Garbage screen on Pet 2001

[kodawari]

Hello Pet lovers,

I'm new to this forum, my name is Gerrit and I live in Amsterdam.
I got my hands on a Pet 2001 (mainboard 320008) with the well known garbage screen.

On this board a lot of modifications were made.
First of all the video circuit has been modified. Also there is a second connector
placed on the motherboard.
The pet came with a basic 4 expansion board.

I've cleaned all the pins of the Rom/Ram.Pia ic's. I also bought BixFixer's Romulator, but as soon as I use the NOP generator or the pettester, pin 7 Cpu is low; all adres lines are high, not pulsing.
There is a schematic with the modifications
I took some pictures to clarify some things.

Can someone point me out where to start to get this Pet up and running?

 

[kodawari]

Thanks for your answer, Aldis1ne, but this only happens when I plug in the Romulator. If I set the romulator to basic 1 for instance, the fault is still the same, but the Pin7 & adresses are then pulsing again. I cannot get in touch with the Bitfixer so I thougyt I give this forum a try.

 

[xp0nentz]

Hello Pet lovers,

I'm new to this forum, my name is Gerrit and I live in Amsterdam.
I got my hands on a Pet 2001 (mainboard 320008) with the well known garbage screen.

On this board a lot of modifications were made.
First of all the video circuit has been modified. Also there is a second connector
placed on the motherboard.
The pet came with a basic 4 expansion board.

I've cleaned all the pins of the Rom/Ram.Pia ic's. I also bought BixFixer's Romulator, but as soon as I use the NOP generator or the pettester, pin 7 Cpu is low; all adres lines are high, not pulsing.
There is a schematic with the modifications
I took some pictures to clarify some things.

Can someone point me out where to start to get this Pet up and running?

Maybe check RAM or ROM, I don't know if I'm qualified for this situation. Try checking those if you haven't.

 

[MicroCoreLabs]

@ensingg , I believe @kodawari lives near you so maybe you can share an MCL65+ board in case he I interested in using it to debug his PET.

​

 

[Hugo Holden]

@kodawari:

I would not even dream about trying to repair your PET in the condition it is in, with those mods, some of which have been poorly executed from the hardware perspective.

The mods look like they were there to drive some sort of expansion card with a Euro style connector, the types used on the Rockwell RM65 computer setups.

Since one thing the PET was missing was an RS-232 interface I'm wondering if it might have been for the Rockwell RM65 Async Card, but really it could be for anything, since the PET's video circuit was modified it may have been for the RM65 video card. But that doesn't make a lot of sense, because it would have been much easier to derive composite video from the three pulses which drive the VDU.

With mods as extensive as this, they might have some use or historical value, so it is better to make sure they are documented as accurately as possible. Sometimes they start to make sense, other times if the design was less hardward and more firmware in the cocktail, it can be more awkward.

If there are any special ROMs there that presumably support the mods, they need to be dumped so that they can be disassembled and that can give a clue about the hardware mods and what they might do.

In any case once that is done, if this computer was on my desk, I would remove/undo all of the mods, and put the pcb & wiring back to factory standard. Only then would I attempt the fault finding on it.

Now it may be possible to repair the computer with all those mods in place and you might "get lucky", but it only takes one tiny defect to upset the apple cart.

 

[xp0nentz]

@kodawari:

I would not even dream about trying to repair your PET in the condition it is in, with those mods, some of which have been poorly executed from the hardware perspective.

The mods look like they were there to drive some sort of expansion card with a Euro style connector, the types used on the Rockwell RM65 computer setups.

Since one thing the PET was missing was an RS-232 interface I'm wondering if it might have been for the Rockwell RM65 Async Card, but really it could be for anything, since the PET's video circuit was modified it may have been for the RM65 video card. But that doesn't make a lot of sense, because it would have been much easier to derive composite video from the three pulses which drive the VDU.

With mods as extensive as this, they might have some use or historical value, so it is better to make sure they are documented as accurately as possible. Sometimes they start to make sense, other times if the design was less hardward and more firmware in the cocktail, it can be more awkward.

If there are any special ROMs there that presumably support the mods, they need to be dumped so that they can be disassembled and that can give a clue about the hardware mods and what they might do.

In any case once that is done, if this computer was on my desk, I would remove/undo all of the mods, and put the pcb & wiring back to factory standard. Only then would I attempt the fault finding on it.

Now it may be possible to repair the computer with all those mods in place and you might "get lucky", but it only takes one tiny defect to upset the apple cart.

Didn't notice the card. Yeah, this is probably correct. I'm not qualified in the PET field since I don't own one... Yet.

 

[xp0nentz]

There seems to be alot of jumper wires in the machine, this machine was probably modified.

 

[MicroCoreLabs]

I would:
1) Take pictures of the rework in case anybody cares.
2) Remove all of it, replace all of the original chips, and repair any trace cuts. You want the motherboard back to original configuration with ICs that match the schematic,
3) Build/acquire an MCL65+ so you can perform focused tests on every RAM, ROM, and peripheral on the motherboard. It will be a lot more effective and informative than running NOPs.

Having an oscilloscope would be ideal, but a volt-meter could also serve if necessary. The MCL65+ can toggle the CPU IOs and generate bus cycles as slowly as needed to register on a volt-meter.

 

[Hugo Holden]

After you remove the mods and check for any obvious cut surface traces, make sure to take a note of every solder point to any track or any IC pin. You need to follow each one to all its destinations, the reason being, that one methodology used to cut tracks, to make mods, is not to actually cut them on the surface (where you can see it) but to run a slighlly over-sized drill through a via and disconnect the tracks that way. You don't want to miss an open track when this method was used. Most of your vias look solder filled, so that is helpful to see they have not been tampered with.

In addition, there is yet another place cut tracks can hide, for the IC's in sockets, sometimes was to cut a track running under the IC, on the top of a pcb, a small drill is applied to the surface, to countersink it, and cut the track that way. You have to be a thorough Detective and miss no disconnected tracks. So all IC's in sockets require removal and inspection underneath them, if there is a view of any tracks, and if some might have been disconnected.

Also use good solder Wick, like Goot, to remove the solder from the gold plated fingers, you don't want the solder to spread down those fingers on the edge connector any more than it already has.

 

[MicroCoreLabs]

If I am not mistaken this looks like a trace was cut and rerouted to a neighboring pin. There may be lots of little details like this that need to be reversed.

 

[kodawari]

I would:
1) Take pictures of the rework in case anybody cares.
2) Remove all of it, replace all of the original chips, and repair any trace cuts. You want the motherboard back to original configuration with ICs that match the schematic,
3) Build/acquire an MCL65+ so you can perform focused tests on every RAM, ROM, and peripheral on the motherboard. It will be a lot more effective and informative than running NOPs.

Having an oscilloscope would be ideal, but a volt-meter could also serve if necessary. The MCL65+ can toggle the CPU IOs and generate bus cycles as slowly as needed to register on a volt-meter.

Thanks for your answer, I do have an oscilloscope, but my question is: What step do I have to take to acquire a pcb of your MCL65+? I also wil ask @ensingg. If I can get a MCL65 I'll think I'll first try that and than afterwards I'll remove the modification. I bought the PET very cheap with a lot of literature. Anyway, thanks again.

 

[kodawari]

Thanks everyone for the answers, I'll try to get hold of a mcl65+ board and the after that I'll remove all the mods of the cheap bought PET. This PET came with a lot of books and 2 boards.