Hello,
You may regret opening this can of worms, but I would be happy to oblige
your curiosity!
The game is a 1989 Gottlieb System 80B pinball called Big House. I have
performed the following:
Replaced all the electrolytic caps on the sound board power supply board, no
electrolytics on sound board itself.
Repinned the edge connectors carrying the sound lines from CPU board to
solenoid driver board to sound board.
Upgraded the circuit grounds; approximately 40 wires. Gottliebs of this
vintage are notorious for having crummy grounds.
Replaced various TTL chips that Gottlieb gurus suggested I change--no
improvement.
The sound board has a dual CPU system using 2 Rockwell 6502 processors.
Each processor has its own sound rom, amplifier chips, and various TTL
chips; both circuits share a DAC. When certain playfield switches close the
CPU circuit that provides speech cuts out. These playfield switches are
associated with transistors that control 24 (actually 36) volts DC either
through light bulbs or a solenoid. The diode on the solenoid tests ok. I
was thinking the 24 VDC bridge was the troublemaker. This is the bridge
that actually outputs 36 VDC, discussed in my first post. This 36 VDC is
pared down to 12 VDC on the sound board power supply board using a 12 volt
Zener; this 12 VDC then goes to the sound board to power the amplifier
chips. My oscilloscope shows all the address and data lines cut out when
the speech crashes; the other CPU circuit seems immune to the crashes. The
crystal still ticks, and the 5 VDC is good. I have replaced the speech CPU
with a new one--no change.
There are probably other things I have done and not mentioned, but those are
the major ones. Good thing this is a hobby, and not my day job--my family
and I would be starving. Most of the Gottlieb gurus I have spoken with
think it is a flaky sound board; perhaps, but what bothers me is that it
seems an insult external to the sound board is causing the sound board to
crash. Ah well. I have spent way too much of your time; I apologize for
the length. Thank you for humoring me!
Regards,
Dan
P.S. I have a fresh batch of bridge rectifiers. When I test them their
forward voltages (juction drop) are 0.490 volts as well; does this mean they
are bad?