Yamaha REX50 mods

[soggybag]

A friend of mine gave me two Yamaha REX50 multi effects unit. I’ve read about mods that change the clock speed. Sort of “circuit bending” the unit.

This page describes using a LTC1799 to set the clock speed in place of the original crystal or resonator. These are hard to get. I was wondering if there was another option like a 555 or clock mad from some hex inverters or digital logic chips?

If I’m reading the schematic correctly the clock is two hex inverters and a 4MHz crystal. I need to figure out what would be a reasonable clock range and then figure out how to build an adjustable clock source.

[anotherjim]

The VCO in a 74HC4046 PLL chip might get you somewhere. The ordinary CD4046 won't do much better than 1MHz. I've used the HC4046 to clock an Atmel chip up to 8Mhz.

[anotherjim]

Here you go, this was actually only needed to run around 4Mhz. The tuning gubbins on the right was to give faster or slower clock around a centre detent pot for R11 with R8 tuning for standard pitch at that detent.
It's really pretty stable (if the 5v supply is) so you won't miss the crystal/resonator osciilator.

[soggybag]

I ordered some 74HC4046 chips, Just got them in the mail, to give this a try.

[soggybag]

I got my parts. I wasted some time thinking about how to do this. Then got to work. I redrew the schematic above in Eagle and made a couple small changes. Check my work and let me know if I got this right.

- Got rid of bend range because I'll just have a single knob.
- Added a LM7805 for 5V.
- C6 looks like 100h in the first drawing I'm guessing this is 100n?

I wasn't sure what CLOCK_TP is? I'm guessing I don't need this and I'll be using MPU_CLOCK?

[anotherjim]

Yes, 100n.
D3 is there to raise the minimum control voltage above 0v. The VCO behaves oddly at the extremes and nosedives to glacial speed near 0v.
D4 and D5 create a dead zone for the bend pot mid position to make it easier to set a "normal" frequency. D5 is Schottky to let the maximum control voltage go a bit higher to better equal the minimum *. You may not care and just use the same as D4.
Clock TP is a test point output to measure the frequency. R14 actually helps reduce ringing on the scope waveform due to probe inductance.
* My application is clocking an MCU that can run up to 20Mhz (not that this VCO can run that fast). Your application might crash if clocked too fast. IIRC, this should reach 8Mhz max.

 

[soggybag]

Thanks again! I’ll breadboard this and check it with my scope next.