The Roland Rhythm Plus PB-300 was manufactured around 1983, and was designed as a complete accompaniment device, similar to the ones found on home organs from the same era. A keyboard can be connected via midi and trigger bass lines, arpeggios or chords along with the preset rhythm selected. The owner’s manual specifically mentions the HP-300/400 Keyboard and PR-800 Roland devices to go along with this unit. We’ve never hooked up the PB-300 to its siblings, only using it as a standalone drum machine.
The PB-300 has 16 preset rhythms, plus a “fill-in measure” knob that can insert a break every 2,3,8,12 or 16 bars. The break can also be triggered manually with a button on the front panel, or with the use of a foot switch. On the back of the unit an “accent” knob allows some amount of volume variation within the rhythms.
The machine sounds somewhere between CR and TR-era Roland drum machines. The bass drum is nice and crisp, sitting very easily in any mix, although the snare lacks some punch. The cymbals and hi-hat are quite unique — very soft and ringing, sounding more like a bell than the typical noise burst of early analog drum machines. The 8 sounds are: bass drum, snare drum, low tom, high tom, cymbal, open hi-hat, closed hi-hat and clave.
Technical Info / Service Notes
Roland PB- 300 Separate outputs modification
One of the main problems with this drum machine, like many others from that era, is that all the sounds are bundled into one mono output. Furthermore, this machine doesn’t have any mixing capabilities, so the factory set drum balance cannot be changed. Fortunately, it’s not too hard to modify the unit for individual outputs, in a completely reversible way, with minimal work and only two solder points.
Most vintage drum machines have three jacks at the back: two outputs (same signal, high and low impedance) and a jack for the start/stop foot switch, so the quick and easy way to add more outputs is to use these jacks. In the case of the PB-300, an additional ‘mix in’ jack is also present, allowing an external signal to be blended in with the drum sounds, and output through the main out. Both the start/stop foot switch and ‘mix in’ are of limited use, so losing these features isn’t problematic.
Using four jacks does not allow for complete separation of all the drum sounds available in the unit; for that, one would need to add even more jacks to the back panel, a more extensive and time-consuming modification. Our priority is always to isolate the bass drum, which increases the versatility and usefulness of any old drum machine dramatically.
The following output configuration was chosen for this mod:
Output 1: Bass drum
Output 2: Snare
Output 3: Low and High Toms, clave
Output 4: Cymbal, Hi-Hats
Most of the individual sounds are located along the following 12-pin cable, labelled “connector A” for clarity:
Here’s the pinout (pin 1 is the white cable on the left):
- ??? (supply voltage maybe)
- Low tom
- High tom
- Bass drum
- Full mix
- Full mix (low volume)
- BD trigger in
- Snare trigger in
Unplugging this cable completely is not an option, because the BD and SN will disappear, so another way must be found to pull out the sounds in a non-destructive manner. Fortunately, the individual cables can be pulled out of the connector fairly easily. On the side of the connector (not the one that actually plugs in), there’s a row of small holes, one for each cable. By using a tiny screwdriver, the tiny metal strip that’s keeping the cable in place can be pushed, after which the cable can be pulled out. As it turns out, each of those cables is actually a little connector in itself, that can be then be inserted somewhere else very easily, allowing for a very convenient way to reroute signals.
Next step is to free up the three jacks: mix in, fill in, start/stop. Connectors C and D below are responsible for these, but simply unplugging them didn’t work; some extra rerouting is needed in order to keep the front panel “start/stop” and “fill-in” working (connector B below).
Here’s the complete procedure for rerouting the sounds to the four back panel jacks:
- Pull out connector C, and remove the three wires going to it
Remove the middle wire from connector B, and insert the middle wire you just pulled out of connector C in its place. Solder the two remaining wires from connector C together. This will allow the start-stop button on the front panel to continue working.
- From connector A, take out pins 2,3,4,6 and 7.
- Plug the pin 6 wire into connector C, pin 1
- Plug the pin 7 wire into connector C, pin 3
- Plug connector C back in place. Now the BD and SN are routed to the play/stop and fill-in jacks.
- Remove pin 5 (the white wire) from connector D. This is the “Mix in” input wire. Solder this wire to wires 2,3 and 4 you removed from connector A. Now the toms and clave are routed to the “mix in” jack.
- Finally, wrap exposed wires with electrical tape to avoid short circuits.
After this mod, the original output jack only has the cymbals and hi-hat left, because all the other sounds have been pulled out and rerouted. With four outputs instead of one, this machine becomes much more versatile.
You can hear the results of this modification in the videos below, which feature the PB-300 prominently.
|Sounds||8 (bass drum, snare drum, low tom, high tom, cymbal, open hi-hat, closed hi-hat, clave)|
|Controls||- Fill-in measure (# of bars)
- Rhythm break button
- Accent knob (back of unit)
|Trigger out||Midi in/out (for bass/chord/arpeggio section)|
- Fill in measure
|Related models||Roland HP-300/400 Keyboard
Roland PR-800 Digital Keyboard Recorder
I want to do this with mine. But what about the bass accompaniment? I still want the bass from it.
I can’t say for sure, as I’ve never tried the bass accompaniment, so I don’t know where the bass signal is. For that matter, I’m not even sure if the bass sound is meant to output from THIS device, or the other one… So my mod might need to be modified to accommodate the bass signal line, wherever that may be… You’d have to look for it to find out, and make sure you route it appropriately.
By the way, how do the bass accompaniment sound? I’ve never heard them!
The bass comes out of the mix-out of the PB-300 only when you have a keyboard plugged into it’s MIDI In. However, the chord/arpeggio plays whatever is plugged into the MIDI Out of the PB-300. It’s a lot of fun to mess around with. The bass sound is alright I guess. Nothing mind blowing but could sound cool driving it through a bass amp distorted. I’m going to try at some point.
wonder if it is possible to tweak the filter of the bass synth part that comes from the PB-300? Just one knob addition for the filter would be super cool. Possible?
Perhaps it is, but I’ve never really played with the bass synth section of the circuit, so I’m not sure. I’ve actually never heard it at all! 😉
can you mix preset rhythm’s? like press Rhumba and Bossa Nova at the same time?
I wonder if it could be modded to allow individual drum triggers.
I’m pretty sure it could, although I haven’t tried. Most of these old analog sound generator boards just need a simple voltage pulse at the right place to trigger the various voices, so if you bypass the program circuitry and just use the sound generator board, tapping at the appropriate place for each sound, it shouldn’t be that hard to accomplish.
Thank you so much for all of this.