Dunlop crybaby troubleshooting

[potul]

Hi

I got a non-working Dunlop crybaby GCB-95 with the hope to fix it myself. After initial troubleshooting with an audio probe I could see the issue is in the input buffer. If I bypass it completely and just send the input to the base of Q1, everything works as expected. This is the reference schematic I'm using:


As I could not see anything wrong, I thought it might be the transitor, so I changed it for another darlington. But, still not working. When measuring DC, looks like the transistor is not biased correctly. In the emitter of Q0 I get 0V instead of the 3V that the troubleshooting guide suggests. In the Base of Q0 I get around 0.6V.  The collector gets a good 8.8V.



Now the weird part comes. I discovered that if I briefly short the collector and base of Q0, the buffer works for some seconds. If I measure during this transient the voltages, the emitter goes higher than 2v, and then drops gradually. When it goes below 1 V or so, the sound  fades.

So, to me it looks like for some reason the transistor is not biased correctly, and when I short base and collector I'm probably charging a condenser that then discharges slowly.

Before starting to change components, I want to be in the ballpark of what to change (I'm bad at desoldering, and already damaged the PCB when changing the darlington). Any ideas?

Mat

[GibsonGM]

I'd make sure the 10k to ground at the emitter is ok; 
is the .01u coupling cap after it ok? 
That 1.5k resistor after it?   

Yes, some odd symptoms, but there are things to check...

and you are sure your new darlington went in with correct pinout, right?  Have to ask!

[thermionix]

Just FYI, that Electrosmash schematic is not 100% correct, but probably good enough for the troubleshooting you're doing.

IMO, you'd end up with a much better sounding wah if you bypass/remove the input buffer.  Then wire for true bypass to avoid "tone suck" when the wah is off.  You'd need a DPDT foot switch for that.

[potul]

Just FYI, that Electrosmash schematic is not 100% correct, but probably good enough for the troubleshooting you're doing.

IMO, you'd end up with a much better sounding wah if you bypass/remove the input buffer.  Then wire for true bypass to avoid "tone suck" when the wah is off.  You'd need a DPDT foot switch for that.

That's my plan B. .... I know it works because when I bypass the buffer it works fine.
But I still prefer to fix it, I hate to have something not working without knowing what.
Regarding the schematic, I will hunt for something else... In fact, I realized I have some extra capacitors not shown in the schematic.

I'd make sure the 10k to ground at the emitter is ok; 
is the .01u coupling cap after it ok? 
That 1.5k resistor after it?   

Yes, some odd symptoms, but there are things to check...

and you are sure your new darlington went in with correct pinout, right?  Have to ask!

The darlington pinout is reversed compared to the original (I used a BC517), but I did consider it when installing.
I did not check C1, but when I sound probe at the emiter of Q0, no sound.... SO it looks like the problem occurs before C1. The 10K resistor seems to be ok. I will check the other  suggestions.



Mateu

[GibsonGM]

I ask about that cap because MAYBE your thought about a bias change could be related, if that cap was very leaky...maybe.  You sure should have more than .6V at the base, and you don't, so WHY?  And that is telling us something...could someone have got in there and "modified' it?

Me doing things the way I do, I'd lift one leg of the coupling cap to isolate the buffer (after checking all connections to be sure they are solid, that the ground there isn't 'interrupted' somehow...the little things...values of all parts etc).   I'd look for DC in the INPUT jack (leaky input cap?)
Admittedly this is 'easter-egging' but it's a simple stage, and would take only a few minutes to dig thru it.

Thermionix is spot-on, you don't need the buffer, so if you get stumped you can omit it      Or build a different, new one....

[potul]

I ask about that cap because MAYBE your thought about a bias change could be related, if that cap was very leaky...maybe.  You sure should have more than .6V at the base, and you don't, so WHY?  And that is telling us something...could someone have got in there and "modified' it?

Me doing things the way I do, I'd lift one leg of the coupling cap to isolate the buffer (after checking all connections to be sure they are solid, that the ground there isn't 'interrupted' somehow...the little things...values of all parts etc).   I'd look for DC in the INPUT jack (leaky input cap?)
Admittedly this is 'easter-egging' but it's a simple stage, and would take only a few minutes to dig thru it.

Thermionix is spot-on, you don't need the buffer, so if you get stumped you can omit it      Or build a different, new one....

I will do further checks tonite... Pin lifting party!

In fact, I bought it thinking that worse case I can use the rocker pedal to build something... But now that I see it's only the buffer, I will try to fix it first, and if not I will modify for true bypass without buffer.

[GibsonGM]

Cool, a wah pedal can be used for many great things!       

Forgot to mention - for testing you're only using guitar > wah > amp, right?  Good to keep things isolated when they're buggy, as a pedal before could potentially affect its operation...

[potul]

...  I'd look for DC in the INPUT jack (leaky input cap?)

Now that you mention it... it's weird. My Crybaby does not have in input cap (Cin1 in the above schem), instead it has a resistor... So most probably I will have some DC in there   

Initially I thought the previous owner screw it up, but I've seen some pictures in the net of pcbs like mine, with a resistor instead of a capacitor...

I will try to replace it for a cap.

This is my pcb. The highlighted part is where the input capacitor should be.

[potul]

Cool, a wah pedal can be used for many great things!       

Forgot to mention - for testing you're only using guitar > wah > amp, right?  Good to keep things isolated when they're buggy, as a pedal before could potentially affect its operation...

Instead of a guitar I use a looper playing a neverending riff... This way I have 2 free hands.

[GibsonGM]

I suggest you try it with guitar > wah > amp...I know, I know, but maybe it's working with the looper in some bad way.

There REALLY should be a cap at the input!!  Look hard, try to "zap out" the path with your meter...really ought to be there!  Can't fathom someone removing it...so measure for DC across the input jack lugs...be aware that 1 lug likely goes to ground, 1 is open til a plug is inserted, and 1 is the 'hot' signal wire.   So you need to measure from signal wire to gnd.    Just put one probe on the signal wire, one to battery "-" with a plug inserted if you like.

That would account for a lot of things, if true, but I'll be amazed if someone pulled the input cap!   Should be simple enough to sound out with meter on 'continuity', or ohms, or by finger-tracing...it's usually something stupid like that which causes these things!

[thermionix]

That resistor is smaller than the others on the board.  I have two GCB-95 boards on hand and they both have caps there.  My guess is that someone added that resistor.  I suspect they got confused by a mod they found online.

[pinkjimiphoton]

shorted cap could do exactly what you're saying. not much there to go wrong tho. temp lift one side of the picofarad cap and see if it works.

also, you say you replaced the darlington and reversed the pinout, BUT in the pic you posted, the pinout isn't reversed, so the q could be reverse beta'd and that too could do exactly what you describe.

when there's a short, the cap somewhere is charging and feeding enough energy to temporarily bias the q on. it may be the q, but my bet is on the cap or the resistor you have circled being open.

quite often them shit resistors that are so ubiquitous these days "internally decapitate" and look fine. they even FEEL solid if ya tug on 'em but if ya check with a meter no continuity at all. usually after you unsolder them, the cap with the lead on one side will fall right off.

first thing i'd do is replace that resistor. use somewhere between about 39k and 68k and that should give ya some sqwahck.

then i'd replace the electros on the board just because the odds are they are shite. in fact, most of the dead wahs i've encountered it's either the diode is blown from bad power, or the electos are bulging. while electros are partially "self healing" to a point, over time shitty power from shitty power supplies in shitty rooms and clubs and venues takes its toll. they're 2 cents a piece at tayda so ....

but sounds to me like you already got it sussed. well done man.

[potul]

also, you say you replaced the darlington and reversed the pinout, BUT in the pic you posted, the pinout isn't reversed, so the q could be reverse beta'd and that too could do exactly what you describe.

I knew this would create some confusion... the picture is from the pcb BEFORE replacing the darlinton. As it arrived home.

As you said,.. the previous of the previous owner did some changes in components. I can tell it from the solder joints. I will need to recheck everything.

This weekend I will do some more testing

[deadastronaut]

just a heads up...there are a few 'revision' versions of the gcb95 pcb.

http://stinkfoot.se/archives/546


http://www.electrosmash.com/crybaby-gcb-95

[GibsonGM]

If the pF cap was shorted, how can we explain the mysterious ".6V" on the base?   It could just be the meter can't resolve well enough, shows 10ths of volts when there's nothing...I"m seeing that as a diode drop, which doesn't make sense.   

Basic testing...if there's a short, you'll have continuity with ground from the base.  Often I list these things that seem like a big laundry list to do, but in reality takes like 4 minutes...dc on the jack...continuity to ground from each transistor leg, etc.    If some myron got in there and changed things, then it didn't work, maybe he used a really strange resistor value, so you need to be sure of THAT, too...

[pinkjimiphoton]

another potential heroic effort in true bypassing gone bad


thats almost sigworthy lol

po, you will no doubt sort this out bro, just keep replacing parts and you'll get there. look at the links rob posted,
find your model board and you can look up the specific circuit, or make up your own dream wah.

[GibsonGM]

Basically, yeah! LOL   

WTH does it need a Darlington in there for??  Or a buffer at ALL?   I like the idea of ripping all buffer things out, keep the Darling for another project, feed it into the 2nd section.   But I do understand that needing to know what's going on, it's good practice.  Somebody jiggered it up good, I bet...

[antonis]

WTH does it need a Darlington in there for??  Or a buffer at ALL?   

I would dare to whisper, MAYBE for a high imput impedance, Sir... 
(considering bias resistors values, an ordinary BJT with 10k Emitter resistor would dominate the total imput impedance..)

[GibsonGM]

WTH does it need a Darlington in there for??  Or a buffer at ALL?   

I would dare to whisper, MAYBE for a high imput impedance, Sir... 
(considering bias resistors values, an ordinary BJT with 10k Emitter resistor would dominate the total imput impedance..)

I see.     That does make sense, Antonis.   

I play a Les Paul with a very bright bridge pickup...for me, the tone becomes much nicer with the lower input Z of my wah.   And a long cable. 

[antonis]

<off-topic ON>

Rhetorical question, self-aimed by Sir Mike:
Shouldn't be better to "load" a little my Les Paul output instead of desperately looking for effects with relatively low input impedance and long cables with stray capacitance..??

Rhetorical answer, answerd by his Highness:
WTF, I'll just place a MosFet booster at the end and go for another beer..!!

<off topic OFF>