Colorsound SupaWah

WelshWonder · March 10, 2007, 01:37:37 PM

I'm trying to find out the problem with a freinds colorsound supa wah. The symptom is that there is no of very little output. I've looked at it with a scope and noticed there a only a small amount of signal every reaching the output tip of the jack. I've also noticed using an audioprobe that there is no signal reaching the other end of the inductor which leads me to think the inductor is faulty. However, I also noticed that there is no output on the collectors of the 2 BC209 NPN's. Are the transistors bad or is it the inductor?

tcobretti · March 10, 2007, 08:24:47 PM

What are the voltages on the transistors?

WelshWonder · March 11, 2007, 02:21:59 PM

Voltages are as follows...

Battery: 9.23V

Q1 (BC209)
Collector: 9.23V
Base: 2.31V
Emitter: 2.74V

Q2 (BC209)
Collector: 3.63V
Base: 0.58V
Emitter: 0V

Inductor
In: 0.58V
Out: 0.58V

Well it seems that current is flowing through the inductor albeit a small voltage so I guess I can rule out a faulty inductor. How about the Transistors though? Q2 seems well under powered compared to Q1?

tcobretti · March 11, 2007, 02:41:54 PM

From the wiki:

Remember that for an NPN silicon transistor to be working as a linear amplifier, it MUST have its base higher than its emitter by about one silicon diode drop, 0.5 to 0.7V, and the collector must be higher than either base or collector.

So there's a problem with Q1, and Q2 looks possibly a little off also. Have any mods been done to the pedal?

WelshWonder · March 11, 2007, 02:49:05 PM

Thanks for that. No I don't think any mods have been done on it. But someone before me did have a go at soldering a new + lead from the battery so yo never know. Looking at the board though, everything appears to be the original solder etc. As for Q1, the 9V from the battery has no components in front of it to resist or stop DC going to the collector so maybe the +9V is soldered to the wrong hole.

WelshWonder · March 11, 2007, 02:52:35 PM

Quote from: WelshWonder on March 11, 2007, 02:49:05 PM
As for Q1, the 9V from the battery has no components in front of it to resist or stop DC going to the collector so maybe the +9V is soldered to the wrong hole.

Actually, scrap that thought, that's incorrect, maybe there is a dodgy resistor incorrectly biasing the base of the NPN's. I may try putting new one's in, I got BC109's, do you think they'll be a good replacement?

tcobretti · March 11, 2007, 03:21:35 PM

The resistor at the base of Q1 might be a problem. Just measure it with a multimeter to see if it is doing what it should. The transistors could be bad, especially if the battery clip is wired backwards. So, I'd check that the battery leads are wired correctly. Then I might try new transistors. I'd heat sink the old ones as I pulled them so if they aren't the problem you can put 'em back in and restore the pedal's mojo. Another thing you might want to try is replacing the electrolytic cap by the inductor. Electro's can go bad with age so it is another culprit; however, if it failed I don't think you'd have the problems you are having.

These are ideas about what could be wrong. That said, I don't know how likely I think they are. Usually component failure is not likely, but on older pedals it is a possibility. I would look very, very closely at the way the pedal is wired, and I'd check for solder bridges and debris on the PCB's top and bottom that might short out the circuit.

WelshWonder · March 11, 2007, 03:28:19 PM

Well the pedal is an original from 1977 so I guess some componants can fail. I checked the resistor biasing the base of Q1 and it is reading 480k so I guess this is okay. I've checked the wiring and it does seem fine. The positive is soldered to the board runing up to Q1 and the negative is on the ring of the output jack so it acts as a switch. I'll try replacing Q1 with a BC109 and see how that goes, then I think I'll try the cap near the inductor. I'll let you know how it goes. Thanks for your advice,

Kevin

tcobretti · March 11, 2007, 03:50:13 PM

If the resistor on the base of Q1 is actually measuring 480k that's a problem. 480 ohms would be ok.

WelshWonder · March 11, 2007, 03:59:46 PM

As an update, I changed Q1 to a BC109 - No change

Replaced the 4.7uF cap with a new one - success! Signal now coming through but it's significantly higher gain than I expected. , this may be due to the BC109 reading something like 480Hfe so I may change it to something smaller or reinstate the BC209 I removed.

I also noticed that the switch is faulty, the output pin is slightly loose and cuts out signal when I move it with my finger. Another strange obesrvation is that it is picking up stray radio frequencies and they are playing through my amplifier as I type. I'll change the switch now to see how that goes and reinstate the BC209.

Funnily enough though that resistor I checked biasing Q1 is now not showing any reading, so this may be faulty too.

Readings - Q1 (BC109)
Base: 2.26V
Collector: 9.11V
Emitter: 2.65V

WelshWonder · March 11, 2007, 04:02:33 PM

That Q1 biasing resistor is now fluctuating between 1M and 1.1M

tcobretti · March 11, 2007, 04:07:36 PM

If it's the resistor on the emitter of Q1 that's acting weird, I would certainly replace it with something around 500 ohms.

WelshWonder · March 11, 2007, 04:40:21 PM

I put the BC209 back in and replaced the switch for an identical one and the switch problem is fine now although there is a harshness about the wah when it's engaged. You mention replacing the resistor to something around 500ohm's, well I found a schematic for a coloursound wah swell and the biasing resistor on this one is 470k. So I did replace it for a newer 470k resistor. Would you still say try a 500R type resistor?

http://www.schematicheaven.com/effects/colorsound_wahswell.pdf

tcobretti · March 11, 2007, 05:37:04 PM

I am mistaken. In most wahs there is a 500R resistor from Q1's emitter to gnd, but there isn't one this pedal. I misread your original post about the resistor. What confused me is you said Q1 when I'm pretty sure all the 470k resistors connect to Q2.

Anyway, I'm glad you got it working. So, it wahs when you move the pedal, but is harsh sounding? By harsh you mean bright and slightly distorted?

WelshWonder · March 11, 2007, 05:40:19 PM

Quote from: tcobretti on March 11, 2007, 05:37:04 PM
I am mistaken. In most wahs there is a 500R resistor from Q1's emitter to gnd, but there isn't one this pedal. I misread your original post about the resistor. What confused me is you said Q1 when I'm pretty sure all the 470k resistors connect to Q2.

Anyway, I'm glad you got it working. So, it wahs when you move the pedal, but is harsh sounding? By harsh you mean bright and slightly distorted?

No problem bout the misunderstanding, it was partly my fault...

As for the harshness, it doesn't have that nice a sweep and could go more trebly in my opinion and yes it's slightly distorted. What do you think this would be down to? any of the transistors?

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