another round of "Name that Mod!"

[jacobus]

Can anyone ID this mod?

 

I'm assuming it's a homebrew 48K upgrade for the 400 (the jumpers are soldered on the motherboard), can anyone confirm?

 

 

thanks!

 

Bonus question - if I toss an unmodified 16K card back in the 400, do I need to remove the jumpers?

Edited March 9, 2013 by jacobus

[Rybags]

There's 64K onboard there actually. But accessing the extra Ram would rely on the logic being present to switch among 4 banks to appear @ $C000.

 

Given the complexity of that loom, I'd say that's a real possibility. Fairly sure the mods needed to allow smaller Ram boards to go in a 400 are much less complex than that one.

 

IIRC at least one bankswitching scheme uses $CFFF to select the active bank.

 

To put an unmodded board back in, unsure. There's the chance a motherboard modded to allow >16K might get repeat images if a 16K only board is present.

 

To check for that, ? FRE(0) - if on a 16K system you get more than expected then that would be the case.

Edited March 9, 2013 by Rybags

[BillC]

Most likely just a homebrew version of the Atari 400 48K upgrade board, which actually has 64K but only supports 48K. The official Atari 48K upgrade board also requires jumpers to allow the 400 to access >32K.

http://www.best-elec...m/800.htm#48K 2

 

There was also the Mosaic 64K Select RAM upgrade which has 64K, and allows access to the extra 16K in 4-4K banks, but I doubt there were many homebrew versions of it.

http://www.atariarch...4K_RAM_Card.php

 

For the 800 there are also homebrews to upgrade an original 16K board to 256K, there is some dispute about which was first.

These are the Byrd and Windhover upgrades.

[ClausB]

Definitely a 48K mod but not one of mine (http://www.atariage....0/#entry1488304).

 

Not enough circuitry there for 64K. The piggyback 74LS158 is used for multiplexing the additional address lines. The other piggyback chip appears to be a 74LS04, which has six inverters. The chip underneath appears to have some mods too and might be something other than the original 74LS10.

 

It would help to see the motherboard jumper connections too.

[ClausB]

It's a Crowell 48K mod:

 

http://www.atariage....e/#entry1326701

 

An unmodified 16K board will NOT work unless you remove the motherboard jumpers.

Edited March 9, 2013 by ClausB

[jacobus]

Thanks to everyone for your replies! I've replaced the RAM card with a 16K and now I have another stock 400

 

Jumpers on the back: