3086 Tremolo (needs some help)

[anotherjim]

Bad memory, it wasn't a PSO but a Twin-T, same sinewave outcome but narrower range than a PSO. It was the CD4007 based Chopper/Trem I posted a few years back which Duckarse developed into the "Rowdy" with a PSO.
Funnily enough, I've had zero clicking problems with the CMOS inverter version of the sq/tri function generator. There still is a Schmitt trigger and it doesn't seem to need an edge slewing cap, but I've been more careful with the layout since the Tremelo debacle.
I just noticed I've coined two cool names. The "Surefire Tremolo" and the "Tremolo Debacle". 

[soggybag]

What is going on here? Note the area in red.

The trimmer R33 is supposed to reduce the feed through. It's source comes from the bias voltage 4.5v. 9V through R13 meets the bias voltage at the junction of R12 and 13.

What happens here? I'm guessing have a voltage divider between R12 and 13. With a potential of 9v at the top and 4.5v the bottom. How do you calculate that?

My multimeter says there's about 7 to 7.7v at the junction.

Second question: R33 didn't seem to have enough range to remove the LFO. I reduced R13 to 7.5K and this seems have given R33 more range to adjust the LFO feed through.

Would it be better to replace R13 with a trimmer here?

[PRR]

R12 + R33 should equal R18 +/- tolerances.

Yes it does seem like R12 could be 15k and R33 could be 10k, cover the slop and be less fussy. But maybe he had a heap of 50k trimmers.

[soggybag]

Thanks for the insights PRR. The parts list shows 5k for R33 but the schematic showed 50K! I didn't understand what was happening here so I rolled the dice and went with the schematic.

In the build without the single op-amp IC2, I could not dial out the LFO thumping until I changed R12 to 7.5k. With this change R33 had enough range to remove the thumping.

The point E on the schematic is used for the power of IC2 in the original schematic. Which looks a little strange?

[PRR]

> point E on the schematic is used for the power of IC2 in the original schematic. Which looks a little strange?

E is Batt+ with 100r and 470uFd filter. What's odd?

[soggybag]

Thanks PRR, that's great. I used 5K for R33, and 18K for R12. This made the trimmer work well. I put R13 back to 10K.