520SFTM 1MB upgrade fail C07085 9-001 Rev 2.0

[NikiD]

Last month I had a fully working Atari ST 520STfm (Good times!).After a few mods I was feeling confident and decided to order 16 DRAM chips plus the extra resistors required from the exxos store. HUZZAH! Finally a 1040 after all these years.

So, I installed the chips and I no longer have a working Atari 520 STfm. Now, I believe I can say with 99% accuracy that I broke it. The problem is after finding the root cause of the issue I still do not know how to fix it.

I have the Atari ST plugged into an OSSC and the OSSC just scans and scans but the little ST that could doesn't give it a signal.

Can someone advise me on the best next course of action? I took the mult-I-Meter out and check for continuity down the lines and two lines on both sets of new chips don't beep. It is the same two lines. Could this be the issue?

Can I just rip out the new chips and go back to 512k? Is there some kind of blueprint I can use to check connections with the multi meter? The board is C070859-001 REV 2.0 if that helps.

[DoG]

Depends where you are beeping and checking for fault of course. Some pins on the chips doesn't connect to other stuff other then a unique point. For example pin 2.

Otherwise you can deactivate your upper ram by lifting/desolder R71 and R72 on the end that is closest to the MMU chip. Then solder a wire to these lifted resistors and connect/solder to 5V somewhere. You don't need to desolder all the ram chips.

Deactivate RAM on STfm.png (553.81 KiB) Viewed 102 times

[DoG]

Pin 2 on each chip can be checked that is connected to D0-D15 on the SHIFTER chip (the big chip in the metal can in the middle of the motherboard. Pin 3-Pin19. Not pin 11).

Shifter.png (169.44 KiB) Viewed 99 times

How it is connected.png (788.57 KiB) Viewed 99 times

[DoG]

Here you have the resistors and how they are connected from MMU and to RAS, CASL and CASH lines.

MMU and resistors.png (415.6 KiB) Viewed 95 times

[rubber_jonnie]

I've run into a similar issue with one of these boards and I suspect that it was good for 512kb but not 1MB when tested. When upgrading these always use sockets so you can just pull out the RAM if it doesn't work.

I'd buzz out all connections and then you can map anything not connected and bodge wire on the underside to fix.

[NikiD]

Here you have the resistors and how they are connected from MMU and to RAS, CASL and CASH lines.

MMU and resistors.png

This is all really useful information. Thank you so much!

[NikiD]

I've run into a similar issue with one of these boards and I suspect that it was good for 512kb but not 1MB when tested. When upgrading these always use sockets so you can just pull out the RAM if it doesn't work.

I'd buzz out all connections and then you can map anything not connected and bodge wire on the underside to fix.

Yes, I honestly don't know why I didn't use sockets. I guess I wanted that factory look but now I might end up with a spaghetti junction of cheat wires that idea seems foolish

[rubber_jonnie]

I've run into a similar issue with one of these boards and I suspect that it was good for 512kb but not 1MB when tested. When upgrading these always use sockets so you can just pull out the RAM if it doesn't work.

I'd buzz out all connections and then you can map anything not connected and bodge wire on the underside to fix.

Yes, I honestly don't know why I didn't use sockets. I guess I wanted that factory look but now I might end up with a spaghetti junction of cheat wires that idea seems foolish

Make sure you use the schematic to understand which lines go where as just flying by the seat of your pants can leave you scratching your head.