Exxos 4MB Ram Upgrade for C070789 strange issue

[kbjunkie]

Hi all,

I purchased Exxos's 4MB upgrade board for my C070789 STFM which already had the 1MB upgrade applied (4 banks of 8 DRAM chips).

I removed all the DRAM chips and checked the PCB thoroughly for any broken traces or other issues (solder shorts etc.) and all was good under my scope. I then followed the instructions and fitted the 4MB board. I also connected the MMU wire as instructed.

If I leave the 1/4MB switch open and power up the machine, I get 1MB RAM just fine. If I short out the jumper with a bit of wire, I get 4MB just fine.

I then fitted a toggle switch (3 pin toggle switch with the 2 wires connected to one side of the switch and the centre pin) across the 1MB/4MB pads and select 4MB (switch 'ON' position) I get 4MB no problem. When I switch off the STFM and switch to the 'OFF' position, I get a black screen with a white border.

It gets even stranger though.. If I desolder the switch side and just leave the 2 wires connected to the 4MB board, with the wires twisted together, I get 4MB fine, but if I separate them I get the same black screen/white border as with the switch connected.

Is the wire picking up some interference maybe? I'm a bit confused by this..

The board has the appropriate 68R address line resistors fitted.

Many thanks,

KbJ

[dgw]

I just did a mod to my Atari (070789) using 256K Eproms (27C2001) to switch between TOS 1.2 and 1.4 by using switch on highest address line. To avoid having long lead off a CMOS input I made use of one the spare gates in U68 (74LS11) to buffer the long lead to switch on back of case. LS inputs being more suitable.

[exxos]

The switch just connects A9 to the RAM which gives you 4MB. With A9 disconnected from the MMU, it will only see 1MB. BUT the RAM side of A9 gets pulled high by the 10k resistor to stop the RAM address line floating.

It's odd a link works but not a wire to a switch. Twisting wires and it having that effect suggests on of the wires is floating. Maybe check the right side of the link wire and make sure it measures 10k to 0v. With the power off of course. If there is a problem with the 10k pullup then 1MB would be unstable.

[kbjunkie]

The switch just connects A9 to the RAM which gives you 4MB. With A9 disconnected from the MMU, it will only see 1MB. BUT the RAM side of A9 gets pulled high by the 10k resistor to stop the RAM address line floating.

It's odd a link works but not a wire to a switch. Twisting wires and it having that effect suggests on of the wires is floating. Maybe check the right side of the link wire and make sure it measures 10k to 0v. With the power off of course. If there is a problem with the 10k pullup then 1MB would be unstable.

Just checked the resistance to ground and it's 100K not 10K - pic of the resistor confirms the wrong value (1003)

4MBRes.jpg (219.97 KiB) Viewed 5211 times

I take it that this resistor needs to be changed to 10K and that this will be the reason the 1MB switch isn't working?

Thanks,

KbJ

[exxos]

I take it that this resistor needs to be changed to 10K and that this will be the reason the 1MB switch isn't working?

100K is probably fine. Its only a pull up. You could try tagging a 10K across it, but its only a guess as to what your problem is. I can't really think of anything else. A lot of those boards have sold and nobody else has reported any issues. Your probably could be anything, most likely a bad connection somewhere.

[kbjunkie]

I take it that this resistor needs to be changed to 10K and that this will be the reason the 1MB switch isn't working?

100K is probably fine. Its only a pull up. You could try tagging a 10K across it, but its only a guess as to what your problem is. I can't really think of anything else. A lot of those boards have sold and nobody else has reported any issues. Your probably could be anything, most likely a bad connection somewhere.

I just replaced it with a 10K SMD resistor - now measuring 4.85K to ground and getting a corrupt screen with bombs when 1MB is selected with a switch.

What I can't understand is if I have a bad connection somewhere, why does the board work fine with a jumper in both 1MB and 4MB but not when connected to a switch though? If It's a bad connection, then surely I would get the same whether using a switch or a link..

I'll go check all of the connections again with a meter and microscope.

KbJ

[exxos]

Have you ran a memory test on 4MB ?

Also if you got the switch to hot during soldering it could have bad contacts.

[kbjunkie]

Have you ran a memory test on 4MB ?

Also if you got the switch to hot during soldering it could have bad contacts.

I'll go run the tests again, but they both passed without using a switch. I've also tried a couple of different toggle switches - one where I soldered the connections on and the other I used spade connectors.

Not sure where to go here apart from checking every trace once again..

KbJ

[exxos]

Is it worth even having a switch ? Just leave it on 4MB ? A lot of games are patched to run from hard drive and 4MB machine anyway. So not sure 1MB is worth the trouble anymore.

[kbjunkie]

Is it worth even having a switch ? Just leave it on 4MB ? A lot of games are patched to run from hard drive and 4MB machine anyway. So not sure 1MB is worth the trouble anymore.

I just wanted max compatibility really - and the board has the option to fit the switch so it should really work.. I'd like to get to the bottom of why its not working..

KbJ