GGG Brassmaster: Significant Output Drop After R9 and R10

[bt2513]

I completed this build and noticed that there is a very large volume drop when the pedal is engaged. Using an audio jack connected to an amp (with a tone generator), I'm able to determine that the drop occurs significantly after R9 and R10 in the chain (they are connected at a junction). I'm considering snipping them and just connecting them with a jumper. I'm a novice at this. Everything else in the circuit appears to be correct. The resistors are 10k and are both striped that way. Cannot test them without removing them.

What would cause this?

Schematic:
http://www.generalguitargadgets.com/pdf/ggg_brassm_sc.pdf

[PRR]

> just connecting them with a jumper.

Shorting the two pots together will screw-up both functions.

Using your signal tracer, snip one end of C4. Does signal level come WAY up? Then Q2 etc is loading-down the R9-R10 junction. It shouldn't. Miller in R6 gives about 13K, R8 times hFE is about 50K, so Q2 should be about a 10K load. Without that (C4 snipped) signal will come up a little. If it comes up a lot, look for the problem around Q2.

[bt2513]

The signal is still low. I clipped C4 (actually, ended up having to remove altogether) and the signal raised significantly. I flowed new solder over the connections at Q2 and double checked all the components around there. I installed a new C4 and still have the same issue.

Any thoughts? Thanks for the help!

[PRR]

> double checked all the components around there

Nevertheless, something isn't correct.

Does it have any Voltages? Way-wrong voltages at Q2 Q3 could be A Clue.

[bt2513]

Will take voltage readings at all transistors when I get home tonight and will post them here. Although, I don't have a point of reference for knowing if they are out of the norm.

[bt2513]

All values are in the order of B,C,E:

	B	C	E
Q1:	.81	1.98	.20
Q2:	.97	.97	.34
Q3:	.97	8.26	.40
Q4:	.67	3.71	.07
Q5:	3.71	6.32	3.06
Q6:	3.96	9.37	3.36
Q7:	.88	8.75	0.00

[bt2513]

dup'd post by mistake

[PRR]

> I don't have a point of reference for knowing if they are out of the norm
>         B      C       E
> Q2:   .97   .97   .34
> Q3:   .97   8.26   .40



Don't need a lot of authority. Transistors in good circuits are predictable things.

One thing is: it is quite unlikely that Vb and Vc should be the same. There's a few circuits where that can happen, even forced to happen, but not this one.

Q2 is shorted base-collector. Maybe a sick transistor, though that is extremely un-likely. More often a solder-blob, or a 470 Ohm where should be a 470K-Ohm. If strip-board, forgotten or bad cut.

This short shorts-out (about 470 Ohms, very low) the signal coming from the 10K mix-resistors, also kills the gain of Q2.

[bt2513]

This was just the info I needed - there was some sort of stray tracing or solder beneath the surface of the board. The black epoxy made it pretty much invisible to me. The only way I found it was by scratching the surface between the base and collector of Q2 hoping it would break any shorts I couldn't see.

Works great now! Thanks!!