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

From: Philip Lord <>
Date: Thu, 12 May 2011 17:15:52 +0900
Message-id: <>
Hi all,
I'm trying to get my second 2001 motherboard working.
Ethan recently sent back my two boards, one fully fixed and working, and one partial fix with lines running vertically down the screen, probably caused by a stuck bit somewhere.

The working board still works, but the one with the stuck bit is not looking good yet.

Now I must mention that I think I'm battling two issues here. This board is missing all of it's RAM (it does have the video RAM), and instead of tracking down more of the 6550's, my intent was to install Nicolas Welte's RAM/ROM board. I built this board up over a year ago now, but have only had a chance to try it out recently. Well, it doesn't work :-( 
I have confirmed its lack of operation of the RAM/ROM board in my working PET board. I have run through Nicolas's troubleshooting guide at the bottom of the page here:

…Basically I can get the computer working with all IC's removed from the RAM/ROM board (except the 6502 of course) and the jumper wire from pin 10 to pin 16 of the GAL socket. The next step in the guide suggests putting in the GAL and trying again…this is where it fails with a screen full of seemingly random characters (although the characters are basically the same on subsequent restarts). So the issue is probably a funky GAL or my soldering. I checked all the solder joints and connections with a multimeter and they seem good. 
Nicola's, if you are listening, I'd love to get a new GAL from you as I have no means to program one myself. I've emailed you previously about the issue, but I have had no reply yet. Please help. 
I am not sure if any of the other components (Flash chip and 74HCT157) have problems as I can not progress to the next step in the troubleshooting guide until the GAL issue is fixed. One thing I am uncertain of, and the guide mentions nothing of it, is at what point should I remove the PET's onboard RAM and ROM.

Now onto the non functional PET. As I mentioned, there is no RAM. The screen appears as a bunch of random blocks  (and they change position each time on restart). with vertical lines through them (probably the lines caused by the stuck bit that Ethan mentioned). Can someone tell me if these blocks are what you would expect without any RAM installed? Again, I'm not sure if I can make any progress until I get the RAM/ROM board up and working first.

I have put pictures here:

Any help or advice would be greatly appreciated.

Thanks again

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Received on 2011-05-12 09:00:03

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