Commodore PET 2001-8 stuck on garbled screen

[RetroGadgetMan]

Hi after debugging the video logic with help from some people here which I am really grateful for we now have a garbled screen.
Character set has been replaced and I now get the following image pic 1.
Sometimes some of the characters will change if I press down on the board in various places or even flicker indicating a bad socket or maybe a bad trace.
I have cleaned all the sockets which made no difference.
Pic 2 shows the tynemouth pet rom ram substitute.
This can be configured to replace either the rom or ram or both and allso contains basic 1 2 and 3 selected by dip switches.
Pin 7 of the CPU with this rom ram board is at hi lo and pulsing.
With the original CPU installed with out the ram rom board pin 7 stays at lo.
Moving forward I think it's best to keep using the rom ram board until we get the correct welcome screen and a flashing curser. Then we can start looking at the rom and ram section.

 

[Nivag Swerdna]

Remove both PIAs and the VIA

Remove the replacement RAM/Board for now

Power on the machine and check the RESET and SYNC pins on the CPU. What do you see?

Do you have a NOP Adapter?

 

[RetroGadgetMan]

Remove both PIAs and the VIA

Remove the replacement RAM/Board for now

Power on the machine and check the RESET and SYNC pins on the CPU. What do you see?

Do you have a NOP Adapter?

Which numberd pins please. And what's a nop adapter. Sorry .

Just found them.

 

[RetroGadgetMan]

Pin 40 hi pulse
Pin 7 lo no pulse
These readings are with the original CPU, not the one in the ram board.

 

[ajgriff]

Character set has been replaced and I now get the following image pic 1.

I assume this means you've replaced the character ROM and that you still get the garbled screen when using the Tynemouth board's ROM & RAM. Presumably this is the early static RAM logic board with socketed RAM chips. If so it looks like there's a problem with at least one of the video RAM chips or associated circuitry. Have a look at this link which describes how you can easily carry out some checks:

Troubleshooting Common Problems with the Commodore PET 2001 | D'Asaro Designs

www.dasarodesigns.com

Alan

 

[eight088]

Pin 40 hi pulse
Pin 7 lo no pulse
These readings are with the original CPU, not the one in the ram board.

I would think if pin 40 is pulsing then that is a problem. Pin 40 should start of low when the machine is first switched on, and then go to a solid high shortly after being switched on.

A NOP generator is a device that ties the data lines to a specific instruction, which makes the CPU doing, basically, nothing. No OPeration. By doing this the address lines will pulse and if you have a way of measuring the frequency on each address line you will be able to tell if there is a problem with a specific address line.

I would be interested in seeing what results you get when you switch the tynemouth board to the pettester.

 

[Nivag Swerdna]

OK let's concentrate on pin 40 which is nRESET. i.e. when LO it holds the CPU in reset (during power on) and then high it let's it run.

It should not pulse... should be low for a fraction of a second and then high for the duration after.

You could look at D9 pin 1 and 2. These should be opposites and not pulsing after the first 1second of turning on.

A NOP Adapter is a small board that you insert between the CPU and its socket which causes the data bus to always be NOP (EA for the 6502); you can achieve the same effect by bending a few pins and soldering a few wires. Since the CPU sees only NOP instructions the net effect is that it turns the CPU into a counter which simply increments the address bus... useful for fault finding address decoding issues.

 

[ajgriff]

These readings are with the original CPU, not the one in the ram board.

Have you got 2 CPUs?

Alan

 

[RetroGadgetMan]

I assume this means you've replaced the character ROM and that you still get the garbled screen when using the Tynemouth board's ROM & RAM. Presumably this is the early static RAM logic board with socketed RAM chips. If so it looks like there's a problem with at least one of the video RAM chips or associated circuitry. Have a look at this link which describes how you can easily carry out some checks:

Troubleshooting Common Problems with the Commodore PET 2001 | D'Asaro Designs

www.dasarodesigns.com

Alan

Hi yes it's the pet with the 6540 static ram that's very sensitive.
Before I replaced the character set I didn't get any characters at all.
I have seen the link already and spent some time yesterday swapping out ram chips to the video chips sockets and found some dead ones doing this, a checkered screen.
The pic I have added here is from that site. Should I be getting an identical character set like this? or can they change position on a restart?. As they do appear to change sometimes especially with different ram pairs as i tried them all.
I do have two cpu's

 

[RetroGadgetMan]

I would think if pin 40 is pulsing then that is a problem. Pin 40 should start of low when the machine is first switched on, and then go to a solid high shortly after being switched on.

A NOP generator is a device that ties the data lines to a specific instruction, which makes the CPU doing, basically, nothing. No OPeration. By doing this the address lines will pulse and if you have a way of measuring the frequency on each address line you will be able to tell if there is a problem with a specific address line.

I would be interested in seeing what results you get when you switch the tynemouth board to the pettester.

Pin 40 starts lo on initial power up then goes to hi with pulse after one second.
On test mode switch 2 on the Tynemouth board I get the following image with jittering lines all over the top of the characters.
In test mode I noted the following:
Pin 40 hi pulse pin 7 hi lo pulse.

 

[ajgriff]

The pic I have added here is from that site. Should I be getting an identical character set like this? or can they change position on a restart?. As they do appear to change sometimes especially with different ram pairs as i tried them all.
I do have two cpu's

No, the characters' positions will not be identical to the example image and they will change on restart. If you're sure you've identified all the faulty 6550s and that both CPU's are ok try removing the PIAs if you've not already done so.

Alan

 

[RetroGadgetMan]

No, the characters' positions will not be identical to the example image and they will change on restart. If you're sure you've identified all the faulty 6540s and that both CPU's are ok try removing the PIAs if you've not already done so.

Alan

Pias and via removed for these tests.
I am just retesting the rams again in the left socket of the video ram location C4.
Pic 1 is an example of one ram displaying a screen of the same character. I guess that means it's faulty? A few are like this.
Pic 2 is what I get using a pair of new rams in adapters.
Pic 3 of the new rams.mm2114n
Pic 4 a single 6540 in left socket of C4. Looks good or bad?

 

[Nivag Swerdna]

You need pin 40 of the CPU to be high to have any chance of anything running. I am not familiar with the Tynemouth board but looking at the photo it doesn't look like it has reset logic so I wouldn't expect it to interfere with the RESET line.

Maybe scope pin 40?

 

[RetroGadgetMan]

You need pin 40 of the CPU to be high to have any chance of anything running. I am not familiar with the Tynemouth board but looking at the photo it doesn't look like it has reset logic so I wouldn't expect it to interfere with the RESET line.

Maybe scope pin 40?

Pic 1 pin 40 with rom ram board cpu1
Pic 2 without rom ram just cpu 2
I leave one CPU on the rom ram board and use a 2nd one for separate tests. Both CPUs have been tested in another pet today and boot fine.

 

[Nivag Swerdna]

Doesn't look like pulsing to me. Looks HI.

 

[RetroGadgetMan]

Doesn't look like pulsing to me. Looks HI.

It doesn't make sense.
When I power up pin 40 starts low on the scope then jumps up to the top after one second

 

[Nivag Swerdna]

That's the correct reset behaviour. Time to move on...

If it was me I would build or buy a NOP adapter at this stage.

 

[ajgriff]

It doesn't make sense.
When I power up pin 40 starts low on the scope then jumps up to the top after one second

That's an acceptable Hi signal as far as I'm concerned and it doesn't look like there's a problem with the reset circuitry.

Overall I'm a bit confused and wouldn't be testing the RAM chips with the Tynemouth board in test mode. Also I think it's worth sticking to using just one CPU.

Alan

 

[RetroGadgetMan]

My thoughts were to use the Tynemouth board until we get this pet booting, hopefully. as I don't know if the original roms are working. And I don't have much confidence in the ram chips that are left supposedly working. I know this model pet can boot with just 4 ram chips installed.
I have the pet rom ram board set to replace the ram and rom and basic 1 when installed.
To me it makes sense to use it.
Thoughts?

 

[Nivag Swerdna]

Overall I'm a bit confused and wouldn't be testing the RAM chips with the Tynemouth board in test mode. Also I think it's worth sticking to using just one CPU.

That is a fair comment. With RAM and ROM substitution from the ROM/RAM board it would reduce to a bad CPU or decoding issue... and then buffers across to Video RAM.

Remove all the RAM and ROM except for the Video RAM and try the ROM/RAM Board... and scope pin 37 and pin 7 of the CPU. Looks like ON ON ON OFF OFF which you have is a good setting.