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 using an external oscillator without a load of weird crunchy noise as they fade out. Anything a bit below the normal clock rate sounds perfectly fine but as soon as i approach the normal playback pitch everything goes crunchy again.

Theres a better scan of the 707 schematic than in the files section here http://www.gzifcak.net/eim/tr-707-schematics.jpg

Up 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, which then goes 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.
You do on my machines anyway. If i just install the crash clock on pin 10 if IC18 as recommended by various posts on here, the sound keeps playing over and over again.

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 everywhere 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 can't see where.

Having trawled the archives i've come across a couple of mentions of this issue but no solutions. I can only assume it doesn't happen on every unit, but it certainly happens on all three of my TR707's!

Heres a quick tip i just discovered. If you disconnect pin 9 of IC21 and IC24 and wire in a new oscillator you appear to be able to control the bit rate of the crash and ride sounds.