small clone trim voltage control question

[jmasciswannabe]

So I am under the impression that the 100k trim is responsible for adjusting the voltage to the unit. The unit is supposed to run at 4.5v, but I am getting about 9.5 on pin 1's regardless of where i position the trim pot....could this mean that the trim pot is broken? Should I be able to read 4.5v on a multimeter where the trim pot goes to vcc? Below is the schematic. I am getting signal at 3 7 and 8 on the mn3007, so I don't think thats the problem. Of course I am not getting the chorus sound. Also, do the dry and wet signal mix together at r19? I am new at reading schematics and could use the help of someone more experienced.

http://img229.echo.cx/my.php?image=reissuesmallcloneschematic4wt.gif
Thanks!

[puretube]

interesting schemo... tnx.

which "pin1" ?

Vcc = is the voltage the circuit is supposed to run on (~9V).

Yes, mix is @ R19, if Q4 is conducting...

[jmasciswannabe]

Someone graciously led me to that schematic. All right, I think I am starting to understand the vcc thing...on the lm324 on pins 3 adn 6 I am getting 4.2 and 2.4 should be 4 and 2....adjusting the trim bias doen't effect these numbers....I can't seem to get other voltages that ask for 4.5 there either. I guess my question is where should I see a change in voltage reading when I adjust the 100k bias trim.

thanks!

[puretube]

the bias trim is not doing the 4.5V thing...
(this is being done by R35/R36/C2, who divide Vcc by2);

the bias trim enables to set the largest available headroom for the BBD,
and that bias voltage appears in the end-effect at pin3 of the BBD;

(in this circuit, it also sets DC bias for Q1...);
the exact voltage can`t be predetermined, but can be "heard" while adjusting (or monitored on a scope) for best setting.

Do you hear a difference in sound, when you hit the footswitch?

[sean k]

essentially its exactly the same schematic as the one at tonepad.
http://www.tonepad.com/projects.asp?projectType=fx
and under small clone you'll find some mods to do as well.

[puretube]

which one is truebypass? :lol:

[H.Manback]

Well, this one sure isn't

Looks like the tonepad version has a modified output circuit, notice the FET on this one? That's omitted because the FET is used for the footswitch, so having it in true bypass isn't necessary.

Other than that, I would say the schematic is the same.

About the trimpot, 9,5 on pin 1 seems pretty logical to me... as stated, it's Vcc, which is your battery/adapter +. The trimpot sets the bias point for the input to the BBD I would say. Adjusting the trimpot will result in a small range where the thing works, and the rest you don't hear anything (or just the dry signal, can't remember exactly).

About the wet/dry mix, yes it is R19, but adjusting R19 will not influence the mix. The voltage divider in front of R19, R17/R18 is the mixing ratio.

[jmasciswannabe]

Bascially, I got this pedal on a trade for a few mods. Upon inspecting the pedal all the wires were attached and whatnot. Nothing has been done to the board....it just stopped working according to the previous owner. Bypass was of course fine, but nothing was coming out when switched. I suspected the switch so I took it out out and wired an input jack into the in on the board and an output to the out on the board so I could just examine the effect. And of course grounded both jacks. It passes the dry signal , but not the effect. I have read that if the trim isn't right then you wont get effect and was hoping that was the case or that the mn3007 had died. I think the lfo is working cause I can hear the clicking from the lm324 and it increases with the rate knob when I probe the lm324. I get no change in sound when I switch the depth knob. I want to try to trace down the problem with an audio probe, thats why I have asked where the mixing is so I can trace the wet signal backwards and see where it might not be coming through. Anyway, thanks for everyone's help.....and more importantly the info.....any more suggestions or questions are more than encouraged!

[puretube]

put the probe on emitter Q3: you should here a delayed, vibratoed audio-signal.
If not: adjust the bias-trim until you do hear it.

Once you hear it here, the BBD-section works.

Now the wet (delayed) audio has to be passed through Q4 (which works as a switch) to R17, when the footswitch is pushed to "ON" (which ought to put Q4 into conducting status).

Edit: ahh, you wrote you have (delayed?) signal at pins 7/8.
so it does work without needing to tinker with the trimpot...

then on again with Q4...

[jmasciswannabe]

well....I am starting to see the problem....I am not switching the q4 on...because i don't have a switch plugged up to it.... :oops: (should have noticed that puretube! "Yes, mix is @ R19, if Q4 is conducting".... ) it acts as a fet.....which I have read a little more now that i have encountered it....the one on tonepad doesn't use the fet, right? So let me get this straight....I need to hook up an led poistive leg to vcc and neg leg to r24/r25 to get q4 operating?

[puretube]

err, FETs don`t like me...

If it were my circuit, I`d connect the plus of C13 to R17 directly, and use a 3PDT to real true bypass the complete circuit...

...and I`d add a toggle switch to lift R18 (dry signal) from R17/19:
voila - vibrato-mode...



(this mod would save the expense of R16/Q4/D2/C16/R24;
the LED via R25 would be switched by the3rd footswitch-pole for on/off indication).

[EDIT]: folks that are recently being talked about in this thread:
http://diystompboxes.com/sboxforum/viewtopic.php?t=32790
might "sell" this mod as a "new" circuit...
:lol:

[jmasciswannabe]

yes and cut out the fet all together....makes total complete rational sense....now why didn't I think of that! geez....puretube, thanks a million for the help, I feel you've taught me a lot! and yes vibrato laong with some of the other "wavy gravy" mods are a must....can't wait to get home and get my hands on this sucker!

[jmasciswannabe]

did as advised and connected c13 to r17, adjusted the trim and tadah!!! chorus!! thanks again puretube! now on to mods!

take care!
ian

[puretube]