Atari Tetris

Atari Tetris Repair log

The board itself looks in quite good nick so I decided to rescue it from the scrap parts pile.

Symptom garbage on screen

After a quick inspection of the board all I noticed was a track that needed patching up but that made no difference to the problem. I then decided that a quick check of the rom’s was a good way forward but they came back as perfect matches to the mame rom set.

I had a quick look and noticed that the ram located at F30 (video ram Part No. V61C16S55L) and the ram at B40 (programe ram Part No. HY6264-P15) were in sockets. The video ram had obviously been pulled before and someone had used a wide socket cut up to make a slim. I removed the ram to check in my programmer but both came back fine. I hen checked the soldering on the socket and all seemed to be fine as well even if it was messy (I’m not calling too much Atari boards I fine are a paint to de-solder and solder).

At this point I decided that it was time to break out the scope and find out what was going on with the 6502 CPU. Inspection on the clock line showed it to be healthy (see pic) but the addresses lines were showing no activity that pointed me to the reset line.

The reset line was being held low but me having very little knowledge of the 6502 made me go dig for the datasheet and from what I could understand (correct me if I’m wrong) that the line should be held low for at least two clock cycles on power up then held high once the voltage has reached over 4.75 volts. So I decided to force the reset pin high with a probe connected to 5 volts on the jamma harness and the game jumped into life.

Well so now I knew there was a problem with the reset or watchdog circuit preventing the cpu from initialising. I traced the reset line to pin 1(1CLR) on 2 SN74LS74AN and also pin 13 (2CLR) on the second after it also goes to pin 12 on a SN74LS197N (output Qd).  The connections to the two LS74 were inputs so were more than likely to connect other parts of the circuit to reset (don’t hold me to that) so the LS192 must be responsible for the power on reset. I’ve seen this before with other Atari hardware from around the same era and remember that in the system1 schematics Id seen a similar circuit used.

New symptom now it seems the game reports a bad video ram probably why it had been pulled at some stage. Even with the video ram error the game seemed to have all the right colours and looked fine. I placed some pressure on the area the ram was located and fired the board up straight into test and found that the video ram problem had gone. Recycled the process without the pressure and the video ram fault was back. Well remember about the messy soldering I said was fine. A little touching up and the board fired up and worked fine.

Well I made that job harder than it had to be.

I was hoping to get some better pic’s of the board but my Iphone camera is not really up to the task.

Ive no idea how ive made it look like this but if moderator can sort it cause I cant work it out

Cheers Kris