Mod for DR55 that provides trigger in's

nc0839 mymoog at
Thu Jan 1 11:49:38 CET 1998

I got the schematics for the DR55 this week. In looking over them it
appears that a modification to allow a device like the MAP ADV-MIDI, PAIA
MIDI2CV8 and TR series drum machines to trigger the DR55's voices via
trigger pulses is possible. In bouncing back and forth between the PCB
sheet and the schematic I discovered the pins on the "Logic" chip that
control the firing of voices. I used an analog VOM to verify I had the
correct pins (these being 12, 14, and 16). The meter being analog made it
difficult to pin down the actual voltage of the spike and if the spike is
positively or negatively biased. Another piece of luck developed when I
discovered that the trigger lines on the board pass thru jumper wires just
after the chip (a good 3cm of exposed jumper). This would make lifting the
trigger lines from the chip and providing trigger in jacks instead pretty
easy. I tride the whole thing with a crudely fasioned trigger cable which
seemed to actually work. 

For this test I made a trigger cable with a 1/8" phone plug on one end and
an alligator clip on the ground and a wire probe on the lead for the other.
I plugged the 1/8" phone plug into my TR606 trigger out and clipped the
ground to the DR55's shell. I then played a pattern on the 606 that would
fire it's trigger output and touched the prob wire to one of the three
jumpers inside the DR55. Every time a TR606 trigger would fire it would
trigger whatever voice I was messing with. This seems to suggest that using
the DM55 in a setup with a MIDI/Trigger interface or a Roland TR type drum
machine is possible.

The only thing I am not sure of is if the TR trigger is a negative pulse
and if the DR55's internal triggers are biased the same direction? I wish I
had a scope so I could see the pulses from the two machines for
comparision. Analog solutions does a mod for TR/CR drum machines to allow
the internal drum voices to be triggered via jacks. Has anyone ever
attempted a mod like this on a TR? What is the trigger voltage/shape used
in these machines to control the firing of voices?

If I could get the pinout of the "Logic" chip I might be possible to modify
the DR55 to sync to an external source. Say DIN or 24ppq pulses. The
"Logic" chip appears to get it's timing from another IC (IC2 in the
schematics). If it were possible to prevent the timer from advancing until
a pulse occures on a curtian pin, one might be able to make the DR55 accept
external sync (In theory).

P.S. I tried the decay mod for the Kick. It makes a very 808'ish sound when
the resistor is bridged.


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