resistor value question on tonepad board

[CoolJohnny]

building one of francisco's boards (the mxr dist+/dod od250) and am having trouble getting anything besides less than unity low gain fartiness out of the thing. there are two resistors in the power section that call for 20K. i had some 22Ks laying around and used them. this isn't enough to cause a problem like i've described is it? cause it sounds like when the voltage is off when i'm breadboarding something with transistors. or is it a wiring thing?

[jkokura]

substituting a 22k resistor for a 20k is only %10 - it shouldn't be a big deal, but if you've got problems I wouldn't rule it out yet. Rule all other factors out and then it could be the issue, but at first theory I'd think it's fine to substitute those resistors.

Jacob

[anchovie]

Those resistors are used in the voltage divider that provides the V/2 bias for the op-amp. 22K won't be causing problems here - if you look at the parts list for the MXR circuit that uses the same board you'll see that in the MicroAmp they are 1Meg!

Follow all the steps in the "debugging" sticky thread and post back.

[petemoore]

  Another way to test the voltage divider R values is
measure the 1/2v divider made from 2 equal value resistors,
if it's reading about 1/2v .
 

[CoolJohnny]

thanks for the responses, guys. turns out, i think my lm741 was fried. i switched in lm301 and lm411 chips and the pedal worked fine. i simply thought that since those resistors were the only part i altered that would be the culprit. bought a new lm741 from radio snack today, and while the circuit works, it still doesn't seem to be getting the same gain/distortion i remember from my old one (even if it was the yellow reissue) and certainly less gain than the clips out there. not that i play very much ultra-high gain stuff, i'd still like to know why. for diodes i have a ge/si configuration, everything else is stock. any ideas, or tips for juicing more gain out of this thing. as it stands now, its a really good booster for a 'heavy clean' kind of sound.

[CoolJohnny]

here's a stupid question i should know the answer to by now. wiring up the 500K gain pot in reverse order, that would make it never acheive anything more than minimum gain, correct? cause i think thats what my dumbass did....

[anchovie]

If it was simply a case of reversing the wires, the pot would work backwards, i.e. fully anti-clockwise = maximum gain.

[CoolJohnny]

shite...poking around with a bare wire in the area of the drive pot joints i can make some kind of connection to get that full distortion but somehow cannot figure out what has gone wrong. all my joints look good and i don't want to risk resoldering and burn out traces uneccessarily.

its almost as if with the drive pot in its current state, fully counterclockwise gives me minimum distortion, and fully clockwise gives me nothing. i'm thinking something is wrong with the pot at this point. its been in a number of prototypes and breadboard configurations. i might have fried it internally somehow. guess ill have to dig out the multimeter. haven't had to use that thing in awhile.

frustrating, but i know this pedal/circuit is worht it in the end....

[anchovie]

i don't want to risk resoldering and burn out traces uneccessarily.

It won't be such a brutal operation if you're gentle! There isn't necessarily a need to take old solder off and put new on, just melt the existing joints and let them solidify again. A shiny joint is a healthy joint!

[CoolJohnny]

i know, but after all these years doing this, the intricate work of getting a good solder joint in the right place, with the right amount is what still trips me up; then there is the inevitable screw up that i have to repair. and i've found commerically-made boards less forgiving in this regard. i end up having to cut lead wire to bridge pulled pads and traces...ugly work. honestly, i'd love to spend my time xbreadboarding until i can't see straight, decide on a final circuit, then pay somebody else to assemble it. but i don't have that capability. some DIY-er i am.

[CoolJohnny]

replaced 500K pot. no difference in distortion amount. pleasing, but still way milder than it should be.