Feb 282019
 

I got the same Black Tiger bootleg board back as in my repair log #3.

The sound had stopped working completely.
I did all the usual visual checks and reseating where I could but no joy.
I probed the Z80 CPU and found following a brief flicker at power on the /HALT pin was always asserted. I pulled the ROM and checked it in my programmer but it came back as good. There is nothing in the way of buffering in between the CPU and ROM.
I decided the quickest way to see what was happening here was to attach the logic analyser to the CPU address bus with the trigger set to the /M1 signal.

From here I could see that it started off good but failed when returning from a call. As the return address is pulled from the stack I decided to remove the RAM and test it. This turned out to be faulty reporting that bit 3 had become disconnected.
Replacing the RAM brought the sound back.

 Posted by at 8:42 am