Stern Hot Hand / MPU-100 - Scoring Issue

Author: Wayne Eggert
Date: 3/26/2011

Problem Description
A Stern Hot Hand MPU-100 was scoring wacky.  When turning the machine on, the intial high score was "443300".  It should be "0".  Even after removing the RAM entirely and putting it back in the MPU board, the high score still showed as "443300".. a clear indication that something was wrong since when disconnected fully from the board, the data in the RAM is no longer powered by the battery and should clear.

Additionally, any points scored were incorrect.  If a 200pt switch was activated it would score as "2200".  If activated again it would display as "4400".  So the scores appeared to be "SCORE + (SCORE x 10)".

Problem Resolution
I swapped the PIAs out, game ROMs, cpu.. even tried putting the MPU board in another Hot Hand machine.  The scoring issue remained.  I also tried swapping the 5101 RAM out to another board, at least I thought -- but it must have been late at night cause it turns out the 5101 RAM was indeed bad.

Here is a comparison of good ram versus the bad 5101 ram when triggering the same "200pt" switch:


Picture: Good versus Bad RAM when activating 200pt switch

I think the reason I overlooked the RAM was bad is these 5101 ics are expensive since they are no longer made and I had been working on other issues with the Hot Hand machine as well (hitting flipper causes machine to boot into self-test mode).  So during one of the times I swapped RAM out I mustn't have tested the scoring issue properly.  Putting this bad RAM in a known working good MPU board also showed the scoring problem, so the 5101 RAM definitely had some issues with its internal circuits.