Arcomatic Rosinator - bowed string effect

[anotherjim]

And the PT2399's arrive...Yay!
Got them from Bitsbox 2days after placing the order.

I've been reading all the threads & articles I could find about the simple chorus circuits using this chip and am having a shot at the OCC type.

The "Merlin" variant is on my BB at the mo'. Got it working but the LFO makes noises at me. Hmm... will pursue further. Biggest problem I suspect is that's it's on a BB. Oh, and my chips read 30ohm between pins 3&4, but won't run unless I short them or fit a lower resistor. Currently I have 100R across them with a separate decoupling cap from +5v to Dgnd. Sounds promising so far.

[anotherjim]

I think I'm happy with the Merlin version OCC on the breadboard now. I have the LFO behaving having "optimized" the timing/filter caps for the 2 different LFO rates I want - that's one at 0.6Hz and the second at 6Hz. I've only heard one at a time, the build will have both in parallel.
Just one already sounds the business though

So, big gratitude from me to Anchovie and Merlin and all who contributed to the OCC and other PT2399 threads I found here.

On the build, I plan on 2 boards, both with the panel pots attached. The main circuit on one and choruses on the other, Chorus will have 4panel pots and carry both PT2399's with separate 78L05 regulators. Each PT2399 LFO rate will be set by internal trimmers. User controls will be for Depth and Delay. Depth is as the Merlin OCC design. Delay control is a modest range with a 1k resistor + 10k pot off VCO pin 6. Increasing delay reduces the depth of chorus while introducing a doubling thru to slapback effect.

I'm praying for freedom from heterodyning between the 2 delays. As an extra insurance, I may cut the "clock out" pin 5 leg off the chip and ground that socket pin. I hate the thought of it sat there, doing nothing but transmitting at over 4Mhz! We don't need the clock out and you can still get a probe tip on the cut leg for testing.

[Mark Hammer]

Unlike so many other clocked things, one of the nice aspects of the PT2399 is that the clock frequency is high enough that many kinds of heterodyning that do occur are largely inaudible.

[anotherjim]

Ok, well I've got my fingers crossed. But as I shall relate, Heterodyning? I should be so lucky...

I got sidetracked trying some other ideas for this. The bass voice will have to go thru it's own VCA, but it can be before the lo-pass filter. I'd noticed that it does sometimes audibly glitch on final fade out without a VCA.
I've found a simple 1 transistor pulse doubler circuit so an octave up might make an appearance. This does sound a bit synthy, but a chorus covers a multitude of sins

Ah yes, the chorus...
Having decided the breadboard situation was getting out of hand, I decided to build 2 PT2399 chorus modules on perfboard. One with the choral 0.6Hz LFO and one with the Vibrato 6Hz LFO. The first Choral completed it tested ok but I thought the LFO was a bit lumpy. Scoping showed some more filtering needed. I changed a cap and it hasn't worked again, even after changing the cap back to the original value. Everything checks out -  no shorts or open circuits. The PT2399 is clocking and the LFO is running, but no delay or effect. No signal out of the demodulator. It isn't locked up, it's dead, as were the next 2 chips I tried; including the one I'd left in the breadboard version and was saving as a "known good part". A 4th new chip tried only in the breadboard works, the dead ones are still dead, even back in the breadboard circuit.

I don't know what it is for sure, but I strongly suspect driving the PT2399 pin2 (Vref) to modulate it is the culprit. Maybe the Vref source in the chip isn't a passive resistor network and doesn't liked being yanked around? Maybe pulsing the pin is charge pumping the substrate via a parasitic diode that MOS devices are full of. Maybe fitting clamping diodes on pin2 will fix it? I don't know, but I think I'm going to have to go back and use conventional sweeping of the VCO resistor and separate LFO.
BTW, the breadboard circuit was much the same as the circuit I built on perfboard, but had been set for the Vibrato 6Hz and didn't have an LFO filter cap (wasn't needed for the faster rate).