Got for a repair this Raimais PCB , an obscure maze game released by Taito in 1988 :
Board played fine but sound was totally missing.If I put my fingers on solderside of the amplifier I could hear some noises, sign that it was still good.Someone replaced invain the YM3016 DAC but this was not the cause.Ruled out a problem is the analog section, I went to check the digital circuitry.Sound CPU is a Z80 with a ROM and a RAM.When I went to check the RAM @IC25 (a Toshiba TMM2063, 6264 compatible, almost prone to failure in my experience like Fujitsu TTLs) I found irregular activity on some data pins (on the left of the below picture an healthy signal) :
I desoldered the chip:
and tested it out-of-circuit where it failed (my programmer went in protection due a possible short , indeed I could measure few Ohms of resistance across VCC and GND pins of the chip)
Fitted a good chip restored the sound.End of job.
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