GGG ITS8 Volume drop when engaged

[Bassbob]

Hi you all out there! I'm seeking help.

The ITS8 is my second build and unfortunatly it has a volume drop when the effect is engaged.
Schematic can be found here http://www.generalguitargadgets.com/pdf/ggg_its8_sc.pdf
Because it was missing from the kit I had to put a 240ohm at R11 instead of a 220ohm.
Here's the voltage measurments:

Power supply 9,37

IC1
p1 4,66
p2 4,83
p3 4,58
p4 0
p5 4,66
p6 4,66
p7 4,66
p8 9,37

Q1
C 9,37
B 2,84
E 3,52

Q2
C 9,37
B 2,84
E 3,51

D1
A 4,81
K 4,66

D2
A 4,66
K 4,83

D3
A 4,66
K 3,29

D4
A 3,29
K 4,66

D5
A 0
K 9,37

Are those number normal for this circuit?
If yes what can cause the volume drop when the pedal is engaged?
If no what is there to do to correct the volume drop issue?

My guitarist really enjoy the sound of this unit BUT when he wants to go for a clean sound and disengaged the effect the volume boost is annoying.

Please I need advise and am willing to learn.

[thermionix]

For Q1 and Q2, the bases should be a bit higher than the emitters.  Are you sure you're reading those correctly?  What transistor types did you use?  Are they EBC or ECB pinout?

[Bassbob]

Both are 2N4401, the pin out is EBC.

[GibsonGM]

Double-check those voltages and make sure B and E are not reversed.  If the voltages are accurate, what is going on in the area to make them opposite each other?  If you reversed B and E, the numbers would look correct, so...  Base bias?

[Bassbob]

I have no clue what Base Bias is. Can you please tell me about it?

[GibsonGM]

The voltage on the base of a transistor should be about .7V higher than the emitter, as thermionix said above.  This is accomplished generally by using a voltage divider of some sort to apply a 'bias voltage' at that point.   This allows the device (transistor, FET, MOSFET, Opamp, etc) to to swing its output both positively AND negatively, by 'mixing' a dc voltage level with the incoming AC signal (which alternatives both positively and negatively, remember).   Without addressing this need to 'go negative', the signal will be partially clipped off and sound ugly.    So you are setting where on its curve the device operates.  "Vr" located above R2 at the first Q is the bias point for this circuit...provided by R9/R10 10k resistors with C7 to make the reference stable.

More to the point with our problem here...it REALLY looks like the base and emitters are swapped.  It would be wicked unlikely to see that nearly exact .7V difference there, an error causing this at both Qs.    If you built and used an audio probe, you might find that the signal gets low right there, not being amplified (less than unity...we'd expect about unity from them, they are used as buffers).  That would be my guess. 

Quadruple-check the pinout vs. the circuit board, re-measure the Q's and post back, ok?    Next up I would make an audio probe.
https://www.diystompboxes.com/pedals/debug.html
Listen at R3/C2, and R14.   Circuit might be cranking at C8 and then go quiet at the output....

[Bassbob]

I have looked on the internet for several 2N4401 datasheets and the pinout is really EBC. I also checked on the board and the Qs are put in right.
I have checked again voltage measurements, they still are E: 3,52 - B: 2,84 - C: 9,37 for both Q.
I have built the audio probe then listen at R2/C3. The sound is barly audible even with headphones. C8 sounds as elsewhere on the circuit.

Q2 Emitter sounds louder than anywhere on the circuit, Q2 Base sounds a lot quieter than Emitter.
Q1 E, B and C sond barerly audible again, even with headphones.

Here are pictures of the actial PCB.

[Bassbob]

I meant "...E, B and C sounds barely audible..." and "...pictures of the actual PCB."

[bluebunny]

I meant "...E, B and C sounds barely audible..." and "...pictures of the actual PCB."

(Aside: you should find there's a "Modify" link on your own posts - for a while, anyway.  So you can fix typos without having to make a second post.)

[jsleep]

Looking at the photos, I don't see anything wrong. Let me think about it, I can send you some replacement parts and see if that fixes it.  on rare occasions I had bad a resistor and probably might as well replace the transistor.  I'm talking about the input buffer, which would appear to be the problem.

I don't always get it the analyses right, anyone else have any ideas?

JD

[Bassbob]

I have built the audio probe then listen at R2/C3. The sound is barly audible even with headphones. C8 sounds as elsewhere on the circuit.

Q2 Emitter sounds louder than anywhere on the circuit, Q2 Base sounds a lot quieter than Emitter.
Q1 E, B and C sond barerly audible again, even with headphones.

I did it already. Where else should I listen to try and spot the fault?

[duck_arse]

Because it was missing from the kit I had to put a 240ohm at R11 instead of a 220ohm.

Bob, please be looking very carefully, and meter the resistor above the 240R, half hidden in your pic. I think it looks like 220R [red-red-blk-blk-brn], and it it is where the 10k should be, you WILL loose signal.

as for the transistor voltages, power off, remove the trannies from the sockets, power on and measure the voltages on the empty socket holes, please.

[Bassbob]

You're right duck_arse!
I read the value backward  (brn-blk-blk-red-red=10k w/ 2% tolerence).
Voltages on both Qs empty sockets are: E: 0,02 - B: 3,12 - C: 9,37

I changed that 220R with a 10k. No more volume drop .

But both Emitters are still at 3,52 and  Bases at 2,84. 

[PRR]

> But both Emitters are still at 3,52 and  Bases at 2,84.

If it is working, I think that Base reading is meter-loading.

Those Bases are fed by 510,000 Ohm resistors. Any meter will load them down. Many low-price DMMs are 1 Meg (1,000,000 Ohms). So *while you are poking* the Base is dragged-down to about 2/3rd of the voltage it sits at when you are not looking at it.

[Bassbob]

The unit is working great now and I owe it to all you guys.

Thank you!

[duck_arse]

nice work, bob. there are some of us that will install all our resistors with the tolerance band to the right, or bottom of the board. doing so makes it easier to translate the value, and easier to spot a wrong value.

and there are also people here that laugh at us  resistor aligners.

[Bassbob]

Thanks again Dave! I think I'll become one of those resistor aligners.