Wah debugging help bad inductor?

[deke99]

Hi all,

I've tried searching on this and can't find anything.  I'm trying to replace the dunlop wah circuitry using the standard wah project from general guitar gadgets.  I have the board populated and installed and I've triple checked the component placement and wiring.  I'm only getting sound when the wah is bypassed, nothing when it's engaged.

I'm wondering if the inductor is bad.  Using my meter's direct connection feature, I'm showing direct continuity through the inductor.  Is this normal?  I'm okay with trouble shooting the other parts, caps, resistors, transistors, etc.  but I really don't know anything about the inductor.  Is there any way to confirm if it's good or bad?

Also, the ggg site shows an install grid for a 4 prong inductor, I'm using the yellow fasel reissue with 2.   I have one leg going up to the jumper and the other goes to the 33K resistor.  With the direct continuity through the inductor I can put my meter leads on each side of the 33K resistor and get direct continuity.  Is this right??

Thanks in advance for all help!!

Derek

[R.G.]

Can you go up and read the sticky "what to do when it doesn't work" and provide that info, plus the voltages on each side of the inductor? This is easier if the info is not piecemeal.

[deke99]

Thanks R.G., sorry I didn't do this from the get go.  Here is the info:

1.What does it do, not do, and sound like? When switched off, signal passes through fine, when switched on, no sound
2.Name of the circuit = GGG Drop in wah replacement
3.Source of the circuit (URL of schematic or project) = http://www.generalguitargadgets.com/index.php?option=content&task=view&id=98&Itemid=126
4.Any modifications to the circuit? No
5.Any parts substitutions? If yes, list them. No
6.Positive ground to negative ground conversion? No
7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? => 9.74
Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:
Voltage at the circuit board end of the red battery lead = 9.56
Voltage at the circuit board end of the black battery lead = 0

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

Q1 MPSA18
C = 9.16
B = 1.37
E = 1.11

Q2 MPSA18
C= 8.04
B= 8.30
E= 8.03

Inductor Fasel Yellow reissue
1.59 V to 33K resistor
1.59 V to jumper

Please let me know if any additional info is needed.  At this point it kind of looks like Q2 is suspect and the inductor voltages match on both sides.  I'm not sure if that is good or bad.  Please advise.  Thanks again!!!!

[R.G.]

Q2 MPSA18
C= 8.04
B= 8.30
E= 8.03

OK, this is why there's no effect - Q2 is saturated on.  But how did it get that way? It's base looks awful high. Q2 is fed its DC bias from Q1, but it does not feed *back* to Q1. Therefore, Q2 cannot be the cause of this misbiasing.

Let's look back upstream.

Q1 MPSA18
C = 9.16
B = 1.37
E = 1.11

Hmmm... Collector looks awful high. Let's play ohm's law for a minute. The battery voltage reads 9.56V on the board, and the collector reads 9.16V. That means that 9.56-9.16 = 0.5V is dropped across the collector resistor. If the resistor is really 22K, then the current flowing there is 0.5V/22K = 23uA. But the emitter resistor has 1.1V across it, and if it's really 510 ohms, there is 2.12ma flowing through it. ACK!! The emitter current is bigger than the collector current by 100 times!?!

OK, we can do some more snooping. The collector of Q1 is at 9.16V and the inductor end of the 470K bias feedback resistor is at 1.59V. So there's
(9.16-1.59)/470K = 16uA flowing through it. Of that, 1.59/100K = 15.9uA flows through the 100K to ground and 0.1uA is left to bias Q1's base. Even more importantly, we notice that the base is at 1.37V and the emitter is at 1.11V, only 0.25V across the base-emitter.

These taken together are highly suggestive of a problem with Q1. Q1 has almost no base drive, and it's only because of the insanely high gain of the MPSA18 that it's conducting at all. While I can't just say "the murderer is...", something is quite wrong with Q1. Possibilities are
- incorrect component value (Q1 collector resistor, or that 1.5K input resistor are likely suspects)
- bad solder joint
- solder short on the board
- hairline fracture on the PCB traces.

I would bet on the first or second one before going any further.

The inductor may or may not be bad, but it almost doesn't matter until we clear up the problem with the transistor biasing.




Inductor Fasel Yellow reissue
1.59 V to 33K resistor
1.59 V to jumper

Please let me know if any additional info is needed. At this point it kind of looks like Q2 is suspect and the inductor voltages match on both sides. I'm not sure if that is good or bad. Please advise. Thanks again!!!!

[R.G.]

Inductor Fasel Yellow reissue
1.59 V to 33K resistor
1.59 V to jumper

Please let me know if any additional info is needed. At this point it kind of looks like Q2 is suspect and the inductor voltages match on both sides. I'm not sure if that is good or bad. Please advise. Thanks again!!!!

I copied this in for quoting and neglected to remove it. Ignore this stuff at the end of my note.

[deke99]

THANK YOU for all your input R.G.  It looks like my 1.5K input resistor is bad along with a few others.  I bought the $.05 KOA resistors from mouser and I think that may have been a mistake.  I'll fiddle around and report back.  Thanks again!!!

Derek

[deke99]

UPDATE:  The wah now works.  It really helps to get away from the project for a day or two and then go back.  In doing so I realized that I had a 510K resistor where a 510 resistor was called for (this is the resistor off of Q1's emitter that goes to ground).  This was screwing up the voltages everywhere.

I was triple checking the schematic in a tizzy, as soon as I walked away and came back a few days later, it stuck out like a sore thumb!

Thanks again R.G. for all of your help with this project!

Derek