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Commodore PET 2001-8 stuck on garbled screen

RetroGadgetMan said:
Some interesting parts on your board. The ram and video ram. I ordered the last two 6550 rams from little diodes today. expensive. Thought it would be a good idea for the video ram if these buffers don't change anything.
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Like https://www.tindie.com/products/nivagswedna/pet-video-ram-wrangler/ for example. I devised these whilst fixing my PET a few years back. I'm trying to take a break from supplying these so I can catch up on my arcade restoration projects... but I'm probably negotiable (via private message). Little Diode is a good source of 6550, the price just goes up over time!
FYI 6550s fail in a variety of ways... in particular their chip select logic might die but otherwise they work... failures of this type may actually work for the video RAM.
 
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Nivag Swerdna said:
Like https://www.tindie.com/products/nivagswedna/pet-video-ram-wrangler/ for example. I devised these whilst fixing my PET a few years back. I'm trying to take a break from supplying these so I can catch up on my arcade restoration projects... but I'm probably negotiable (via private message). Little Diode is a good source of 6550, the price just goes up over time!
FYI 6550s fail in a variety of ways... in particular their chip select logic might die but otherwise they work... failures of this type may actually work for the video RAM.
Click to expand...
That's very interesting about the 6550s.
 
Speaking generically here, not related to any particular seller:

Imagine an episode of Lost in Space, with Dr. Smith up to something (because he would sell the Robinson family out for a penny, if it was in his own best interest) and the Robot waving his arms, saying warning warning, danger Will Robinson.

If you check, you will find that there are certain internet/ebay sellers that do not offer a picture of the items they are selling, or a "generic photo". This is because they do not have the actual parts on hand. It is not just that they have "not got around to posting the picture of it" and say the image is coming soon. They wait until an order is received and then they send that out to their "procurement department" to try to find a part (or perhaps an ill considered re-labelled equivalent of it, as I have been the victim of) to supply for the customer's order.

My advice is, if you are buying vintage semiconductors & IC's, of any kind, that you buy from an actual stocking seller and more importantly, that what they send you, is from their own stock/warehouse and the photo is representative of what they send. To avoid fakes/copies, it is often necessary to study the auction photos to confirm that it is new old sock. As we probably both know, it has been a very long time since IC's like the 6550 have been manufactured from new. And if they were, you would not want one.

After a while, you will learn who the actual stocking sellers are and come to trust them for genuine vintage parts.
 
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Hugo Holden said:
Speaking generically here, not related to any particular seller:

Imagine an episode of Lost in Space, with Dr. Smith up to something (because he would sell the Robinson family out for a penny, if it was in his own best interest) and the Robot waving his arms, saying warning warning, danger Will Robinson.

If you check, you will find that there are certain internet/ebay sellers that do not offer a picture of the items they are selling, or a "generic photo". This is because they do not have the actual parts on hand. It is not just that they have "not got around to posting the picture of it" and say the image is coming soon. They wait until an order is received and then they send that out to their "procurement department" to try to find a part (or perhaps an ill considered re-labelled equivalent of it, as I have been the victim of) to supply for the customer's order.

My advice is, if you are buying vintage semiconductors & IC's, of any kind, that you buy from an actual stocking seller and more importantly, that what they send you, is from their own stock/warehouse and the photo is representative of what they send. To avoid fakes/copies, it is often necessary to study the auction photos to confirm that it is new old sock. As we probably both know, it has been a very long time since IC's like the 6550 have been manufactured from new. And if they were, you would not want one.

After a while, you will learn who the actual stocking sellers are and come to trust them for genuine vintage parts.
Hugo Holden said:
Speaking generically here, not related to any particular seller:

Imagine an episode of Lost in Space, with Dr. Smith up to something (because he would sell the Robinson family out for a penny, if it was in his own best interest) and the Robot waving his arms, saying warning warning, danger Will Robinson.

If you check, you will find that there are certain internet/ebay sellers that do not offer a picture of the items they are selling, or a "generic photo". This is because they do not have the actual parts on hand. It is not just that they have "not got around to posting the picture of it" and say the image is coming soon. They wait until an order is received and then they send that out to their "procurement department" to try to find a part (or perhaps an ill considered re-labelled equivalent of it, as I have been the victim of) to supply for the customer's order.

My advice is, if you are buying vintage semiconductors & IC's, of any kind, that you buy from an actual stocking seller and more importantly, that what they send you, is from their own stock/warehouse and the photo is representative of what they send. To avoid fakes/copies, it is often necessary to study the auction photos to confirm that it is new old sock. As we probably both know, it has been a very long time since IC's like the 6550 have been manufactured from new. And if they were, you would not want one.

After a while, you will learn who the actual stocking sellers are and come to trust them for genuine vintage parts.
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Ah yes Dr Smith, oh the pain..the pain...much what I am feeling with this Pet. Seriously though, Very good advice. I have actually been watching this listing for a very long time from little diodes. New old stock from 1981. Adrian from Adrian's digital basement bought some from them when they were just £4.99ea 😯 and Dave from Daves workshop has allso acquired some so I know they will work...for now. Of course they are very old and can and will fail at any time. 🙁
 
Nivag Swerdna said:
Keep going RGM.... You cannot be far away from having something working soon....
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Ok so I have replaced B3 and there was no change but when I changed B4 I now get this image using the new substitute rams. Pic1 More characters and stable. Before it was just white blocks with new ram. Pic 2 shows what is installed ic wise to get this image.
I also now have that Dmm with HZ feature.
 

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I don't know if you have seen it, but we have another 'interesting' thread where the Commodore PET character generator ROM works fine. But, when it is replaced by an EPROM, all manner if 'weird stuff' happens to the video display.

We haven't got to the bottom of it yet, but I then thought of the potential similarity to what I am reading here...

Dave
 
daver2 said:
I don't know if you have seen it, but we have another 'interesting' thread where the Commodore PET character generator ROM works fine. But, when it is replaced by an EPROM, all manner if 'weird stuff' happens to the video display.

We haven't got to the bottom of it yet, but I then thought of the potential similarity to what I am reading here...

Dave
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Just had a read. At this moment in time I would be happy with those results 😆
 
I would continue with removing and socketing the buffers. This will help us a lot with investigation. Unfortunately for this to work reliably we really need to socket all six.

For my amusement can you put the original character ROM and tell me what you see.

I would remove H8 and H9 and leave the new sockets unpopulated.... and the remove, socket and replace G5/G6.
 
I've just read the first page of this thread again. A couple of observations:

1. The PET didn't display an image at all using the original character ROM.

2. Things seemed to go seriously awry when the 2114s were first used in the video RAM positions.

Alan
 
Nivag Swerdna said:
I would continue with removing and socketing the buffers. This will help us a lot with investigation. Unfortunately for this to work reliably we really need to socket all six.

For my amusement can you put the original character ROM and tell me what you see.

I would remove H8 and H9 and leave the new sockets unpopulated.... and the remove, socket and replace G5/G6.
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Already ordered 🙂
 
ajgriff said:
I've just read the first page of this thread again. A couple of observations:

1. The PET didn't display an image at all using the original character ROM.

2. Things seemed to go seriously awry when the 2114s were first used in the video RAM positions.

Alan
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I think it was just coincidence. These new ram chips have been tested and confirmed working in the same model pet by the seller.
 
Forgive me for saying so, but it was to the seller's advantage to tell you that. Do you have any independent method of testing the 2114s?

If you have an Arduino in your tech toy box, here is an Arduino project which tests 2114s. I don't know how comprehensive the tests are but it will certainly give a dead / not dead indication.

github.com

GitHub - Danjovic/2114-Tester: Inexpensive Arduino based tool for testing 2114s

Inexpensive Arduino based tool for testing 2114s. Contribute to Danjovic/2114-Tester development by creating an account on GitHub.
github.com github.com
 
SiriusHardware said:
Forgive me for saying so, but it was to the seller's advantage to tell you that. Do you have any independent method of testing the 2114s?

If you have an Arduino in your tech toy box, here is an Arduino project which tests 2114s. I don't know how comprehensive the tests are but it will certainly give a dead / not dead indication.

github.com

GitHub - Danjovic/2114-Tester: Inexpensive Arduino based tool for testing 2114s

Inexpensive Arduino based tool for testing 2114s. Contribute to Danjovic/2114-Tester development by creating an account on GitHub.
github.com github.com
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I don't no. I have not used Arduino yet. They sound interesting.
 
Hello...just been thru battling these 2114 RAMs in my pet, and being a Commodore C64 enthusiast, here is how I tested mine.
Unsoldered the 2114 in my C64 and installed a socket. When 2114 is reinstalled, the boot screen should come up normal and readable characters.
Now, I unsoldered those 4 - 2114s from my PET and tested them on my C64...found a faulty one where some of the characters in C64 boot display are missing.
If you have a C64...this might help...
 
RizThomas said:
Hello...just been thru battling these 2114 RAMs in my pet, and being a Commodore C64 enthusiast, here is how I tested mine.
Unsoldered the 2114 in my C64 and installed a socket. When 2114 is reinstalled, the boot screen should come up normal and readable characters.
Now, I unsoldered those 4 - 2114s from my PET and tested them on my C64...found a faulty one where some of the characters in C64 boot display are missing.
If you have a C64...this might help...
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Thanks for the tip. Which one did you unsolder in the c64 to test
 
RetroGadgetMan said:
Oh I didn't realise that. I have the bread in and the last model they did.
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Probably not helpful but.... testing 2114s is something I tripped myself up on (you shouldn't have a problem in a C64)... 2114 really do use TTL levels so you can get very spurious results if you use something like an Arduino or any modern CMOS uP for that matter since the levels don't quite line up. Always read the small print of your chip tester! I almost junked a pile of 2114s thinking they were fake/broken before realising my mistake.
 
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