Re: PET 2001 fix Part 3 - RAM/ROM board etc.

From: Philip Lord <>
Date: Mon, 16 May 2011 18:38:02 +0900
Message-id: <>
I received an email from Nicolas Welte. His diagnosis of my RAM/ROM board problem was spot on. Although I had a 10k resistor pack instead of a 4.7k, he thought that it shouldn't cause any problems, but instead he asked me to test the orientation of the pack. I was unaware that they were directional, and true to form, I had installed in backwards. A quick zip with the desolder station and reinserted it the correct way proved that he was indeed right. The board fired up perfectly the first time.

The RAM/ROM board is now installed in the bad PET motherboard and there is much improvement overall.

Basic 1 boots, Basic 2 boots, and Basic 3 is just weird characters, but I'm not sure if Basic 3 actually works in a PET 2001 anyway. Characters are shifted, which is consistent to the repeating characters we were seeing previously.

I'll hook up a keyboard and reinstall the 6520's to see what happens when I actually start typing something.

Anyway…I can now see the light at the end of the tunnel.


On May 16, 2011, at 5:00 PM, Philip Lord wrote:

> Hi,
>> - can you program in ASM and produce BINs?
> Unfortunately I have no skill at all in 6502 (or any other kind) assembler :-(
>> - you can program EPROMs, can you program EEPROMs as well?
> I have an Needhams EMP-20 programmer and a stack of 2708, 2716, 2732, 2764, 27256, 27512 and 2532's (maybe about 8 of each type)
>> - if yes, do you have EEPROMs laying around?
> I might have a 59V65A (28C64 equivalent) somewhere, but I couldn't get it working for what I bought it for (ZX1541). I ended up resorting to a real 28C64 and it it might be dead!
>> - if yes/no or no/no, do you have enough EPROMs laying around?
> I have plenty of EPROMs
>> The reason for these question, we know my little program works fine so
>> the next thing you should do IMHO is to step things up. 
>> You could start with a little program that checks various things like
>> the RAM (at least Zeropage + stack) and the hardware (6522, 6520, etc.).
>> Another thing I'm curious about is what happens around the last and
>> first columns. First only fill the first page with 0..$FF. Then check
>> the columns by copying the addresses $8000+$4C to $54 to somewhere in
>> the next page in the middle of the screen, for example from $8128 on. If
>> you see 'lmnopqrst' then this would mean that the video RAM is written
>> with the correct valus and the display circuit is to blame. But if you
>> see 'ghijhhijk' again then would mean IMHO that the filling of the video
>> RAM already went wrong.
>> You see, enough work to do :)
> Sure is...but unfortunately none of the above means much to me as I have no assembler skills at all...I can understand basically what you are describing, but the technical aspect just sounds like martian :-o
> Now I mentioned before that I didn't have a scope...actually I was lying. I do have an old Kikusui 20mhz COS 5020 that was given to me. I have never used a scope before or know how to use one exactly (I understand the basic principle of course). So I've just pulled it out of it's box incase it comes in useful. I have just tested it for a square wave, and It seems to work, but the beam tends to wander from time to time, but should be ok.
> Thanks again.
> Phil
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Received on 2011-05-16 10:00:08

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