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1200XL ram chips


whaledriver
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Hello all.

I received a 1200XL from a friend. He tried, unsuccessfully, to do several mods to it. I am trying to put it back to stock just to get it in working condition for now. One of the mods he tried was the 256K ram upgrade. The board has the MN41256A-08 ram chips installed. Since I don’t have the original ram chips, is there a way to just use the first 64K bank with these chips? I thought I read somewhere that pulling pin one low on each chip would allow this.
post-62318-0-18484100-1508963582_thumb.jpg

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I think pin 1 on each DRAM is tied down (or up, don't remember). The 256K chips should work if that is still true. Do you see any extra wires?

 

A picture?

 

Bob

According to the schematic in the 1200XL FSM R30 is a 3k pull-up resistor for pin 1 on the 1200XL RAM chips, the 256k upgrade would have needed to connect A8 instead.

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No extra wires. I attached a picture. Do yo not see it?

The picture only covers a small area of the board, making it impossible to see whether there is wiring or modifications in other areas.

 

Pull-up resistor R30 is just above RAM chip U5, since it's still there the 41256 chips should act the same as the original 64k chips. The connection can be confirmed by checking from the output of the voltage regulator in the back right corner to pin 1 of the RAM chips with an ohm meter with power off, the reading should be about 3k ohms.

 

Perhaps a previous owner installed the 41256 chips in preparation for a memory upgrade he never got around to installing, or they were all he had when replacing the original chips. It doesn't matter since they are compatible when pin 1 is tied to +5V or ground.

Edited by BillC
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The board is kind of a mess. I didn't power it on before I had to leave the country for work. I will be back in a week or so and will try to fix it then. I will attach a full picture of the board. Things I know for sure. He had tried to install the 800XL rom. I had the chips to go back to the original roms. I soldered a jumper on W6 and put the jumpers W9,W8 and W7 back. He tried to do the clearpic mod but I think it was done poorly. I see that C115 is missing a resistor from the clearpic 2002 mod. Also the most obvious error is the fact that the CPU socket has a PIA chip in it. :? I noticed that right before I left home. I have two C0148806C-29 chips which I believe are the Sally CPUs. I will put one of those in when I get home.

 

So if I am reading this right, I should be able to just leave the 256K ram chips like they are and use them as 64K without any modding. The data sheet shows that pin 1 is A8.

 

I am gonna have to figure out what to do about the clearpic mod. I am thinking about trying to convert it the the latest clearpic. Either that or get the UAV board from Bryan.

 

 

post-62318-0-66298600-1509054695_thumb.jpeg

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Also the most obvious error is the fact that the CPU socket has a PIA chip in it. :? I noticed that right before I left home. I have two C0148806C-29 chips which I believe are the Sally CPUs. I will put one of those in when I get home.

Maybe the prior owner though the PIA (which often has “6520” marked on it) was just another CPU in the 6502 family. But either way, there’s no problems with SALLY in 1200XL. I don’t recall if that’s stock for the system but I know I replaced a bad CPU in one of mine with a 6502 salvaged from a 600XL.

 

I am gonna have to figure out what to do about the clearpic mod. I am thinking about trying to convert it the the latest clearpic. Either that or get the UAV board from Bryan.

 

I have UAV boards in two of my 1200XL’s and one of my 800XL’s. Looks great and very easy to install. :)

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  • 1 month later...

Update:

 

Well I found the problem. I replaced the ram chips with the original ones and that didn't fix it. I removed the OS rom chips and found the high OS chip select had been cut underneath the chip (OS upgrade gone bad?). I ran a bodge wire from pin 15 of the MMU to the chip select pin 20 of the ROM and voila she works perfect!

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