Another PET 2001 motherboard needs your help...

[MikeS]

Yes, you are right, I still have not tested the original ram in the video sockets. Can you explain the process a little more clearly so that I can get some genuine results.

Well, it's easy now that the computer's working off the adapter; just replace the video RAM with each of the RAM chips and see if there are any anomalies.

Boy, this fix is proving to be a challenge.

All the more satisfaction when it's finally working 100%.

You don't need the on-board RAM, so really only the video problem left.

 

[Nama]

...so really only the video problem left.

Oh, I hope so.

I'm also hoping someone could possibly make some educated guesses as to where I could look for problems. Then I could break out my scope (hopefully it works) and take some readings. Ruud mentioned the 74177 (74LS197's) and I have a couple spare, but I don't want to just start shotgunning the repair as I've noticed these old PET boards lift their traces very easily, and I'd hate to damage something and make things worse. I'd rather keep the board level desoldering and soldering to a minimum if possible.

Cheers again
Phil

 

[MikeS]

I hear ya...

I wouldn't think it's the '177s because I'd expect any problem there to occur at binary multiples and not every 40 characters, but the timing and decoding logic is too complicated for my old brain so who knows; as to what it is instead of what it isn't, I don't have any good ideas.

Dave: Any ideas?

I suppose one thing you could try is put both PETs side by side and go through the various ICs on the bottom and top left of page 3, comparing the signals in the good one to the one with the problem; if you do find a difference maybe that will give us a clue where to look.

And c'mon, test those 4 RAM chips in the video sockets to see if any of them look different; I wanna check 'em off my mental list so I can start sleeping well again ;-)

 

[Nama]

I suppose one thing you could try is put both PETs side by side and go through the various ICs on the bottom and top left of page 3, comparing the signals in the good one to the one with the problem; if you do find a difference maybe that will give us a clue where to look.

Oh boy, I thought you may say something like that. I'll have to have a good think as to how I can make some space, and not annoy the wife. These small Japanese apartments really aren't cut out for working on multiple machines with large test equipment [scratches head].

And c'mon, test those 4 RAM chips in the video sockets to see if any of them look different; I wanna check 'em off my mental list so I can start sleeping well again ;-)

Last night i built up a 6550 RAM adaptor:

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

I've had the board sitting around for ages, but as there was a small pause in the conversation as everyone ponders the screen glitch issue, I thought I'd build it up. Seems to work great, now I don't have any 6550 ram on the board at all. However, if it means more sleep time for you, I'll try to test the original RAM tonight ;-)

Thanks again for your time.

Phil

 

[MikeS]

Oh boy, I thought you may say something like that. I'll have to have a good think as to how I can make some space, and not annoy the wife. These small Japanese apartments really aren't cut out for working on multiple machines with large test equipment [scratches head].

Hold off on that for a bit; I can actually duplicate your symptoms but I'm scratching my head trying to pin down the exact cause ;-)

Last night i built up a 6550 RAM adaptor:
http://vic-20.de/x1541/hardware/adaptors.html
I've had the board sitting around for ages, but as there was a small pause in the conversation as everyone ponders the screen glitch issue, I thought I'd build it up. Seems to work great, now I don't have any 6550 ram on the board at all. However, if it means more sleep time for you, I'll try to test the original RAM tonight ;-)

No need, unless you want to know if they're all good or why the PET won't work with the onboard RAM & ROM; I actually slept quite well last night ;-)

Besides, now that you have no more use for 6550s who knows, you may be able to make someone else happy. (e.g. I need one... ;-) )

 

[Nama]

Hold off on that for a bit; I can actually duplicate your symptoms but I'm scratching my head trying to pin down the exact cause ;-)

Wow! I'm curious how you can duplicate these symptoms?

Besides, now that you have no more use for 6550s who knows, you may be able to make someone else happy. (e.g. I need one... ;-) )

It's a deal, if you can help solve my screen glitch issue, I'll certainly send you a 6550 ;-)

 

[tezza]

These small Japanese apartments really aren't cut out for working on multiple machines with large test equipment [scratches head].

Phil, You've seen the space I've got. If I can do it in a matchbox so can you :D

Tez

 

[dave_m]

I wouldn't think it's the '177s because I'd expect any problem there to occur at binary multiples and not every 40 characters, but the timing and decoding logic is too complicated for my old brain so who knows; as to what it is instead of what it isn't, I don't have any good ideas.

I'm struggling with this area too. Does anyone know the the flip flop called NEXT does. It looks important but I am lost.
(I'm looking at schematic 320349 sheet 7) I hope it's at least the right schematic.

 

[Nama]

Phil, You've seen the space I've got. If I can do it in a matchbox so can you :D

Tez

Terry, I think your space is like the Australian continent compared to my space. In my old place I used to have an entire room like you, now I have a small closet as storage, and the top of two dressers as a work area and no space or place to sit, so I have to do everything standing. The PET keyboard, motherboard, transformer, IBM laptop and burner take up one and a half dresser tops! The remaining half is used for a place to put IC's and tools that I'm presently using.
Since moving to my new place I've had to cull my collection by 2/3's...it's all good though as I can keep and concentrate on only the good stuff that I really like.
By the way, your box arrived yesterday, and looking very beat up. I've checked inside and everything looks ok though. Once I finish off this PET fix, I'll give the System 80 disk drive a whirl...I do have some question though so I'll fire through an email in a while.
Thanks again.

 

[Nama]

I'm struggling with this area too. Does anyone know the the flip flop called NEXT does. It looks important but I am lost.
(I'm looking at schematic 320349 sheet 7) I hope it's at least the right schematic.

I think those schematics are for the 2001n. Maybe either 320130 or 320008 may be better. Can anyone confirm this? Basically it the board with 6550 ram and 2316 ROMs, so it may be a combination of the two???

 

[MikeS]

...(I'm looking at schematic 320349 sheet 7) I hope it's at least the right schematic.

Wish it were so; those are much easier to read. No, I think we're dealing with 320081-3, and that older video logic is somewhat different.

 

[tezza]

Terry, I think your space is like the Australian continent compared to my space. In my old place I used to have an entire room like you, now I have a small closet as storage, and the top of two dressers as a work area and no space or place to sit, so I have to do everything standing. The PET keyboard, motherboard, transformer, IBM laptop and burner take up one and a half dresser tops! The remaining half is used for a place to put IC's and tools that I'm presently using.
Since moving to my new place I've had to cull my collection by 2/3's...it's all good though as I can keep and concentrate on only the good stuff that I really like.
By the way, your box arrived yesterday, and looking very beat up. I've checked inside and everything looks ok though. Once I finish off this PET fix, I'll give the System 80 disk drive a whirl...I do have some question though so I'll fire through an email in a while.
Thanks again.

Gawd. Yes I guess at least I don't have to share the space with Annette or any other aspects of living.

I was wondering if the System 80 expansion unit had arrived. Lemme know when you get around to it. It SHOULD be just a matter of plug and play.

Tez

 

[Nama]

Wish it were so; those are much easier to read. No, I think we're dealing with 320081-3, and that older video logic is somewhat different.

Ok, I think you are correct. I believe the complete set is:

320130-1
320081-2
320081-3

...or am I wrong again?

 

[dave_m]

Wish it were so; those are much easier to read. No, I think we're dealing with 320081-3, and that older video logic is somewhat different.

OK, I am on the right page now. Yes, it is quite different than the 2001N schematics and looks a little simpler because there is no need for dynamic refresh logic for the main RAMs.

However, I agree that they are worst schematics I have seen in a long time. Most of the gate outputs are pointed to the right as is normal but some are pointed up and some down and even a few are output to the left! There are few signal names to help with the logic deciphering. And a few function tables would have been nice. I guess Chuck Peddle was in a real hurry at the time.

I have found these Commodore schematics in my file. If anybody needs B size (11"x17") hardcopy, let me know.

 

[Nama]

So I've busted out the old scope that was given to me many years ago. I've never used a scope before, so with the help of MikeS I think I have it somewhat calibrated.

I've posted some videos showing the working board (top) compared to the non-working board (bottom). Presently I have only tested C5 pin 5, C9 pin 8, 9 and 11.

I've posted video's here again:

http://web.me.com/lord_philip/other_computers/20012.html

Phil

 

[Nama]

No I haven't disappeared, I've been working with my scope and help of MikeS and André from cbm-hackers to try to narrow down the cause of the funky screen. Andre had a great hypothesis for my troubles. Here is what he says:

While displaying a line, the video circuitry first loads this memory address into the registers D6/D7, which get incremented every cycle. When 40 characters are shown, the "DIS ON" (from C5 in the horizontal timing circuitry) signals goes off, but the counter still counts on.

The counter is reloaded with the DIS ON signal. I.e. every time a new raster line starts, the memory address counter is loaded new from the 74100 latch. If the latch would not be reloaded, the screen would show a single line over and over.

The characters are made by using the same memory address, but selecting different raster lines addresses in the character ROM A2.

Only if the three raster line counters (output of A1, input into the A2 6540 charrom) are all ones (7 = last rasterline of a character), AND the DIS ON signal is on (not sure about the exact timing though) the buffer 74100 is loaded with the next address (triggered via the C7 NAND - shown as OR with inverted inputs) - the last character address in the current line + 1, which is the first address in the following character line. This is how lines are counted in memory.

In the current problem we see the following:
- the character lines still show 40 characters
- For each line, the character address is advanced only 36 characters. So the 4 last characters of a line are (somewhat) repeated in the first 4 characters of the next line
- It seems the first character of each line is repeated
- This happens per character line, not per raster line

So the memory map in screen coordinates would be:

0 0 1 2 3 4 5 6 .... 35 36 37 38
36 36 37 38 39 ...
72 72 73 74 ...

As I said this happens per character line, which is interesting, as the "mixup" of memory addresses must happen each raster line.

Now I see that the 74100 and D6/D7 counters actually loads the _upper_ 8 bits of the 10 bits video memory address. The lower two bits are handled by the D5 dual-JK-flipflop and the following multiplexer at D2.

Looking at the fact that the lower two bits show a strange behaviour in the memory map:

00 00 01 10 11 00 ...
00 00 01 10 11 00 ...

my assumption is there is a spike in the DIS.ON line within one cycle after it becomes active that clears the lower two address bits (so the "00" is repeated). This would also explain the 36 character line length. Due to the repeat of the first cycle, the last character of a line has an address of 38, the one after that (which is stored in the latch(!)) has 39, which is binary 0000100111. Of that the upper 8 bits are stored in the latch for the next line, and the lower two (in the D5 flipflops) are cleared (as they are always 0 at the start of the line for 40 chars/line). So after clearing the lowest two bits is 00100100 = 36 - the starting memory location for the next line!

So the summary is: look for a spike in the DIS.ON signal.


Unfortunately, I have NOT been able to find any spike on DIS.ON...most upsetting.

After looking at my scope images there was also some discussion as to the issue maybe being noise on the 5V line, this has not yet been fully ruled out but may not be likely as I have recently retested Vcc on various IC's using the corresponding GND (scope GND) from the same IC's. The noise seems much reduced when testing like this.

I have posted a bunch of new scope images on my page. Please take a look and if you have any thought we'd love to hear them as we are running low on ideas :-(

Phil

 

[dave_m]

Now I see that the 74100 and D6/D7 counters actually loads the _upper_ 8 bits of the 10 bits video memory address. The lower two bits are handled by the D5 dual-JK-flipflop and the following multiplexer at D2.

Phil,
I think I am following what you are saying. To me, the problem is related to the two LSBs of the address counter. Looking at the 1 MHz clock (1 clock per character in each scan line) to the D5 flip flop (D5-9), it is being loaded down to 2V rather than the normal 3-4V. I would start by replacing D5, the 74LS107 flip-flop.

EDIT: Still looking at that clock. Does the pulse on the bad board look a bit more skinny than on the good board? and a bit more glitchy? And of course the amplitude is lower. I wonder it if is the clock source flip-flop C8 pin 3 rather than loading?
-Dave

 

[Nama]

Hi Dave,
Let me re-test D5-9 on both machines and see if I can't get a better look at the clock pulse. Can you think of any other IC's and pins related to this this clock signal that I can compare between the two machines?

Thanks again

 

[dave_m]

Hi Dave,
Let me re-test D5-9 on both machines and see if I can't get a better look at the clock pulse. Can you think of any other IC's and pins related to this this clock signal that I can compare between the two machines?

Thanks again

If you have two probes, can you sync and display the positive edge of (horizontal) Display On at D5-10 (or D5-13)and and the 1Mhz character clock on D5-9?

I never realized until examining this schematic closely that the characters are output at a 1 MHz rate which means the dot clock is going at an 8 MHz rate (see the clock at the character ROM shift register). Not bad for an old Commodore machine.

 

[Nama]

I've just posted a picture of an enhanced image of the clock pulse. Please take a look. To me it looks like the broken boards pulse is reduced by 1/3, both in time and voltage.

So are you saying I should use D5-9 as a trigger for D5-10 (or D5-13)?

EDIT: Actually now I re read your message, I think you maybe saying just to show D5-9 and D5-10 (or D5-13) on the scope at the same time, Hopefully I'm correct. I'm a newbee when it comes to scopes.