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Repairing pillaged PET 2001-8

bitfixer

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Apr 6, 2011
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San Francisco, CA
Hi all,
I finally got around to attempting to fix a 2001-8 that I got years ago. This one has had a bit of a rough ride, the case is very rusted and the board itself had all of its ROMs pillaged. The CRT is also disconnected, it's in storage somewhere and need to dig that out.
Using a ROMulator with a BASIC 1 rom set, I attempted a startup. The character ROM as well is missing, which is not replaced by the ROMulator, so i halted the CPU and dumped the shadowed Video ram to see what the CPU had written there. The full video ram space was filled with 0x20 (space) characters. Normally this would contain the "Commodore basic" message and ready prompt.
I confirmed that the PETTEST rom and my own ram/rom test do show the appropriate contents in video ram during execution.
It looks to me that a portion of the startup sequence completes, including clearing the screen. But another condition is not met which prevents it from getting to the basic prompt. Any guesses as to why the startup could be stuck after clearing the screen? Thanks
 
Assuming page 0 and page 1 RAM are OK, the next culprit would probably be the /IRQ pin of the CPU being stuck low.

How far have you run my PETTESTER?

You can replace the character generator with a conventional EPROM if memory serves me correctly.

Dave
 
Thanks Dave,
I had not run it for very long, in this case just long enough to confirm that something was running and the CPU was writing to video memory.
Well, I tried swapping the 6520s and 6522 with a known working PET, and with those swapped in from the working pet, the 2001-8 did write the commodore basic message to video memory.
After checking which combination of chips caused that to happen, as far as I can tell, both 6520s in the patient PET are bad. If I include either or both, the basic message will not come up. 6522 does not have an impact. I'm not deeply familiar with the PET startup sequence but I imagine some interaction with the 6520s doesn't work correctly and prevents basic from starting.
Next, writing a character generator ROM replacement.
 
There are adapters for the 6540 character ROM. See Nicolas Welte's design. Forum member dabone built several assemblies specifically to replace the character ROM. I bought one from him. Perhaps he has some extra ones for sale. There may be other forum members with similar adapters.
-dave_m
 
Hey Bitfixer, I'll ship you a set of ram/rom replacement boards for the 2001-8 video section tomorrow.

I've already got some made up after I made one for myself.
 
Thanks very much! Appreciate the help. Now just have to write the EPROM.
I didn't mention before, but this is a rev C board which uses 2114s for both video and main ram. As far as I can tell the video ram is ok, or at least it passes the ram test I ran on it.
Any recommendations for a reasonably cheap EEPROM programmer? Last time I needed to write an EEPROM, I used a breadboard circuit and little C program to write it with a Raspberry Pi, which was fun, but would be nice to have a dedicated one. If it can test logic chips too that would be pretty nice.
 
We assumed you had an old board with 6540 ROM and 6550 RAM and needed adapters for RAM and ROM. What kind of ROM do you have for your character generator in the A2 socket? The old 6540 (28 pin) or the 2316 (24 pin) package? If the 24 pin package, you can get by with a 2716 EPROM or a 2816 EEPROM with no adapter. I could send you a programmed 2816.
 
I have a https://www.tindie.com/products/nivagswedna/romulan-omni-6540-rom-replacement/ that works well as a video ROM replacement; it's not the cheapest solution (the EEPROM it uses is relatively expensive)... I could do you a deal :eek:)

I'm also working on a cheaper variant that uses a cheaper flash.... you could try one of those but I haven't assembled it yet so would take a few days to get together.

The adapters and an old UV EPROM are the cheapest solution but IMHO aesthetically less pleasing.

PM me if interested
 
Any recommendations for a reasonably cheap EEPROM programmer? Last time I needed to write an EEPROM, I used a breadboard circuit and little C program to write it with a Raspberry Pi, which was fun, but would be nice to have a dedicated one. If it can test logic chips too that would be pretty nice.


For cheap and some IC testing, the TL866II Plus Programmer works for most people.
I use my sometimes, but I also have a gq-4x that I've had for ages and I'm much more comfortable with the software.

For writing old eproms, (Like 2532 and other ancient things) I've got a Husky multigang programmer on my Tandy 1000sx. This is handy for newer pets.
 
Thanks dabone for the adapters! Thanks to that, and using a ROMulator to replace the missing 6540 ROMs, the 2001-8 lives again. The only fault that was preventing it from booting earlier was a bad 6520. I luckily had a spare. I'll post a picture here next time I'm near it.
Next up when time allows will be to try some form of restoration on the case, it's pretty rusty and would probably need to be repainted/power coated. Debating whether to try to go for an original look or something more out there.
 
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