Help needed with 32K PET 2001 - makes a chessboard pattern on startup

[kiwwy]

As mentioned previously, I've recently bought a PET 2001 with a 32K expansion board.
http://i1130.photobucket.com/albums/m523/hq-retro-computers/pet2001k.jpg

Unfortunatly it got damaged during transport. I've replaced the busted fuse holder:
http://i1221.photobucket.com/albums/dd475/KiwwyPet/DSC07838.jpg

But unfortunatly, something else has gone bust as well. All I get is this:
http://i1221.photobucket.com/albums/dd475/KiwwyPet/DSC07847.jpg

I've tried several times, so its a consistent result. If I unplug the RAM board I get a screen full of X'es instead. :crazy::crazy::crazy:

Any ideas where to start?

 

[Nama]

Isn't the chequerboard pattern something you see when there is no Video RAM installed?

 

[MikeS]

Isn't the chequerboard pattern something you see when there is no Video RAM installed?

That certainly is the usual cause; the display circuitry and the character generator are obviously working and displaying the checkerboard character. It has the numeric value of 255, binary 11111111, so the inputs to the character generator are all high, suggesting that either the two 6550 video RAM chips are missing/not properly inserted/defective, or they're not being read by the character generator for some strange reason.

So, are the two chips at C3 and C4 fully and correctly inserted? If so, what happens when you remove one?

 

[kiwwy]

I tried tapping them in place, no result. I've now removed the right one of them, same result still.

What excatlly happends at power on is is this:
Datasette motor runs
Screen comes on with horizontal bars
Screen scrambles for about 0.5 sec, then the checkerboard comes on

 

[Nama]

I'm no expert, but it sound like the CPU is reseting at least.
Does the Datasette motor continue running?

 

[kiwwy]

I'm no expert, but it sound like the CPU is reseting at least.
Does the Datasette motor continue running?

Yes, also my conclusion that the CPU does do some kind of init. And I would also guess that the video RAMs are ok, as it is atleast able to show scrambled characters for half a second before it switches to the checkerboard.

Yes, datasette motor keeps running.

 

[Nama]

Maybe you could try carefully removing the ROM's and see if that effects how the machine starts. If there is a difference in the sequence, then it's possible that the ROMs were being read...at least initially.

 

[kiwwy]

Maybe you could try carefully removing the ROM's and see if that effects how the machine starts. If there is a difference in the sequence, then it's possible that the ROMs were being read...at least initially.

Fiddling with the ROM chips doesnt seem to make much difference. Is there a squence of wich ROM chip gets initialized first? Or in other words, can anyone tell me wich ROM chips contain wich part of the system?

I'm suspecting the CPU to be at fault. It seems very much like the video RAM gets filled up with checkerboard characters the instant that the CPU initalizes.

I was wondering, could I use CPU and/or RAM from a VIC-20 to test with?
That way I could remove the 32K RAM expansion, and do some tests without it.

 

[Pet Rescue]

I was wondering, could I use CPU and/or RAM from a VIC-20 to test with?
That way I could remove the 32K RAM expansion, and do some tests without it.

You certainly can, from my time looking at my pet problems the checkerboard pattern was a problem with the video ram 2114 chips gone down. Plenty to go at in the VIC 20! You could put your video ram and CPU into the VIC 20 to check your pet ones! Oh ! hang on you have 6550 and they are not compatable with the 2114 rams in the VIC 20

 

[kiwwy]

You certainly can, from my time looking at my pet problems the checkerboard pattern was a problem with the video ram 2114 chips gone down.

Yes, the two video RAMs are 6550. But on power up, I get the usual random stuff, untill it "resets" to the checkerboard screen. This makes me think that the video RAMs are ok, atleast so far as that they can contain random values.

I'm suspecting the CPU, if I move my fingers near it (without actually touching), the screeen flickers radom characters, then resets to the checkerboard after a bit.

Wich RAM chips would I be able to use in the original system RAM slots on the mainboard? Only 6550?
There werent any chips installed here as all 32K are on the expansion board.

 

[carlsson]

If I'm not mistaken, the PET will start in the last ROM ($F000), whichever part number that is. This is because the 6502 by definition looks for some pointers located at $FFFx to know where in memory to start execution. As others have written before, the 6502 CPU tends to be one of the most resilient chips so if yours is broken, you might find yourself in need of replacing a lot more. Of course if you can test it in a VIC-20, 1541 or whichever 6502 based computer/device you have handy, it is a nice way to eliminate it as the cause.

Rather than just reseating the ROM chips, you should try read them. Then again I understand those are 6540 chips, not the later 2332 ROMs? Perhaps not as easy to dump in an EPROM reader then, to verify their content.

Also, while I tested before that pulling 6520 and 6522 chips still will make (newer) PETs boot but without input and some flickering characters, perhaps if you have fried chips those would make matters worse. Therefore as a kind of hardcore testing, you could try to remove the 6520 and 6522 chips to see if there is any difference. For what it is worth, I believe this PET generates the video display independent on CPU content so it could be worth pulling the 6502 too and see if the checkerboard pattern appears at all.

 

[Pet Rescue]

I'm suspecting the CPU, if I move my fingers near it (without actually touching), the screeen flickers radom characters, then resets to the checkerboard after a bit.

Wich RAM chips would I be able to use in the original system RAM slots on the mainboard? Only 6550?
There werent any chips installed here as all 32K are on the expansion board.

Kiwwy you could still try your 6502 in the VIC 20 to test and I think your model pet uses 6550 ram only for video ram and the main ram. Hi Anders you beat me to it!

 

[Nama]

Wich RAM chips would I be able to use in the original system RAM slots on the mainboard? Only 6550?
There werent any chips installed here as all 32K are on the expansion board.

Yes, only 6550's unfortunately. However Nicolas Welte makes an awesome dual 6550 adaptor to use with modern SRAM (6116, 6164 or 61256) as a replacement.

http://vic-20.de/x1541/hardware/adaptors.html

 

[MikeS]

I'm suspecting the CPU to be at fault. It seems very much like the video RAM gets filled up with checkerboard characters the instant that the CPU initalizes.

I was wondering, could I use CPU and/or RAM from a VIC-20 to test with?
That way I could remove the 32K RAM expansion, and do some tests without it.

Actually, I doubt it; as far as I can tell, your RAM chips are 6550s, not the same as what's in a VIC-20.

You could swap the CPU if it's in a socket, but the 6502 is pretty well the least likely to fail and in any case your symptoms actually suggest that it's not the CPU; it's apparently correctly executing the startup sequence up to the point of clearing the screen, but it looks like no data is making it to the video memory for some reason.

Was it working when you bought it and did you do anything other than replace the fuse holder?

 

[kiwwy]

For what it is worth, I believe this PET generates the video display independent on CPU content so it could be worth pulling the 6502 too and see if the checkerboard pattern appears at all.

If I pull the exapnsion board (with both 6502 and the 32K RAM) then I get a screen full of all same characters. Sometimes its an arrow pointing left, sometimes other thinges, but allways a screen full of same characters, but havent seen the checkerboard in this "mode".

I just got my VIC-20 out, but everything is soldered in. Maybe I need to try with a 1541 instead. Lucky I kept most of the C64 stuff, hehe.

What kind of response should I expect if I try to boot a system with no system RAM installed?

 

[kiwwy]

You could swap the CPU if it's in a socket, but the 6502 is pretty well the least likely to fail and in any case your symptoms actually suggest that it's not the CPU; it's apparently correctly executing the startup sequence up to the point of clearing the screen, but it looks like no data is making it to the video memory for some reason.

Was it working when you bought it and did you do anything other than replace the fuse holder?

My main reason for trying another CPU would be to try to boot the machine without the expansion board, to see if the fault would be there.

Yes, it was working very nicely when I bought it, but apparantly it got bumped around a bit during transport. The fuse holder got crushed, and the expansion board got knocked out of its socket. I took everything out, reseated the expansion board, soldered in a new fuse holder, then reassembled the system. And then kept getting the checkerboard..

 

[Pet Rescue]

I think couriers play football with parcels these days, have you put the expansion board back in right without bending any of the pins as it is easy to do. Also if it has been bumped as badly as you say it could be worth checking the mainboard for any breaks in the tracks.

 

[carlsson]

For what it is worth, the checkerboard pattern is one graphic character as well. In your case I think you get 255 ($FF) all across screen that displays as a checkerboard pattern. If you get a solid screen of another character without the CPU + RAM board.. hm, yes maybe it makes some difference.

 

[kiwwy]

I've tried a 6502 from a 1541, same result, as you guys predicted, always ends with a screen full of one character, though not always the checkerboard.

I'm starting to suspect that there are some broken conections somewhere around the CPU socket. I'll go see if I can get hold of some 40 pin sockets today, then I'll inspect more closely for any brokenness, if nothing else turns up then I'll try replacing the socket.

 

[Nama]

I'm starting to suspect that there are some broken conections somewhere around the CPU socket.

Yes, that sounds like a good avenue to pursue. If you have a multimeter you should check the continuity on all the pins down to the tracks.