Pet 2001 hanging at reset

Hi all,
I have been following several threads on getting my Pet 2001-8 working.
So far i have fixed a problem with the Display, bad solder joint on the VR1 12v regulator.
Now i have a display i can see that it puts garbage on the screen, then resets too a black screen.
The only thing i can see for now is that some of the Sel lines from the 74154 at G2 are being held high.
Any ideas as im thinking the 74154 is bad.

regards
ian

greenoneten said:

The only thing i can see for now is that some of the Sel lines from the 74154 at G2 are being held high.
Any ideas as im thinking the 74154 is bad.

Click to expand...

ian,
Too early to start replacing parts like the 74154. The blank screen after garbage indicates that the program starts OK, but soon gets lost. This is usually due to bad low RAM or a bad ROM which causes the CPU to get into a tight loop or to execute an illegal KILL instruction. Do you have a scope or just a meter? Check the signal SYNC on the CPU. If it is pulsing or reading about 2 volts on the DC scale, then CPU is running but may be in a loop. If SYNC is stuck low, then the CPU has executed the dreaded KIL instruction indicating there is a bad ROM or a problem with data bus.

What type of ROM/RAM devices? Are they on sockets? If you have the oldest type of PET main board, you will have the 65540/6550 chips. Replacements are very difficult to find. You may have to consider a special RAM/ROM board to replace them. The 2332 ROMs and 4116 RAMs have replacements.

Check address lines A15, A14, A13 and A12 to see where the CPU is running. If it is stuck for example in 1111 ($FXXX) then the kernal ROM may be bad, etc.

The schematics can be found on the zimmer site. The people on this site will soon help you get your PET running.

Hi Dave,
Thanks for the reply, Its an early 6540/6550 model.
I have a scope and a dvm.
I have checked the SYNC on the CPU and cold boot its low. Warm boot its pulsing and you sometimes get artifacts and garbage.
Address lines seem to be running but selects a different rom on various startups.

Just checked again and it seems to be selecting H4 and H7.
Ordered a Board from Tynemouth to help.

OK going backwards
Just been checking round and now it doesnt reset.
get the pulse at the CPU but nothing now happens.
Sync now low, got clock pulses but all lines now high.

The H4 and H7 ROMs compose the kernel and if one of those are bad, it would certainly give the results you are seeing. The Tynemouth board is an excellent product and will fix all memory problems other than the 1K video RAM (C3 and C4). If it turns out you do have a problem with the video RAM, you will need to salvage two good ones from the 16 in main RAM which should be a good bet.

The RAM/ROM board plugs into the 6502 CPU socket. The main board you have requires slightly longer pins due to the type of socket. Did you mention to Dave that you have a 2001-8 and will need the longer pins?

The most complicated part of a PET is the addressing circuitry due to all the multiplexers, select logic, and tri-state buffers. It might be prudent to build a NOP Generator to check all the address line paths if the Tynemouth does not fix the all the problems. Also if there is a problem with the 40 pin chips (6502, 6520 and 6522), they are still available. The Motorola 6820 and 6821 are cheap replacements for the 6520. Are the big chips on sockets on your board for easy replacement? Note that people have reported that the original sockets on the PET get very intermittent with old age; yet another concern.

With any luck, the RAM/ROM board will do the trick, but be prepared for a little more troubleshooting.

-Dave

EDIT: just saw your latest post. If you are getting a good long RESET pulse, but no activity on address or data lines, the problem could be a bad 6502 CPU. However they rarely fail. I assume you have checked for clean +5 Volts, and 1 MHz clock? Make sure the Interrupt Request line (F3-pin4) is not stuck low. Depending on the quality of the scope, it may be difficult to see startup activity if it doesn't last for many milliseconds.

Hi,
On a cold start F4 pin4 is stuck low.
If i remove the roms and toggle power quickly the CPU starts running.
So im not sure if i am getting a good reset signal now.
Tynesoft board was shipped today so I guess will arrive Tuesday.
Ordered the board with the long pins.

Seem to be going backwards now LOL

greenoneten said:

Hi,
On a cold start F4 pin4 is stuck low.
If i remove the roms and toggle power quickly the CPU starts running.
So im not sure if i am getting a good reset signal now.

Click to expand...

If the interrupt line (F3-pin4) is held low, you may have a bad PIA. Remove G8 if it is on a socket. It is the source of the interrupt, and see if you get further.

Check for a good negative reset pulse at the CPU (F3-pin40) on power up. I think it is about a 1/2 second pulse. If the CPU is not running, you should see a garbage screen. Hang in there.

Just a garbage screen now, doing strange things now.
wont run cpu with H7 in. Then will.
wont run cpu with ram in I1 then will.
No difference with PIAs removed or Via removed, checker screen and white screen with video ram removed.
Was missing 3 ram chips and had some bent pins when i acquired it, guess someone had a go before.
Board is in a nice condition no corrosion in sockets or on pins and everything so far socket to board checks ok.
swapped CPU for another but that one from a vic20 has no effect, will test it tomorrow in an apple II.
Think i have a spare CPU in an apple II clone that i can use for making a NOP generator. worry about the Clone later as i robbed it of ic's to fix another genuine IIE.
Will keep ploding on.
thanks for the replies

greenoneten said:

Just a garbage screen now, doing strange things now.
wont run cpu with H7 in. Then will.
wont run cpu with ram in I1 then will.

Click to expand...

Make sure you are getting a good long reset pulse at power up, otherwise check 555 timer and associated capacitors.

No difference with PIAs removed or Via removed, checker screen and white screen with video ram removed.

Click to expand...

With PIAs and VIA removed, the Interrupt Request line at F3-pin4 must be high (unasserted). If not, there is a short on that circuit.

With Video RAM removed, you will get 'opens' into the character generator input which will look like all ones. Character Generator will then output a checkerboard pattern only.

I would do nothing until you try that Tynemouth board. Else you may be wasting your time chasing your tail.

Hi,
Tynemouth board arrived, it works with the board fitted but only after removing the roms and ram. so something there causing it not to start up.
Cpu resets but all data lines high with roms and ram fitted.
will keep you updated.

Hi,
I have done some further testing, I have 4K of good Ram and video chips, plus 1 spare 6550!
The Roms in H2, H4, H6,H7 are working, H1,H3,H5 are not working.
This leads me too my original question 74154 as this was what i found originally. Some of the Sel Lines are high.
As originally it didnt appear to select H1,H3,H5.
Ordered one before easter just waiting for it to be delivered.
The Only way to be sure i guess as i cant verify the roms!

You are doing great! How did you determine which ROMs are working? If the Tynemouth board does not run with a particular ROM installed, it may mean there is a short on that part on one of the data lines.

A NOP Generator is great for checking all the SELECT lines as all the addresses are toggled causing the SELECT lines to pulse in a similar fashion.
-Dave

Obligatory Space Invaders

sideways!

Hi,
Changed 74154 at G2 and now Roms H1, H5 work. Appears H3 is bad.
When fitted with Tynemouth Board it causes Garbage in rows vertically but Basic still works in background.
Whats the options on Ram and Roms?
As i would like to get it running on its own.

Also dragged my 8296sk out and thats dead also LOL
Makes the chirp but no display. Job for another day!

dave_m said:

ian,
Too early to start replacing parts like the 74154. The blank screen after garbage indicates that the program starts OK, but soon gets lost. This is usually due to bad low RAM or a bad ROM which causes the CPU to get into a tight loop or to execute an illegal KILL instruction. Do you have a scope or just a meter? Check the signal SYNC on the CPU. If it is pulsing or reading about 2 volts on the DC scale, then CPU is running but may be in a loop. If SYNC is stuck low, then the CPU has executed the dreaded KIL instruction indicating there is a bad ROM or a problem with data bus.

What type of ROM/RAM devices? Are they on sockets? If you have the oldest type of PET main board, you will have the 65540/6550 chips. Replacements are very difficult to find. You may have to consider a special RAM/ROM board to replace them. The 2332 ROMs and 4116 RAMs have replacements.

Check address lines A15, A14, A13 and A12 to see where the CPU is running. If it is stuck for example in 1111 ($FXXX) then the kernal ROM may be bad, etc.

The schematics can be found on the zimmer site. The people on this site will soon help you get your PET running.

Click to expand...

This looks like the perfect place to discuss an issue with my recently acquired Commodore PET 2001 32N. I have worked with several Commodore 64's but I do not know my way around the PET yet.
They seem quite similar. However I do get a little fuzzy on the ROM chips. They are all labeled the same, but I do not believe they are interchangeable.
The white ceramic ROM chip does seem to get a bit hot.

Anyway the problem is I get a garbage screen, but I do see a ready inside the garbage, even though I cannot access the Floppy drive unit. I can see the cursor move down, when I press enter.

I have already downloaded the schematics and the parts list, as best as I can tell, but I need to put all this together in my thimble sized brain.
I have a logic probe and a computer based OScope as well as a nice DMM. So, I am close to getting ready to start, I would just like some suggestions, on where
I should start looking first. Is the possibly a video memory problem and a reset problem?

Thank you

greenoneten said:

View attachment 30392
Obligatory Space Invaders

sideways!

Click to expand...

Nice PET and Space Invaders !!!

KC9UDX said:

I would do nothing until you try that Tynemouth board. Else you may be wasting your time chasing your tail.

Click to expand...

I have a PET 2001 32N. I get a garbage screen, but I can see a READY underneath it all. I did notice that the first ROM chip was getting hot, but all other chips seem to be running cool.
I have not started probing, yet, when I found info on the board you are referring to. It sounds like it could help me diag the problems by replacing all the memory, or all the ROMs.
How do I go about purchasing one of these boards?

Novell2NT said:

I have a PET 2001 32N. I get a garbage screen, but I can see a READY underneath it all. I did notice that the first ROM chip was getting hot, but all other chips seem to be running cool.
I have not started probing, yet, when I found info on the board you are referring to. It sounds like it could help me diag the problems by replacing all the memory, or all the ROMs.
How do I go about purchasing one of these boards?

Click to expand...

In your case, I wouldn't recommend it. You probably have bad video RAM.

PETs are much simpler than C64s and much easier to work on, in my opinion.