This dynacomp is killing me! I need help debugging...

[syndromet]

I' building a Dynacomp from tonepads layout, and all i get is a distorted, gated sound. I have narrowed it down to a problem with Vb created by the 56K/27K voltage divider, but I can't understand why my voltages are so low here. Seems like everybody else is getting close to 4.5v, while mine reads out 2.59 volts.

So, here we go.

1.What does it do, not do, and sound like? It's a weak, distorted and gated guitarsound

2.Name of the circuit = MXR Dynacomp

3.Source of the circuit (URL of schematic or project) = http://tonepad.com/getFile.asp?id=9

4.Any modifications to the circuit? Changed the 2k trimpot to two 1k resistors, added a 10k resistor for the recovery mod from Fuzzcentral


5.Any parts substitutions? If yes, list them. No

6.Positive ground to negative ground conversion? Y or N 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? => 9,00V

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 = 8,83V
Voltage at the circuit board end of the black battery lead = 0v

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
C =8,83
B =2,61
E =2,05

Q2  Something's wrong here, right?
C=0,85
B=1,38
E=0,71

Q3
C =0,14
B =0,00
E =0,00

Q4
C =7,63
B =0,04
E =0,00


Q5  WTF?
C =8,86
B =7,64
E =8,63

IC1 (or U1)
P1 0
P2 2,95
P3 3,55
P4 0,00
P5 0,70
P6 1,37
P7 8,88
P8 0,00

D1
A (anode, the non-band end) = 0V
K (cathode, the banded end) = 8,88

D2
A = 0,00
K =0,1

D3
A = 0,00
K =0,00

I also get 2,59V at Vb, wich is supposed to be 1/2 of V+, right?
Anyone have any ideas of what I should check next? I'm possitive all components are mounted the right way, and I'we cut everywhere there could have been a solderbridge. Guess that is still the problem somewhere, but I don't know where to start looking. Thank you!

[aron]

Vb should be around 2.88 volts in ideal conditions based on 8.84V battery. In any case, it's not 4.42V. There must be a miswire or... have you triple checked your transistor pinouts as well?

[R.G.]

I' building a Dynacomp from tonepads layout, and all i get is a distorted, gated sound. I have narrowed it down to a problem with Vb created by the 56K/27K voltage divider, but I can't understand why my voltages are so low here. Seems like everybody else is getting close to 4.5v, while mine reads out 2.59 volts.

Let's play math for a minute.

A 56k/27K divider can't produce 4.5V from 9V. The calculation goes
Vbias = 9V * 27K/(56K+27K) = 9* (27/83)= 2.9V. I'd say that your divider is very close to right, taking into account loading on it. But there are other problems.

Thank you so much for providing an actual link to the schematic you used so I didn't have to go searching the net for it.

4.Any modifications to the circuit? Changed the 2k trimpot to two 1k resistors, added a 10k resistor for the recovery mod from Fuzzcentral

I'm not familiar with the "recovery mod", so we'll ignore that for a moment.

Q1
C =8,83
B =2,61
E =2,05

This is OK, and it reflects the bias divider we calculated eariler.

Q2  Something's wrong here, right?
C=0,85
B=1,38
E=0,71

Oh, yeah.
The base of the transistor should be tied to the same bias voltage, and should be nearly the same as Q1. The IC's output is a current source and goes to whatever voltage it's told to. So you have one problem here. Check both ends of that 150K resistor from Vb to the base and see if you have the right bias on one end. If not, you have a solder or board problem getting Vb to the 150K. If yes, then you have some problem with the base or emitter of Q2 being pulled down. The collector being so low hints that the path from the collector through the 10K to +9V is open, possibly from a board problem or soldering problems. Look there first. As always check the pinout for the specific transistors and manufacturer you used.

Q3
C =0,14
B =0,00
E =0,00

Q4
C =7,63
B =0,04
E =0,00

Q3 and Q4 collectors should be at the same voltage because according to the schematic, they're connected by a wire. If they have different voltages, there ...can't... be a wire connecting them, can there? Solder or board problem.

Q5  WTF?
C =8,86
B =7,64
E =8,63

Actually, this one is right where I think it should be. 

IC1 (or U1)
P1 0
P2 2,95
P3 3,55
P4 0,00
P5 0,70
P6 1,37
P7 8,88
P8 0,00

This reflects the bias problem with Q2. Fix Q2, then come back to this.

[aron]

Look at the difference in knowledge between my post and R.G.'s hehehehehehe

[R.G.]

Look at the difference in knowledge between my post and R.G.'s hehehehehehe

You did, however, center right in on the biggest difficulty he's having. Not too shabby at all.

[Ben N]

Look at the difference in knowledge between my post and R.G.'s hehehehehehe

You did, however, center right in on the biggest difficulty he's having. Not too shabby at all.

Louis, I think this is the beginning of a beautiful friendship.

Carry on.

[owenjames]

I think gtoudoran had a similar problem. His turned out to be a mistake on the pcb, i think he got his pcb from tonepad to. You might want to pm him to see where the problem was.

[syndromet]

Thank you so much, guys. I found a bridge between q2 and pin5 on the ship, and a bad solder joint on the 10K resitor from 9v. Now it compresses nicely, just like it should. 

[R.G.]

It's almost impossible to believe when your newly-made pedal won't work, but almost all new-construction failures are just that *construction* errors, not bad parts. It takes quite a bit of experience not to get all adrenaline-saturated when that happens.