DIY Crybaby rebuild issues

[MrStab]

hi guys,

so basically, i did something stupid the other day and even minor contact with a heavy power tool seemed to screw up the inductor in my modded GCB-95 CryBaby. upon trying to fix it, i figured the board was so annoyingly damaged on the underside thanks to my former, non-DIY-er self, and much of it was crudely wired point-to-point, so i figured screw it - i've made more complicated circuits than this. i'll make a new one and then some. or not.

i should mention that i've since opened up the red Fasel inductor in a last-ditch effort to avoid buying a new one, put blobs of solder on the tiny wires to the pins, and then it seemed to start wah-ing again. here's the thing though - it only "properly" wahs the buzz when i touch the 2k resistor in the feedback loop (?) going into the NPN on the left (which in this case is a 2n5088). the signal  itself seemed unaffected at first, but now seems weakly and wrongly affected. it's like the NPN isn't letting the wah affect the signal but i can't see anything wrong with it.

also, i thought i'd get rid of the input buffer and replace it with an opamp in/out buffer in the process (it already had a working, retrofitted opamp output buffer before). the buffers themselves seem fine. so now, after some of the usual debug fixes, it passes signal, but upon probing i've run into signal loss/too weak a signal around the base of the first NPN and it seems to be related to a weak, dysfunctional wahing. the collector of the NPN seems fine though. also there's a kinda click when the wah pot is all the way up, like a scratchy pot but the width of the scratch on the taper goes up when increase the resistance in series with the inductor.

hopefully i'm not being too confusing. here's a crude diagram + vero layout (hopefully not too inaccurate):



voltages on opamp (OPA2227 but same problem with TL072):
pins 1,2,3,5,6,7 = ~4.28
pin 4 = 0
pin 8 = 8.59

Q1:
C = 4.43
B = 0.61
E = 0.04

Q2:
C = 6.83
B = 2.81
E = 3.34

there are a few weird artefacts caused by fixing errors/stupidity however i could on the vero, ie. different rails for the wah & buffers.
there's also a Millennium Bypass on the board.

could it be that the inductor's just screwed, or something else?

any help appreciated

[tubegeek]

When I look at that drawing, what stops me is the input circuit - you've got an input impedance at the + input of OA1 of 10K to ground in parallel with 10K to your power supply, in parallel with the reactance of 100nF. At 1KHz that cap's reactance is only 1.6K.

A buffer is supposed to be HIGH input impedance and you have LOW input impedance - where we would hope to have over 1Mohm loading your guitar, you've got 5K at DC and going lower with increasing frequency as the cap passes the input signal to ground more and more.

Try this: put 1M from the junction of R1 with R2 and C4, to the +in of the opamp. Put the right hand end of the .01UF input cap straight to the +in end of the new resistor.

Then, I think, maybe your buffer will buffer? What it looks like to me now is, your bias network is loading down your input like crazy.

[MrStab]

thanks for the response, tubegeek

would there be a more layout-friendly way of accomplishing this, maybe changing the actual voltage divider resistors to 1M+ instead? otherwise i'll do as you advised - it'd just require moving more stuff around.

fwiw, i'd tried just taking the opamp out the socket and jumping the + inputs & outputs at one point, but still had wah weirdness.
i'm having voltage issues with Q1 atm, but hopefully just needs a replacement

[MrStab]

*suddenly learns how to actually work out input impedance* (i think...)

[MrStab]

ok, so i replaced the 10k resistors in the input buffer with 3.3Ms (all i had in the region) - 1.65M as opposed to the original GCB-95's 1M (iirc)....? anyway, it seemed to fix the strength of the signal a lot, and now i'd say i get more or less full clean signal (the output buffer resistors are still 10k, i'll increase them later).

i replaced the transistors, and voltages seem ok again, but still no wah. good news is, when i touch between the 2k feedback resistors to Q1's base (2x 1k in series), and press down the pedal, there's a crackle-free wah buzz. but it still doesn't affect the signal, which leads me to think there must be something up with my vero layout. i can't spot anything, though.

[tubegeek]

Are you sure you fixed the inductor? If there is a short between turns, it will have much, much less inductance than if it is in its original state.

Good news on the buffer though....

What I did to "see" the input impedance was replace all the caps with a short circuit (that's roughly the state for very high frequencies) and see what the resistances to ground and/or power supply looked like. The power supply is a low impedance path to ground FOR AC if it's done right, which it is (big cap between +9 and ground.)

Next you can replace all the caps with an open circuit which will tell you what's going on at DC, for evaluating the bias network.

[MrStab]

i almost took you literally on swapping out all the caps! thanks for the insights. one key difference between the opamp bias with 10k and with 3.3M resistors was the characteristic loading-down of my crap DMM on the + input pin, but solid half-voltage on the - input and output. so when that happened i had a feeling things were on the way up.

i'm not sure if i've fixed the inductor or not, tbh - that's what i'm trying to figure out now i guess. i'm trying like hell to get this working before thursday so i'll have to be quick with ordering and rely on luck to get a new Fasel on time, and the sooner it seems worth ordering a new part, the better. iirc, i measured 16R across it. is that about right?

just seems weird how the wah-ing buzz seems strong but it's just not affecting the signal - surely i wouldn't even get that if the inductor was screwed?

[MrStab]

upon unbolting the wah pot to check the full range (which i'd done before but not post-buffer-fix), it seems that the wah is affecting the signal, but very weakly. so i guess all i can do is order a new inductor - and maybe a cheap pot to make it through a coupla gigs, in case that's dying too - and see if i can rule out any other errors in the meantime.

[MrStab]

after removing the 100nF to ground in the output buffer whilst changing the 10k resisters round opamp B, i got a faint but usable wah. after removing the 100nF to ground on opamp A, i got wah so intense that i had to measure it with a geiger counter.

...if only i had worked this out before paying ~£20 to have another Fasel shipped out

i didn't even think to look at the impedance. thanks tubegeek!

[tubegeek]

Glad to be of service.