Wha-wha trouble Solved

syndromet · February 25, 2006, 09:24:47 AM

Hi,
today I went out to get myself a multimeter, so that I could try to figure out why my Jen Super Crybaby stopped working. It is still giving signal when bypassed, but when I activate the circuit it is dead silent...

I suspect I might have given it 9 v with the wrong polarisation. Could this destroy some of the components?

I started reading the resistance over the different components. That's when I started wondering if you should be able to read voltages over the caps? All the caps in the circuit, exept the input cap, showed some resistance. The input-cap showed nothing. Could this cap be the problem? How do I check if a capacitor is working?

Hope someone could help me figuring this out.

syndromet · February 26, 2006, 03:39:56 PM

The capacitor is a green mylar, with 3k50 printed on it. I have done my searching, but since my english is not that good I could not understand what value it is? I'll just try to change it, and see what happens.. Anyone?

syndromet · February 28, 2006, 01:49:24 PM

Changing the cap did not work at all..... Anyone have any other ideas?

AL · February 28, 2006, 02:05:30 PM

Check the switch.

Reversing the polarity may have blown a protection diode. I'm not familiar with your circuit. A schematic would be helpful.

AL

Paul Marossy · February 28, 2006, 02:14:21 PM

QuoteReversing the polarity may have blown a protection diode.

It shouldn't have, that's what it's there for - protection against accidental polarity reversal. There's only three active components in the typical CryBaby circuit, so that's one place to look for trouble. The non-electrolytic caps don't really ever go bad unless they are physically damaged somehow. If it were the 4.7uF electrolytic cap, then it should be acting more or less like a volume pedal. Completely dead could also mean that the circuit isn't getting power, maybe a bad battery snap or a corroded DC power jack.

My suggestion would be to make an audio probe and go thru the circuit from the input onwards and see where the signal stops. If it's the transistors, worst case scenario is that you will have to replace three transistors.

AL · February 28, 2006, 04:01:24 PM

I'll second, third, and fourth the Audio Probe.

You'll be very happy you made this.

AL

lethargytartare · February 28, 2006, 07:32:12 PM

On the topic of the probe...

The thing I don't understand when trying to trace a circuit with a probe is what to do when you reach things that fork -- like a transistor. I'm debugging a wah I built, and I'm getting odd results (first nothing, then it behaved like a volume, then nothing again). I've swapped the transistors in and out a few times with different ones, new identical ones, etc. So then I got the probe, but when I get to the first transistor, I'm not sure what I should expect on the "other side" -- should one leg have some noise and the other none, noise from both, from none? That's the part that confused me. But I'm still plugging away :-) I used a bunch of really old composite resistors, so, while they all read the right resistances, I'm accepting the fact that they may be the problem...

the debugging continues...

ltt

Paul Marossy · March 01, 2006, 10:31:47 AM

QuoteThe thing I don't understand when trying to trace a circuit with a probe is what to do when you reach things that fork -- like a transistor.

That's easy. Your signal going into to the base would be a certain level. Depending on if it's an emitter follower circuit or a common collector circuit, the signal leaving the emitter/collector should be equal to (if unity gain) or more than the signal that is going to the base. If the signal stops at the transistor, then there is some kind of problem with it, which isn't necessarily the transistor itself. Of course, you need to know the pinout of the transistor in question to be 100% sure of your findings.

Does that help?

syndromet · March 02, 2006, 10:37:46 AM

I did my probing, and found a resistor that blocked all of the signal. This is the third component in the sircuit, and I have not tested the rest of the components. Anyway, I guess I found my trouble, and I'm gonna order a copy of all of the the components onboard, so that I can change any other broken components.

Thanks for the help.

lethargytartare · March 02, 2006, 04:39:23 PM

Immensely -- many thanks, Paul!

ltt

Quote from: Paul Marossy on March 01, 2006, 10:31:47 AM
QuoteThe thing I don't understand when trying to trace a circuit with a probe is what to do when you reach things that fork -- like a transistor.

That's easy. Your signal going into to the base would be a certain level. Depending on if it's an emitter follower circuit or a common collector circuit, the signal leaving the emitter/collector should be equal to (if unity gain) or more than the signal that is going to the base. If the signal stops at the transistor, then there is some kind of problem with it, which isn't necessarily the transistor itself. Of course, you need to know the pinout of the transistor in question to be 100% sure of your findings.

Does that help?

syndromet · March 03, 2006, 06:12:04 AM

I drew up a scheme for the circuit, but I can not guaranty that it is right. I thought it was to bad that there is no scheme of this classic available.

I tried changing the resistor with a similar one, and I still got no sound. It is the first resistor in the scheme, and I get sound from the first pin of it. At the other pin it is oh so quiet... I also measured the original resistor, and it showed 67k, just like it should. Why don't I get sound from the other side??? could a broken transistor do this? I also get the right reading, measuring the resistor in the circuit.

http://www.geocities.com/n_h_k_l/jencry.JPG

nero1985 · March 03, 2006, 08:32:21 AM

can you post a picture of the board? mabye both sides? so we can take a look too

syndromet · March 03, 2006, 02:46:14 PM

Yupp!
Here is the top and bottom of the board...
http://www.geocities.com/n_h_k_l/pcb.JPG
http://www.geocities.com/n_h_k_l/toppJPG

sorry for the bad pics, but my camera is not that good with close-ups.

What transistors would be a good substitution for the bc238b and the bc 239c that is inn there now?
This wha is driving me mad. I think this is a way to big project for a noob like me.....

deadsnake · March 03, 2006, 04:40:20 PM

The top view link of your PCB is dead.
As for the Cap and Res reading you did. Those components need to be ofboard to be acuratelly measured since current will choose the easiest way out. For eg. if you have a Cap in paralel with a Res and you try to read them from their legs you will probably get a wrong value for them. Try desoldering one of their legs and measuring as normal.

syndromet · March 03, 2006, 05:06:10 PM

Quote from: deadsnake on March 03, 2006, 04:40:20 PM
The top view link of your PCB is dead.
As for the Cap and Res reading you did. Those components need to be ofboard to be acuratelly measured since current will choose the easiest way out. For eg. if you have a Cap in paralel with a Res and you try to read them from their legs you will probably get a wrong value for them. Try desoldering one of their legs and measuring as normal.

Thanx for the tip. I Have tried desoldering both the components, and they are working as they should. The signal still stopps after the first resistor, so I understand nothing.

johngreene · March 03, 2006, 05:07:07 PM

Since you have a multimeter now, measure the voltage at each of the transistor's pins.

I think with the exception of a couple of values, this schematic is correct for your wah.

It looks like the battery clip has been replaced before, you may want to check that it was soldered securely as I think I can see a wire end sticking out of the black tape.

--john

syndromet · March 04, 2006, 03:08:32 AM

Quote from: johngreene on March 03, 2006, 05:07:07 PM
Since you have a multimeter now, measure the voltage at each of the transistor's pins.

I think with the exception of a couple of values, this schematic is correct for your wah.

It looks like the battery clip has been replaced before, you may want to check that it was soldered securely as I think I can see a wire end sticking out of the black tape.

--john

Well, I measured the trannies, and I found no voltage on either one of them. That made me check my battery connection, and the wires was all fine. ( I still replaced the battery snap). Then I took a look at the dc in plug, and it showed that the two plates on the jack that should connect didn't. I bended the plates back in place, and everyting worked as it should. That means that I have a working wha, and have learned a lot about debugging abd how things work. Things could not be better, and I could not be more happy!! Thanx for the help, everyone!

Paul Marossy · March 04, 2006, 03:21:54 PM

Yeah, a lot of completely dead FX that have a DC power jack are often dead because of something with that darn DC jack.

Glad you got it working.

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Wha-wha trouble Solved