Recently I found in a lot of untested/faulty PCBs I bought an interesting and unusual Super Space Invaders '91 (also known as Majestic Twelve: The Space Invaders Part IV).Game has been released by Taito in 1990, it's basically an enhanced version of the classic Space Invaders game and runs on F2 hardware :
When I powered the board up it surprisingly booted into game but graphics was glitched :
I was under impression the garbage floating over the screen was actually sprites leftovers and I was right because it changed its pattern when I shorted pins of the Toshiba TC51832 pseudo-static RAMs (pin to pin compatible with 62256) which are part of the sprites circuit along with a couple of QFP custom ASICs that generates address to the RAMs and process their data :
The first suspects were these TC51832 pseudo-static RAMs because I often encountered bad ones during my repairs.So I went to probe them with my scope and I found a couple of them @IC29 and IC33 with unhealthy data signals.You can see in below picture a comparison with an good signal on the left, the data transaction appears to be complete and valid compared to the bad one on the right :
So I removed the two RAM chips, they both failed the out-of-circuit testing :
Board 100% fixed!
What programmer are you using for the ICs to check them? And why pseudo-static RAM? Thanks!
ReplyDeleteI'm using an old Data I/O programmer.The Toshiba TC51832 are pseudo-static because they utilize a one transistor dynamic cell.They are pin to pin compatible with 62256 (32K x 8-bit staic RAM).You can find datasheet here:
ReplyDeletehttp://pdf.datasheetcatalog.com/datasheets/480/493267_DS.pdf