Adding controls/mods to this "Symphonic" BBD circuit?

[Sesh]

Hi DIY'ers

So this is technically not a stompbox, but a circuit in the Yamaha PS-6100 keyboard I just bought dirt cheap. It's just an on-off chorus mode ("Symphonic") by an MN3204 BBD and an IG10090 for BBD modulation.

I was wondering: Would it be possible to mod it with the usual chorus/flange/delay controls like mod depth, delay time etc.? Calling all BBD-nerds!
Here's the chorus-part of the schematic.

[Mark Hammer]

The IG10090 is not a chip I am familiar with.  However, it appears to provide the complementary clock pulses needed to drive the MN3204.  I'm guessing the pair of inverters driving pin 4 of the IG10090 form the master clock.  If that's true then you can perform a simple experiment by adding some capacitance to the 6800pf that is already there.  It's possible you can simlpy tack it on to the solder pads.  If, say, 3300pf, added to the existing 6800pf yields a "thicker" chorus, then you know you've hit paydirt.  That doesn't provide for variable sweep width or rate, but sometimes even small variations in simple delay range yield pleasing changes.  You'd need to actually remove the 6800pf cap, but dropping its value down to , say, 3900pf would put you closer to the range of flanging, or at least Leslie-type swirl.

[Sesh]

The IG10090 is not a chip I am familiar with.  However, it appears to provide the complementary clock pulses needed to drive the MN3204.  I'm guessing the pair of inverters driving pin 4 of the IG10090 form the master clock.  If that's true then you can perform a simple experiment by adding some capacitance to the 6800pf that is already there.  It's possible you can simlpy tack it on to the solder pads.  If, say, 3300pf, added to the existing 6800pf yields a "thicker" chorus, then you know you've hit paydirt.  That doesn't provide for variable sweep width or rate, but sometimes even small variations in simple delay range yield pleasing changes.  You'd need to actually remove the 6800pf cap, but dropping its value down to , say, 3900pf would put you closer to the range of flanging, or at least Leslie-type swirl.

Thanks! Sounds very interesting. Perhaps a switch with selectable cap sizes, then?

[Kevin Mitchell]

Hey you were in the other forum. Was going to look into this but work picked up 

I agree with the cap switch approach. Not invasive, super simple.

[anotherjim]

I have a suspicion the symphonic element is the production of x2 LFO's to simultaneously modulate the BBD clock in the IG10090. One slow and one fast as per string ensemble chorus. There is only one BBD instead of three, but I can assure you this still has some of that symphonic flavour.
OTOH, it could be a single LFO with a selection of fixed rates by those resistors from pins 12 to 15 according to digital selection from the CPU I think is out of sight on the left.
The two inverters feeding the "CLR" (Clear?) pin are I think a one-shot timer for a power-on reset.
Pins RM & CM could well be controlling a master clock oscillator in the IG10090.

I agree the 1000pF caps could be the easiest way to mess with it.

[ElectricDruid]

I had a look in the Yamaha IC Guide Book I've got a copy of, but it doesn't mention this chip. The book is too early, I think.

There are some IG series chips, but they all start 00xxxx

Sorry!

[Sesh]

I had a look in the Yamaha IC Guide Book I've got a copy of, but it doesn't mention this chip. The book is too early, I think.

There are some IG series chips, but they all start 00xxxx

Sorry!

This was all I could find:
https://www.dtech.lv/techarticles_yamaha_chips.html

"CCD (BBD) delay line chorus controller / driver. Despite simple looks, contain two ROMs and two DACs, besides all other stuff."

[anotherjim]

"2 ROMS & 2 DACS", could be LFO wavetables?

[ElectricDruid]

"2 ROMS & 2 DACS", could be LFO wavetables?

That would be my guess too, Jim.

[Sesh]

"2 ROMS & 2 DACS", could be LFO wavetables?

So.. the wavetable LFO are converted to an analog signal via DACS - that then drives the BBD?
Does it have any implication for the mods stated? Mark suggested I try experimenting with the 6800 pf cap, you mentioned the 1000pf cap.

[anotherjim]

Pin9 "CM" of the IG is probably the master clock timing cap. 1000pF. Adding a cap in parallel probably only makes the delay longer.
The use of "C" and "T" in the pin names for the group of resistors feeding pin11 (Min, which could be the master clock modulation input) may suggest the terms choral and tremulant which is organ speak for slow & fast modulation.