single pot mod for vibe question

[rhdwave]

Hey, i've been working with the geofex single pot mod which replaces the dual ganged reverse taper 100k pot.  After much scratching of the head and befuddled looks to my dogs i finally got it working...lfo controlled...lamp blinking in accordance with the speed control and depth control and then all of the sudden...poof it stopped.  Bulb is still lit, but no blinking.  The thing is, it seemed that as this happened, the red led in the circuit got brighter.  I'm just wondering if it's possible i burned out the vactec unit which is packaged led/ldr.  Is there a way to tell if i did this without simply taking it out and trying another one?

Much thanks for any responses!

[Papa_lazerous]

surely you could remove it and use a meter to accertain whether or not its fried...... gona be the simplest way

[Solidhex]

Rad

  I'm working on an eagle layout for that circuit to be mounted on a 3dpdt switch. That way I can switch between an panel mounted 10k speed pot and a stereo jack for an external expression pedal. Probably will try to do another one featuring Rg's leslie ramp up/down circuit. Fun stuff.

--Brad

[rhdwave]

Got everything working a day a couple of days ago!!! Sounds great! This is the third one i've built and each one has its own flavor.  One of the problems i was having was actually with the led indicator.  Stupid me had disconnected the led which i originally had tried to hook up to flash with the speed of the lfo, but then with all the other problems i was having wanted to rule that out and disconnected.  I never reconnected it the normal way and thus i think i was grounding out somewhere.  Hooked up led and wallah!!!
Will post pics and some sound clips soon...i made this one for Dean Wareham...dropped it off yesterday...he was very happy with the sound.  I hope he uses it.  That would be such a thrill to hear something i built being used on a record...

Anyway, thanks for all the help!!!

[danielzink]

Rhd,

I'm working on this same mod for a couple of rehoused Chicken Salads.
If I may ask - what resistor value did you end up with ?
How much did you play with that value ?
I was going to mount a 10k pot instead of the fixed resistor and put the pot on the enclosure with the rest of the controls so you could adjust it as needed.
Just wondering how much effect the value of the resistor had on the sound ?

Thanks, Dan

[rhdwave]

Hey Dan,

I don't have the build with me anymore and i neglected to write everything down, BUT...i'm almost positive i used a 2k resistor and it sounded great.  I didn't really fool with trying different values, but i like your idea of a 10k pot to adjust that. 

[danielzink]

Somewhere else on the board is RG's reply to me regarding this mod and the Chicken Salad.

Here it is:

Dan,

(1) find +9V somewhere on your Chicken Salad. Start where the power comes into the board and trace it to an easy pad for powering the mod. Solder a white wire to the pad. 6" should do it.
(2) find ground somewhere on your Chicken Salad. Start where the power comes into the board and trace it to an easy pad for powering the mod. Solder a black wire to the pad. 6" should do it.
(3) Make yourself up a small perf board with the added circuit on it.

LED1 is a red LED, any kind. It will not be seen outside the box and is used only for its voltage drop.
D1 is a silicon diode, any kind, both 1N4148 and 1n400x will work fine; it's used only for its voltage drop.
Transistor Q1 is an NPN transistor with a gain over 100. 2N4400, 2N3904 will work fine, as will most other NPN silicon transistors you can find. Be sure to check the pinout on the transistor you have before soldering it in.
Re is the scaling resistor that converts the voltage on the emitter of Q1 to a current through Q1. It should be sized so that  the 9V supply minus the approximately 1.5V LED voltage plus the approximately 0.6V D1 voltage gives a workable current through the opto isolator LED.

What's a "workable current"? It depends on the LED/LDR's LED. Over 20ma probably kills the led, so Re is going to be bigge than (9-1.5-0.6)/0.02A = 345 ohms. How much bigger? You need to put a pot in there and diddle it until the control range from min to max is what you like for your LED/LDR.

Some LED/LDRs are very sensitive, needing less than 1ma for full-on to essentially full-off on the LDR. That would be an Re value of 6900 ohms. So you're looking for a value between 345 and 6900 ohms.

Dan

[rhdwave]

Hey Dan, This is what i based my 2k resistor info on (keep in mind that your supply voltage will be different as it's a 9V circuit instead of a 15V or 18V as for the neovibe...actually i probably messed up on this as i used the example in the post but i should have changed my voltage input as well for the formula...but it ended up working fine):

This info was from a post some time back and it's R.G. talking here:

The LED we want a max current in is the LED in the Vactrol. LED1 is already limited to the supply voltage minus one LED and one diode drop, divided by the 10K of the speed pot. When the 10K speed pot is turned all the way up, it goes up to the power supply less the drop of LED1. So its emitter is one diode drop lower than that. And the current in Re is (Vsupply - Vled1 - Vbe)/Re.

We want that current to be less than the max current for the LED in the vactrol. I forget the max spec, but let's just say it's 5ma for the purposes of this example. So if Vsupply is 18V, Vled1 = 2V, and Imax = 5ma, then Re = (18V - 2V -0.6V)/0.005 = 1925 ohms, or 1.925K. I'd use a 2.0K.

I do know from past experience that your vactrol is likely to need currents under 5ma for the best range on the univibe circuit. Vactrols should hit 250K resistance on each side of the centertap with something well under 1ma in the LED.


One thing i have noticed is that the top speed of the lfo is not as high as in the two other builds i did with the normal dual ganged pot...i think someone posted something about this as well somewhere...This is using the same resistors that control speed in all three builds, so the vactrol method definitely has an effect on the speed, but i guess it's a juggling act to get everything right.  The speed though, was fine, just not as fast as the others.  But, i had done the mods on all three builds to increase the max speed anyway, so this probably brings the top speed down to a normal univibe range.

[danielzink]

Very interesting....

I learn something new every day...

Dan

[R.G.]

One last caveat: LDRs dark-adapt. If you turn it down to the loooooowwwwwessst speed you can, you will find that it drifts even lower over a couple of minutes. It's a quirk of the LDRs themselves.