It's A Mystery: LFO Clicking

[heyniceguy]

Hey guys. Allow me to introduce myself. My name is JB. I used to be a pretty active member of this forum 4-5 years back when the forum was all tiled. the good ole' days....

Anyway, i've finally gotten back into tinkering (mostly repairing broken units for myself/friends) and i've hit a road block on this one...

---------------------------------------
UNIT: DOD 406 Chorus Pedal (circa 1980)
UNIT TOPOLOGY: Uses SAD512 for delay generation, TL022 for LFO, TL062 for signal/mixing. 2 knobs: rate/depth.
PROBLEM: Chorus works but LFO clock bleeds through to signal
---------------------------------------

I've scoured these and other forums and have taken the following actions in accordance with what was recommended:

1) Replaced all semiconductor components (diodes, op-amps, transistors, SAD512).
2) Used a strong power supply (12V 1000mA) to counter the increased demand for current on LFO rise.
3) Decoupled power supply to TL022. Used 470ohm series resistor and 47uF to ground. Installed this directly at the chip's +/- pins.
4) Decoupled TL062 as well with same values.

RESULT: Originally, the pedal ONLY passed dry signal and experienced NO clock bleed. After replacing a bad transistor and a bad SAD512 (im currently "borrowing" an SAD512 from another pedal), I got my chorus sound, but with that damned LFO bleed through. So I then went on to do steps 2, 3, and 4. No change in LFO bleed through. BUT i did notice that when i put a large cap (47uF+) across the TL022 +V/-V pins, the bleed went away BUT so did the chorus sound.

I've tried to give a comprehensive history of this pedal. I hope someone here reads this and remembers something they read in Device 15 years ago that will help.

I do have a decent workstation with a scope and simple 1kHz signal generator and audio probe, in case someone thinks of a test i need to do that would require this equipment. Thanks in advance and it's good to be back.

BTW is WGThickPresence still around?

[Mark Hammer]

WGTP is a regular over at the revised diystompbox forum

[R.G.]

Again, if you can supply schematic or a link to schematic and the pin info as requested in "Debugging: What to do when it doesn't work" thread in the diystompboxes.com forum, we can probably help.

Actually, you should post the question over there. This forum is quietly moldering away, the activity is all at diystompboxes.

[heyniceguy]

Again, if you can supply schematic or a link to schematic and the pin info as requested in "Debugging: What to do when it doesn't work" thread in the diystompboxes.com forum, we can probably help.

Actually, you should post the question over there. This forum is quietly moldering away, the activity is all at diystompboxes.

I could not find an exact schem for this. BUT i did find a schematic of another DOD of the same product line (565 chorus), with very similiar delay/LFO/compander sections. Here is the link:

http://www.freeinfosociety.com/elect...dio/dod565.pdf

Its too bad this forum is fading. But i'll post this over there as well. Thanks for all your help thus far. I'll be changing those caps as soon as i get home.

[Mark Hammer]

Nick Boscorelli notes in his Stompboxology newsletters that one way to make 2-op-amp LFOs behave a little better is to convert the initial square-wave (which is responsible for the sudden current drain that produces the click) into more of a trapezoidal wave, with a rise time just slow enough to eliminate the audibility of the current drain.

If you look at the LFO in a Boss BF-2 flanger, you can see this strategy implemented. http://www.freeinfosociety.com/elect...es/bossbf2.gif

What is different about this LFO? Normally, pin 6 of IC5 would be tied directly to its own Vref and there would only be the feedback resistor R37. Instead, feedback cap C26 is added from the output to the other input, and that input is tied to Vref via a current-limiting resistor. You can see the contrast between this arrangement and the more "normal" LFO circuit used in the Boss CE-2 (http://www.freeinfosociety.com/elect...o/bossce2.pdf).