exxos's Atari, Amiga & retro forum

Hi Folks!!

I've seen a few examples of other people trying to upgrade the C103175 Rev.1 motherboard and doing super clever upgrades beyond 1MB.

I'd just like to upgrade my 520 spec to a 1040.

Can anyone who knows please advise on what is required to correctly upgrade this spec board to 1MB as I haven't found it elsewhere (I only want 1mb as this is enough to run most music software, without crazy fixes required to get higher MB counts than this).

I've attached a picture of the board I have in its current state. SYSINFO confirms it's at 512KB RAM.

I understand principally that I would need to solder another 512KB RAM into the unoccupied slots U10, U11, U12, U13.

I have already ordered Exxos's RAM kit for the 103175 board and the capacitor set.

I believe looking at other posts that I also need to solder some capacitors into C25, C26, C27, C28 and resistors into locations R71, R72, R73?

If this is correct, assuming the Exxos capacitor set is correct, do we know what the value capacitors and resistors that should be used please? (It may be useful to others to confirm the value of capacitors.)

Are there any other modifications, jumper wires, etc - more steps that need to be taken for it to work?
Many thanks for taking a look at this!!
mm

So aside from the RAM here: C103175 RAM you'll need these: Axial capacitors

The caps fit at C25, C26, C27, C28 as you've suggested. I would advise against soldering the RAM in directly and instead fit sockets as it's helpful if you have issues in the long run. You might need low profile sockets as it's under the PSU, but I think you'll be OK.

As for resistors, my best suggestion would be to find the schematic for the 1040 version of that board and see what's in use, however I do know that similar upgrades for other machines use 68R resistors. Schematic is the best place to check though.

The store says they are 100nF caps BTW, probably marked as 104.

Just wanted to say thank you for this response!! I've been trying to act on it, and find a schematic for this board but failing. I think I might just have to take a gamble and put 68 ohm resistors, which are what are in the adjacent R59, R60, R61. I'll write up the solution once I find it to help anyone else in future.

Hello all!
I have the same mainboard from an Atari 520 STf and it already has capacitors and resistors in the right places so I think that I can upgrade the RAM from 512 KB to 1MB adding only 4 x 20 pins DRAM modules. I already soldered the sockets for them but I can't find the same type of RAM (MT 4C4256-10) anywhere. Actually there's one guy on eBay that sells them for about 40 euros plus shipping but, as you guess, I can't spend 160 euros for 512 KB of RAM... :D

Would these be compatible?

https://www.ebay.it/itm/165375573844
https://www.ebay.com/itm/323861343006

Also... why those chips seem to be of 256 KB each but the total RAM I have now is just 512 KB instead of 1MB? Sorry for the "noobie" question... :)

jurassicman wrote: 04 Oct 2025 13:57 Hello all!
I have the same mainboard from an Atari 520 STf and it already has capacitors and resistors in the right places so I think that I can upgrade the RAM from 512 KB to 1MB adding only 4 x 20 pins DRAM modules. I already soldered the sockets for them but I can't find the same type of RAM (MT 4C4256-10) anywhere. Actually there's one guy on eBay that sells them for about 40 euros plus shipping but, as you guess, I can't spend 160 euros for 512 KB of RAM... :D

Would these be compatible?

https://www.ebay.it/itm/165375573844
https://www.ebay.com/itm/323861343006

Simplest thing to do is to download the datasheets and compare the specs to see if they match.

jurassicman wrote: 04 Oct 2025 13:57 Also... why those chips seem to be of 256 KB each but the total RAM I have now is just 512 KB instead of 1MB? Sorry for the "noobie" question... :)

They are 256KB x 4 bits, so 2 IC's makes 256KB x 8bits and 4 IC's makes double that, therefore 256KB x 16 bits.

rubber_jonnie wrote: 04 Oct 2025 15:17 They are 256KB x 4 bits, so 2 IC's makes 256KB x 8bits and 4 IC's makes double that, therefore 256KB x 8 bits.

256KB x 16bits?

JezC wrote: 04 Oct 2025 15:21

rubber_jonnie wrote: 04 Oct 2025 15:17 They are 256KB x 4 bits, so 2 IC's makes 256KB x 8bits and 4 IC's makes double that, therefore 256KB x 8 bits.

256KB x 16bits?

Corrected, must pay better attention to what I write...

jurassicman wrote: 04 Oct 2025 13:57 Would these be compatible?

https://www.ebay.it/itm/165375573844
https://www.ebay.com/itm/323861343006

Yes, they will work. Exakt pinout as the original.

Thank @all guys for your prompt replies and above all thanks DoG for having checked the pin layout! :)
I'm going to buy 4 of those and I'll let you know how it goes!

This listing seems the more convenient...
https://www.ebay.com/itm/174489801777

The other says "M5M44256BP-7 DIP20 IC" but in the image there's a "-10" IC... I think it is better to be sure that I get the right one...