4032 cbm pet no video

[Dwight Elvey]

I'm not sure what was done. I suggest moving the ROM to another socket to see if the checksum was correct. I didn't expect the system to work that way. The socket's address is partially decoded. The ROM it self has additional decoding.
Are you saying that the ROM from erratic location now has a good check sum?
If so, it is either a bad socket or there is some type of contention with some other component.
As Daver2 says, the value from the keyboard looks bad. That may be related or just another possible problem. Dave_m suggested moving it to another socket but using the same select signal. Was that tried and what were the results?
Dwight

 

[Desperado]

OK then your file should be OK. Did you make sure to burn it into a 2716 or 2516 (2K). A 4K eprom will not do unless you used a 2732 with data in upper 2K address range.
-dave_m

Yes sure, i burned on 2716 eprom

 

[Desperado]

I'm not sure what was done. I suggest moving the ROM to another socket to see if the checksum was correct. I didn't expect the system to work that way. The socket's address is partially decoded. The ROM it self has additional decoding.
Are you saying that the ROM from erratic location now has a good check sum?
If so, it is either a bad socket or there is some type of contention with some other component.
As Daver2 says, the value from the keyboard looks bad. That may be related or just another possible problem. Dave_m suggested moving it to another socket but using the same select signal. Was that tried and what were the results?
Dwight

I burned file in a new Eprom and now pettester results are good..
How can i do to move eprom in other socket but using same signals?
Thanks

 

[daver2]

What Dwight is asking is what was the fault attributable to in the end?

If you replaced the IC socket - and used the same ROM - then the problem must have been either the IC socket or the soldering and not the ROM itself. EDIT: From your last post, it was the ROM that was faulty by the looks of it. Yes? What type of EPROM did you use for the replacement? Just so that we can check.

If my PETTESTER is now giving you the correct checksums, do *** NOT *** start moving ROMs around. It is likely that that these are now working OK!

Either the problem is the EDIT ROM or interrupts... Is the keyboard PIA in a socket? If so, try removing it and see if you get the READY prompt when you power up. You won’t have a flashing cursor though, but we can work on that later...

If you don’t have a CRTC machine and the screen works OK with my PETTESTER, but the screen is ‘blank’ when you put the EDIT ROM back in - this means that the EDIT ROM is clearing the screen. What do you see in the SYNC pin of the CPU? Is this pin pulsing (indicating that the CPU is still executing instructions) or not?

Dave

 

[Desperado]

What Dwight is asking is what was the fault attributable to in the end?

If you replaced the IC socket - and used the same ROM - then the problem must have been either the IC socket or the soldering and not the ROM itself. EDIT: From your last post, it was the ROM that was faulty by the looks of it. Yes? What type of EPROM did you use for the replacement? Just so that we can check.

If my PETTESTER is now giving you the correct checksums, do *** NOT *** start moving ROMs around. It is likely that that these are now working OK!

Either the problem is the EDIT ROM or interrupts... Is the keyboard PIA in a socket? If so, try removing it and see if you get the READY prompt when you power up. You won’t have a flashing cursor though, but we can work on that later...

If you don’t have a CRTC machine and the screen works OK with my PETTESTER, but the screen is ‘blank’ when you put the EDIT ROM back in - this means that the EDIT ROM is clearing the screen. What do you see in the SYNC pin of the CPU? Is this pin pulsing (indicating that the CPU is still executing instructions) or not?

Dave

Hi sir,
i am not sure where is located kb Pia, i attach picture....only one is socketed...
I tested Edit rom verifying with eprom programmer and results are good...
I don't have crtc machine, pin 7 (sync) on cpu pulse!
Thanks!

 

[dave_m]

I don't have crtc machine, pin 7 (sync) on cpu pulse!

I don't understand. Do you have a pulse on the Sync signal? If yes, please check for skinny negative going IRQ pulse on pin 4 of CPU. It should occur every 16.66 mS (60 Hz).

If no Sync signal then the system is getting stuck and not executing instructions properly.

 

[Desperado]

I don't understand. Do you have a pulse on the Sync signal? If yes, please check for skinny negative going IRQ pulse on pin 4 of CPU. It should occur every 16.66 mS (60 Hz).

If no Sync signal then the system is getting stuck and not executing instructions properly.

Hi Dave_m i have pulsing signal on cpu pin 7 but always low signal on pin 4.

 

[dave_m]

For some reason you are having a constant IRQ (Low)! That is very bad. One of your PIA's or VIA is holding it down. We need to come up with a way to find out which one without having to remove those big chips.

Maybe try to use your voltmeter on low scale and read the IRQ voltages on all the big chips including the CPU. The chip with the lowest reading may be the bad one.

Dwight may have a better method. He knows all the tricks!

Also check the board connections for solder splashes.

 

[Desperado]

For some reason you are having a constant IRQ (Low)! That is very bad. One of your PIA's or VIA is holding it down. We need to come up with a way to find out which one without having to remove those big chips.

Maybe try to use your voltmeter on low scale and read the IRQ voltages on all the big chips including the CPU. The chip with the lowest reading may be the bad one.

Dwight may have a better method. He knows all the tricks!

Also check the board connections for solder splashes.

Ok thanks so much

 

[Desperado]

For some reason you are having a constant IRQ (Low)! That is very bad. One of your PIA's or VIA is holding it down. We need to come up with a way to find out which one without having to remove those big chips.

Maybe try to use your voltmeter on low scale and read the IRQ voltages on all the big chips including the CPU. The chip with the lowest reading may be the bad one.

Dwight may have a better method. He knows all the tricks!

Also check the board connections for solder splashes.

Voltages are low on UC6 (6520) pin 37 and 38! 188mV

 

[dave_m]

That is suspicious as only C7 should be programmed to issue an IRQ.

If you ground the /RESET (C6-pin 34), does C6-pin 37 go high as expected?

 

[Desperado]

That is suspicious as only C7 should be programmed to issue an IRQ.

If you ground the /RESET (C6-pin 34), does C6-pin 37 go high as expected?

Yes!If i ground c6 pin 34, c6 pin 37 go high but i have always black screen....

 

[daver2]

If you look at the schematics (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320349-3.gif) and follow the keyboard connector (J5) backwards, you will find that UC7 is the keyboard PIA.

Looking at your picture back in post #25, is that 40 pin IC socketed or not? It certainly looks different to the other PIA/VIA devices.

If so, remove it and test again.

See if (a) the screen is anything other than black and

(b) the permanent low on /IRQ has gone away.

Dave

 

[Dwight Elvey]

I'm with Dave. If it is working now don't mess with it. I also see that the other ROMs are soldered in.
Dwight

 

[Desperado]

News!

I removed 6520 from UC6 socket and now i have basic screen!!
Thanks so much for your advices!
Unfortunately i don't have 6520 spares...so i must buy one....
But another CBM PET it's alive again

Why i see 15359 bytes?? I have 16 ram 4116 ic...
See you soon when i have new 6520....

 

[daver2]

Because the number indicates the amount of 'free' bytes for the user program.

Some of the 16K you have is used by the BASIC interpreter as working storage, so it is not 'free' for the user program.

The dreaded faulty PIA and stuck /IRQ line again...

Good news on another rescued machine resuscitated!

Dave

 

[Desperado]

Thanks Daver2!
Can i try to exchange place to this 6520?
Thanks!

 

[dave_m]

You may want to buy at least two 6520 or 6821 as your keyboard PIA may not be working.
Youshouldcheck jumpers on the board to make sure it is configured for 32K.

 

[dave_m]

For jumper information, assuming you have a 2001N type board, use this top assembly drawing and look for "Detail A and Detail D" notes. The shunts are toward the lower right of drawing.

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320351-6.gif

-dave_m

 

[dave_m]

News!

I removed 6520 from UC6 socket and now i have basic screen!!

I'm getting confused. Do you mean you removed C6 or C7? It looks like from the photo that C7 is the PIA that may be on a socket.

If it was C7 that you removed, then it may fix both your keyboard issue and the interrupt issue.