Has anyone successfully done the pitch mods on a TR707 or 727? I added pitch controls to a 727 for someone while back and that went alright, so today i had some time free and i thought i'd try adding them to one of my 707's and its driving me insane.
All the pitch controls work perfectly apart from the crash and ride controls which refuse to playback at normal pitch and higher without a load of weird crunchy noise as they fade out. Anything a bit below the normal clock rate of 25kHz sounds perfectly fine but as soon as i approach the normal playback pitch everything goes crunchy again.
Theres a good scan of the 707 schematic here
http://www.gzifcak.net/eim/tr-707-schematics.jpgUp on the top right you can see IC25, IC18 and IC19 which are for playback of the crash sound. I've established that on the normal setup a 50kHz clock is fed into pin 11 of IC25 which is a flip flop that divides the clock down to around 25kHz output on pin 13, to the binary counter at IC18. The output pin is held high when the sound isn't playing, allows the clock signal through while the sound plays, and then goes high again when the envelope closes. This seems to act as a kind of gate as without it you get a weird echoing repeat of the sound.
This appears to mean that the new clock has to be injected at the input to IC25 or you don't get the gating effect. What i've done is to disconnect pin 11 of IC25 and have wired in a new oscillator. If i set the new oscillator to 50kHz i can measure all the inputs and outputs on my frequency counter/oscilloscope and they are absolutely identical to how they would be if it was wired up normally with the internal clock, and yet the sound will be strangely crunchy and distorted. It'll pitch up and down perfectly fine, but over a certain pitch it just sounds crap.
I've tried using a 555, a 40106 and an LTC1799 as an oscillator and i still get the crunchy playback despite every clock signal and voltage apparently being perfectly normal. This has completely baffled me. I have a feeling that there could be a mismatch between two clocks somewhere and the external oscillator may need to be wired in somewhere else as well, but i'll be buggered if i can see where.
I've read a guide on how to do this mod elsewhere and it just doesn't seem to work properly, at least any guide that i've seen doesn't work on any of my 3 TR707's.
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Topic: TR707 pitch mods - crash and ride (Read 5913 times)