exxos's Atari, Amiga & retro forum

I am trying to upgrade a 520 STFM with C103253 rev1 mother board.

Which Resistors do I need to lift and which end of them to connect to +5v to disable the onboard RAM?
The C103253Rev1 board is not covered in the photos & notes.

I anticipate that it is R59,R60,R61 (68R) for 1st 512kb and would also be R71, R72, R73 for the 2nd 512kb on 1 Mb version.

Is there a circuit diagram that has the correct part numbers available for this motherboard?

As you can see from the photos, it is fitted with 41256AT surface mount RAM chips, in bottom left hand corner to the right of the MMU and shifter blocks. (btw ROMs are under the internal PSU and are 2 x 128k x 8 Masked ROMs)

Are there any other aspects to look out for?

Thanks

Peter

Just lift the right side of those 4 68R resistors and tie to 5v somewhere. That's all you need to do.

You mention 4 Resistors which one is the 4th one?

Thanks

Peter

Here you go, someone with exactly the same motherboard with photos of the mod:
https://www.exxosforum.co.uk/forum/viewt ... =15&t=3959

microman wrote: 05 Feb 2022 16:40 You mention 4 Resistors which one is the 4th one?

Not four, It's only three.

Thanks I now have the board working.

However, the module over the MMU does not sit particularly deeply into the MMU socket.

If I remove the MMU then the board pins go in fine. If I have the MMU in the socket then the board is about 3 mm above the socket and although the pins are in to some degree, I am not sure how secure it is if I move the system.

Please advise.

Thanks

Peter

microman wrote: 05 Feb 2022 20:20 Thanks I now have the board working.

However, the module over the MMU does not sit particularly deeply into the MMU socket.

If I remove the MMU then the board pins go in fine. If I have the MMU in the socket then the board is about 3 mm above the socket and although the pins are in to some degree, I am not sure how secure it is if I move the system.

Please advise.

Thanks

Peter

About 1.5-2mm is about where it should be ideally, but if you are going to try and push it further in, then you need to put the board on a flat surface with something like anti-static rubber matting and removed from the bottom steel tray to prevent the board flexing as you push.

In all honesty if it works, you're probably best not to fiddle too much, provided it works fine and isn't unduly affected by moving the machine around.