROM itself, mine would be the 8032 version which is not the same.

Meanwhile, good luck getting the PET running; 'pop's are not usually good sounds, but they do often leave visible evidence of their origin.

As to the model numbers, I think they were a little confusing; AFAIK that dynamic RAM 9" screen board was known as a 40xx over here in North America (not to be confused with the FAT40, which was actually a version of the 8032 board), and in Europe as a 30xx, and also as a 2001N everywhere ;-)

 

[dave_m]

Video board

Video board

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! :-\

Be careful of the high voltage in the video area. As I recall everything is rather exposed. Do you have a schematic of this board? I have one somewhere if you need it.
Regards,
Dave

 

[cosam]

Be careful of the high voltage in the video area. As I recall everything is rather exposed.

Definitely good advice, yes - and you recall correctly ;-) I found discharging instructions on the web and switched off before hooking onto different test points if they weren't easily accessible.

Do you have a schematic of this board? I have one somewhere if you need it.

Thanks very much but I already found a full set of those on the web too. The video board schematic even has pretty pictures of what the waveforms should look like at the various test points.

 

[cosam]

I finally got some good dumps of most of the ROMs. Two of them check out fine, but the other two (leaving the Command-O to one side for the moment) look like they've had it. They start out looking OK, but later on there are all sorts of random errors.

I guess whichever ROM is responsible for blanking the screen works well enough to clear away the garbage, but not to display anything sensible. That would explain the inactive-looking video RAM, anyway.

So... anyone happen to have a spare 901465-02 and/or a 901465-03 lying around? ;-) It'd be great to replace them with original (or at least period) parts. If not I'll need to see about shoehorning some modern chips in there...

 

[cosam]

Looks like I spoke too soon - A10 on my wonderful ROM adapter had an intermittent connection which was the cause of the read errors. I'm relieved to say that all ROMs are intact. No working PET yet, but I suspect whatever's broken is easier to replace than the ROMs...

Now that I'm confident of the accuracy of the dumps, here's an image of the Command-O ROM.

 

[dave_m]

Looks like I spoke too soon - A10 on my wonderful ROM adapter had an intermittent connection which was the cause of the read errors. I'm relieved to day that all ROMs are intact. No working PET yet, but I suspect whatever's broken is easier to replace than the ROMs...

Congrats on good ROMs. If ever needed, I have some 2532 EPROMs you can have, but at the moment my Data I/O Model 29B has a bad UniPak and I can not program PROMs. I hope that the UniPak only needs calibration which I am struggling to do, but it may be really broken and need replacement. Here is a link to a photo of the setup.

Data I/O Model 29B

-Dave

 

[tezza]

I always wondered what these EPROM burners looked like!

Tez