B
bitrex
Guest
I'm trying to repair this vintage programmable analog drum machine on my
bench. It was water damaged, and it looks like it's been sitting in a
basement and hasn't worked in a very long time. I've cleaned up the PCB
and underneath the rust residue it looks OK, and I've replaced all the
electrolytic capacitors and removed any signs of corrosion. It's
showing signs of life now, and occasionally produces sounds that seem
vaguely correct, but still isn't working properly.
Here's the schematic:
http://fa.utfs.org/diy/boss_dr55/dr55_schematic.jpg
It's a pretty crude machine by today's standards - there's no
microprocessor. Just what seems to be a CMOS static RAM chip that the
patterns are manually bitbanged into by the user in "write" mode, and
then clocked out of in "play" mode. The logic outputs of the RAM IC
then trigger four different analog sound generation circuits on the
right...for example the bass drum seems to be based on a transistor
phase-shift oscillator, etc.
The first big problem I see with my scope is that the there's no clock
coming out of the clock generator when I try to set the machine to
"play" - tracing backwards it looks like the output at pin 3 of the flip
flop made up of two sections of a 4011 (what's the deal with those
symbols?) is oscillating at about 4 Hz, and the nominally active-low
inputs of the flip flop are totally unresponsive to button presses or
manually pulling them low with a jumper.
On the scope the oscillation looks like a slow "inverse" exponential
decay from high to low, followed by the output of the flip flop slamming
low for a little bit and then rapidly coming back up and repeating. It
looks like the on-off duty cycle is about 90%.
Any idea what could be going on here?
bench. It was water damaged, and it looks like it's been sitting in a
basement and hasn't worked in a very long time. I've cleaned up the PCB
and underneath the rust residue it looks OK, and I've replaced all the
electrolytic capacitors and removed any signs of corrosion. It's
showing signs of life now, and occasionally produces sounds that seem
vaguely correct, but still isn't working properly.
Here's the schematic:
http://fa.utfs.org/diy/boss_dr55/dr55_schematic.jpg
It's a pretty crude machine by today's standards - there's no
microprocessor. Just what seems to be a CMOS static RAM chip that the
patterns are manually bitbanged into by the user in "write" mode, and
then clocked out of in "play" mode. The logic outputs of the RAM IC
then trigger four different analog sound generation circuits on the
right...for example the bass drum seems to be based on a transistor
phase-shift oscillator, etc.
The first big problem I see with my scope is that the there's no clock
coming out of the clock generator when I try to set the machine to
"play" - tracing backwards it looks like the output at pin 3 of the flip
flop made up of two sections of a 4011 (what's the deal with those
symbols?) is oscillating at about 4 Hz, and the nominally active-low
inputs of the flip flop are totally unresponsive to button presses or
manually pulling them low with a jumper.
On the scope the oscillation looks like a slow "inverse" exponential
decay from high to low, followed by the output of the flip flop slamming
low for a little bit and then rapidly coming back up and repeating. It
looks like the on-off duty cycle is about 90%.
Any idea what could be going on here?