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. 9 fader caps and 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.
Power and first check
Since the power adapter was missing I bought a new one 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 707 simply won't boot up.
When you press and hold the CLEAR and INSTRUMENT buttons while switching on you set the 707 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 machine and checking IC1 (7805) for correct +5 Volts output I switched off again, disassembled and started to investigate.
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.
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.
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 feeded 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 the machine.
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 and fader caps. 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.
Finally, everything is working again now and after a proper cleaning of all plastic parts my rhythm composer is fully functional again. If you clean it, be careful when you remove the LCD, the clips underneath that are holding the display frame seem to be a bit weak.
Just one thing missing... if you have spare fader caps (Aladin, please?) or if you can 3D print new ones, please get in touch with me. I'd need 9.