520ST wrong ram size

[misjah]

Hello, my Atari 520ST is recognizing only 512KB of Ram (with memwatch app and memmaper). There is 1MB installed on board, chips are soldered on top over the old ones, connected with wires. Any tip what could be the problem? There are some soldering jumpers (A16, A17) that are both set to 256K (the other position is 1M which is open)....

[Greenious]

Those jumpers has nothing to do with RAM. Only touch them if you exchange the eproms containing TOS.

RAM is set at boot by TOS, and by a trial and error procedure. It simply tries to write to various memory locations, and read back what it wrote. The memory blocks that reads back what it wrote is valid, and then it sets the size of ram accordingly.

So, that piggyback solution you have is most likely faulty. Either it is installed incorrectly, or one (or more) of the 300 or so solderjoints is faulty.

[misjah]

Thanx for nice info. I re-soldered all legs from bottom side of board, everything looks ok (gonna do that with upper chips, too). I just can`t find any tutorial for memory upgrade and wiring (only method with SIMM upgrade). What happens if only one chip is dead?

[Greenious]

If only one of the new memory chips are broken, or not properly soldered, the RAM will read back faulty, and TOS will not recognize it.

[misjah]

Found it! It was just one bad soldered point. Working good now. Thanx