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Frank's Danish PET

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This picture is "startup" screen with a new 2124 from a C64, placed at UF7.



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This is the screen after switching power OFF and ON.



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The new UF7.
Could UF8 be defect too?
 
frank_DK said:
Could UF8 be defect too?
Click to expand...

Looking at the extraneous characters, I see errors in both the upper 4 bits (UF7) and in the lower 4 bits (UF8 ). So both RAMs may still have a few bad cells or there may be some bad solder joints or perhaps there is a problem in the parts that perform the video RAM read/write function.

As a test, when you are at the last screen shown, on the keyboard type: poke 32777, 100 and see if the '@' turns into a 'd' on the word 'Commodore'.
 
dave_m said:
As a test, when you are at the last screen shown, on the keyboard type: poke 32777, 100 and see if the '@' turns into a 'd' on the word 'Commodore'.
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Boy, this PET ain't gonna give up easily, is it...

You may be right that there's actually garbage being written into (now) good screen RAM.

But I didn't look closely enough at the step 5 picture; maybe it is just a bad UF8.

Frank, please also try removing the (old?) UF8 and compare that to the screen from step 5.
 
MikeS said:
Boy, this PET ain't gonna give up easily, is it...

You may be right that there's actually garbage being written into (now) good screen RAM.
Click to expand...

It is interesting to note that at initialization, the screen is cleared by sending spaces ($20) to all the video RAM clearing them with a routine one screen line at a time and when the first text

*** Commodore Basic 4 ***

is sent to the screen, the routine sends two carriage-returns before showing the number of bytes free. So that tells me that the initial screen clearing left the garbage. Very puzzling.
 
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I have changed the UF8 too, also from an old C64, and now the "startup" screen looks nearly ok



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I wrote this, and then look...

 
dave_m said:
Alex,
The difference between an ASCII "/" and an "o" in the first photo indicates a possible problem on SD06 or LSD06 signals or another bad UF7 RAM chip.
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Yeah, considering how notoriously unreliable the 2114s turned out to be, I'd try another replacement if you can find one.

Meanwhile, try swapping them to see if the symptoms change; if they're both good then it should look the same.
 
Thanks for all the good advices. I changed the UF6 and UF7 (VRAMS), and then the "startup" screen is now OK but there still come some garbage if I write long texts.
 
frank_DK said:
... then the "startup" screen is now OK but there still come some garbage if I write long texts.
Click to expand...

Frank,
That is good news. You may not notice the problem in regular usage. If the RAM is good, there is no fixing the residual problem without a scope or super good luck. If that was my system, I would like to first put a scope probe on the "TV RAM R/W" signal (Sheet 7, UF6 pin 7) and look for a glitch or distorted signal. If that was clean, I'd look at the video RAM data paths.
 
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If you're going to buy a 6520 anyway to replace that defective one, why not get another pair of 2114s while you're at it, just in case; get a pair from the same manufacturer, preferably with a speed of 200ns or less.
 
So we're up and running 100%. Bought two 2114 VRAM and a 6520th It is a joy to have the old machine up and running. Are there some here who have programs to it?
 
frank_DK said:
So we're up and running 100%. Bought two 2114 VRAM and a 6520th It is a joy to have the old machine up and running. Are there some here who have programs to it?
Click to expand...
Excellent! Congratulations, glad we could help, and I'll probably lose some weight now that this thread is closing; every time I read the title in New Posts I had to go and get a couple of cookies (mmm... Danish...) ;-)

There are hundreds of PET programs around; the trick is getting them transferred and loaded.
 
So to summarize, this PET required replacing 1 ROM, a couple RAM and a 6520?

Maybe we should start keeping a "bill of materials" for every salvaged PET to get a figure how much work there can be in the least, average and extreme cases. Ideally also grouped by model and motherboard revision.
 
carlsson said:
So to summarize, this PET required replacing 1 ROM, a couple RAM and a 6520?

Maybe we should start keeping a "bill of materials" for every salvaged PET to get a figure how much work there can be in the least, average and extreme cases. Ideally also grouped by model and motherboard revision.
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... And the number of posts that it took ;-)
Not a bad idea actually, a database of problems, clues and solutions.

What is the easiest way to get programs from the 'net into a PET?
 
carlsson said:
Maybe we should start keeping a "bill of materials" for every salvaged PET to get a figure how much work there can be in the least, average and extreme cases. Ideally also grouped by model and motherboard revision.
Click to expand...

I like this idea.

I know the Commodore forum here has been very heavy with PET stuff recently, maybe it's time to create a new "How To Troubleshoot And Repair Your PET" web site that we can just point people at in the future.

I'll start to write up what I've done for my PETs this weekend, when I'm (hopefully) not quite so swamped with work (I worked until 1 AM last night... er, this morning? Whatever. And I'm back at it now, just taking a break while my unit tests run!)

I'm going to try to help with Eudimorphodon‎'s PETs as soon as we're both free, too. That'll give another opportunity to write up procedures and debugging notes.
 
Twylo said:
I know the Commodore forum here has been very heavy with PET stuff recently, maybe it's time to create a new "How To Troubleshoot And Repair Your PET" web site that we can just point people at in the future.
Click to expand...

Yeah, a one-stop shop, so to speak, with all the links, documentation downloads, etc, all gathered in one place would be super-useful.

I'm going to try to help with Eudimorphodon‎'s PETs as soon as we're both free, too. That'll give another opportunity to write up procedures and debugging notes.
Click to expand...

Heh, if you could come over right now... ;^) (I'm stuck home from work today babysitting some contractors, so I've been hiding out in the garage poking at my PET like an ape, as I'll be updating my thread to reflect.) Given how useful I am technically I'll at least try to take some notes while someone smart is at work.
 
I want to say thanks for the super professionalism there are among the users in this forum. Without this great help I might never have got my PET up and running.
I find the idea of a special PET guide a very good idea.
Maybe I can help with some photos of the scope images.

Thanks a lot!
 
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