When I bought my TR-707 it was not in a good shape and had several problems and issues. The Rim button and four of the step sequencer LEDs weren't working at all. The output of both Ride and Rim/Cowbell were at a very low level and barely hearable. Additionally, some of the faders were crackling. The housing was not light grey but dark with sweaty black dirt all over. Many screws were missing, apparently this drum machine had been used intensly, then something went wrong, then someone tried to fix it and lost both patience and components.
Some years later a TR-727 dropped into my studio. While cosmetically in a better shape, it missed all fader caps, the tempo button and had no function. At least, both machines powered up.
Power and first check
Since the power adapter was missing in both cases I bought new ones from a local electronics store. You'd need an adapter with 12 Volts DC output, center negative, 200mA at least. I took a 500mA adapter since it was cheaper and the device would only consume the power it needs anyway. Supplying 12 Volts center positive will do no harm by the way, there's a diode in the circuit to prevent any damage from wrong polarity and the 7x7 simply won't boot up.
When you press and hold the CLEAR and INSTRUMENT buttons while switching on you set the drum machine into test mode. All LEDs will start lighting up sequentially. Pressing ENTER advances to the next test, all LEDs and LCD dots light up at once. Pressing ENTER once more enters the button test mode to check if all of the buttons work. Not all of the tests succeeded in my case.
So after opening the instruments and checking IC1 (7805) for correct +5 Volts output I switched off again, disassembled and started to investigate.
707: Broken diodes
After a first cleaning (vacuum cleaning of all parts, careful cleaning and spraying of the fader pots) I inspected the PCBs and found, without any measuring, two broken diodes.
While D341 (button 8/Rim) could be replaced by a standard 1N4148 the other one (D17) appeared to be a Germanium diode without further specification, neither on the tiny glass tube nor in the service manual. So I asked around if anyone knew the type of diode that had been used here and got some helpful answers which led to testing the ride output with Germanium diodes of types AA113 and AA143. Basically, the diode's purpose is smoothening the decay of the sample.
- AA113: decay a bit too fast but very soft in the end, volume slightly lower than the crash volume
- AA143: smooth decay over whole sample length, volume slightly louder than the crash cymbal
I decided to go with AA143.
By the way, you could skip the diode also and replace resistor R73 (2,2MΩ) with 4,4MΩ. This is the set-up for TR-727 which plays the chimes on this channel.
707: LED not working
It appeared that the broken LEDs followed a pattern. Not working were numbers 4, 8, 12 and 16. The service manual shows that all of these are on a path between R304 (on Pin 13 of IC 302, M54517 Darlington Transistor Array) and LED D320 (No. 16). Checking the path for continuity gave no signal so it must be broken somewhere between No. 4 and the resistor. I simply bridged it with a wire.
707: Rim/Cowbell with no sound
While the ride cymbal came back by replacing the Germanium diode the case with the very low volume for the rim was different. At least, you could hear something, so I first suspected some fault in the VCA circuit and looked closer at the slide pot, the 5218 OpAmp and neighbored components. All of these are still available easily, that's good news. To check the VCA I fed it with a Snare signal and it worked without any problem. It turned out to be a broken path between R140 and connector CN2 pin 31. So I bridged with some wire again and all was working well.
These two connectors CN2 and CN3 are a PITA by the way. They're meant to make removing the wiring easy. Instead, you'd fail constantly. I'd better replace both of them the next time I have to open one of these machines.
Beating the box
I believe my 707 really has been having a hard time and treated not very well, indicated by the broken glass diode, broken electrical paths and the missing screws. I guess it was part of some sort of beating the box rather than beat boxing. There are two stickers left on the housing: one by a New York City musical equipment trading company, one by a DJ called "Aladin". Hi DJ Aladin, remember your broken composer?
I kept the stickers for historical reasons. And hey, the box is from New York.
All my 727 suffered from was dirt. Sand and dust blocked all buttons to the left so it was impossible to even start/stop a sequence. After disassembling and a thorough cleaning of all components it was back to life.
To clean the machines remove all PCBs and the display. Be careful when you remove the LCD, the clips underneath that are holding the display frame seem to be a bit weak. Brush the electrical parts with a soft brush. I simply used water to rush away the sand from all parts of the housing and the buttons. You can do that in your sink or use a dishwasher at low temperature*. The contacts both on the PCB and the two contact mats can be cleaned best with benzine or Isopropyl alcohol. Don't use WD-40 or something similar and make sure the fluid evaporates traceless. Since the sand made some smaller scratches on the sides of some of the buttons and these were still a bit stiff I used a nail file to make them move smoothly.
Just one thing missing... if you have spare fader caps or if you can 3D print new ones,
please get in touch with me (I sold my 727 recently so I do not need caps any longer). I know that caps from a Roland SH-101 look similar though these are slightly longer in size and have a different color scheme. But if you like you could get spares for it easily since Behringer made that 101 reissue, so spares should be available. You might have to cut the length of the fader caps very slightly, though.
Another approach: you could be looking for a graphic equalizer (you can get them very cheap second hand) with suitable caps.
Instead, I put small switch caps onto the faders which looks very nice actually and gives my 727 some kind of ARP Odyssey feeling. The missing tempo button was replaced by a button from a 1980ies receiver that I found at a local recycling site. The tempo button is still available, however, there are sources in France and the U.S.
707 and 727 in Sync
Finally, the rhythm composers are fully functional again. To use both machines synchronized connect both SYNC ports with a standard MIDI wire. The slave machine has to be set to [S-d] by pressing [SHIFT] and [SYNC MODE] once. Start the Master and both composers should be synced. It's great to let both machines play their own patterns in sync and use the Rim Shot and Agogo Trigger Outputs to drive two synthesizers like a Juno-6 or SH-101 for Arpeggio and Bass. The fun part is to have various trigger patterns from 16th to 8th to 4th or something inbetween and to swop live.