BOSS DR55 - CB55

The BOSS Dr. Rhythm DR-55 was my very first rhythmbox. I bought it secondhand together with a friend (we timeshared it weekly). After some time we sold it, about 40 years ago.

A few years ago I spotted the CB55 PCB from circuitbenders. It was an easy build and the sounds were an amazing time machine.

https://www.circuitbenders.co.uk/newsarchive/CB55.html

Soon after the module was built I started circuitbending and searched the internet for mods.

Here's the result of searching and experimenting:

Kick drum mods:

BD decay - Replace R33 with a 1KB pot and 500R trimmer*

BD pitch - Replace R31 with a 1MB pot and 2M trimmer

BD pitch drop - Replace C10 with switch between 68nf and 100nf

Snare drum mods:

SD decay - Replace R52**

SD pitch - Replace R50 with a 1MB pot and 2M trimmer

SD pitch drop - Add a switch-in 104 capacitor to C23

SD colour - Replace R57 with 10K pot

Rimshot mods:

RS pitch - Replace R43 with a 1MB pot and 2M trimmer***

RS decay - Replace R45****

Hihat mods:

HH tone - Replace R69*****

HH pitch - Replace C35 (6.8nf) with switched capacitors******

HH decay - Replace R66/70 with a 2M pot

HH open - No mod found

* People have suggested a 2K5 pot here

** People suggest replacing R52 with unknown value

*** People have suggested replacing R44 with a 50K pot

**** People suggest replacing R45 with unknown value

***** People suggest replacing R69 with unknown value

****** I couldn't find any reference to what capacitor values

All channels need B500K/A10K volume pots, probably between relevant jumper and switch output socket. A momentary or SPST mute/break switch could be put here too.

All channels need trigger in and output sockets.

Personal findings:

SD decay: replace R52 (330 Ohm) by 220 Ohm + 500 Ohm pot (1k if 500 Ohm not available)

SD noise decay: replace R55 (120k) by 82k + 100k pot (alternative: change C26 (27nF) into 100nF via a switch)

HH noise: replace R69 (10k) by 1k + 50k pot