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.

Owner’s Manual. Click to download.

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:

Connector A

Here’s the pinout (pin 1 is the white cable on the left):

• ??? (supply voltage maybe)
• Low tom
• High tom
• Clave
• ???
• Bass drum
• Snare
• 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).

Connector B

Connectors C & D (after modification)

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.

Related Videos

Specifications

Gallery

Connector A

Connector B

Before modification

Connectors C & D (after modification)

Before modification