Atari Gauntlet arcade pcb repair & dual boot

Gauntlet 1 PCB

Initially booted with corrupt sprites – certainly drawing as the wrong width – then subsequent boots showed no sprites at all.

I went through the ROMs and PROMs to reseat everything and a problem was soon obvious – 136037-103 had previously been inserted with the power pin bent outside the socket – a weird fault but when restored everything was 100%.

Gauntlet 2 PCB

Game played but with corrupted graphics most noticeable on the title screen and test mode. The graphics in Gauntlet are 4 bits per pixel, with each plane stored in a separate ROM. By removing ROMs one at a time it became clear the corruption was on plane 0 only. With the ROM at 1A removed although the title screen had wrong colors there was no more noise or corruption. My initial thought was this ROM had failed – but it tested ok off the board, and a replacement ROM showed the exact same fault. Next to check are the two custom chips next to the ROMs – these are identical and process a pair of planes each. I swapped the two chips but the fault stayed on plane 0 rather than move to plane 2.

I then used a scope on the data pins of the 1A socket and found D7 was noisy whereas D0-D6 were clean (held high by the pullup resistor pack). It seems the ROM at 1C was emitting noise onto the bus even though it wasn’t enabled – with that ROM replaced everything was good.

Dual Boot

The dual boot process is well documented on the Gauntlet site at ionpool – http://www.ionpool.net/arcade/gauntlet/main.html

In case that site ever goes down the process is quite simple as Gauntlet 1 & 2 are essentially the same board and already share some ROMs. For the ROMs that differ – 4 graphics ROMs, alphanumeric ROM, 4 program ROMs, 2 sound ROMs and the settings eeprom – the idea is you replace each with one that is double the size. Then you put the Gauntlet 1 data in the ‘low’ half of the ROM, and Gauntlet 2 in the ‘upper’ half – now by pulling the top address line either low or high you can force one game or the other. You can install double stacked sockets to achieve this, but I went for the simpler route of just pulling the relevant pins outside the sockets and running patch wire between them.