Some bugs in Foxx Tone Machine

[Gabriel Hendrix]

Hi,
I just finished my Foxx tone Machine from GeneralGuitagadgets, but
there are some bugs:
-When i play i hear some pop's, like popcorns . As more bass i play, more pop's
i.e.  E sixth string -> lot of pop's.  E fisrt string - some few pop's
-when i full clockwise the volume knob, no sound.. :/
-when turn on the octave switch, all is ok( no more pop's).

Thanxs for anything,

Gabriel Hendrix, from Brasil

PS:Excuse my english...

[Gabriel Hendrix]

Link to Schematic

http://www.generalguitargadgets.com/diagrams/ftmsc.gif

[Gabriel Hendrix]

DC: 8.8V

Q1                   Q2                      Q3                       Q4
C - 2.8V            C - 6.8V             C - 7.4V            C - 7.9V
B - 0.4V             B  - 2.8V            B - 0.6V            B - 0.5V
E - 0.02V            E - 2.2V             E - 0.1V            E -  0.1V

Diodes                D1            D2          D3          D4
K                        0.0V         3.7V       0.0V       0.0V
A                        0.0V         3.7V       0.0V       0.0V

Mods: All diodes 1n60

Thanxs,

[R.G.]

DC: 8.8V

     Q1      Q2              Q3         Q4
C - 2.8V   C - 6.8V  C - 7.4V  C - 7.9V
B - 0.4V   B - 2.8V   B - 0.6V  B - 0.5V
E - 0.02V  E - 2.2V   E - 0.1V  E - 0.1V

Hmmmm... nothing hugely wrong there. Q1 has a suspiciously low Vbe, only 0.4V, and the voltages don't match up with the currents calculated for Rc and Rc by ohm's law, but that could be measurement error.

I would expect Q1b to be 0.63V and the emitter at 0.13V, or quite close to that. the numbers show that there is about 127uA going through the collector resistor of Q1, and maybe 10uA of base current, so it's showing an effective gain of 12. That's odd.

If it were mine, I'd touch up and resolder the joints of the Q1/Q2 section and the feedback net from Q2 to Q1 - carefully. Maybe look at them with a magnifying glass first.

Diodes D1 D2 D3 D4
K 0.0V 3.7V 0.0V 0.0V
A 0.0V 3.7V 0.0V 0.0V

Mods: All diodes 1n60

I'm guessing that this is in the non-octave mode to get 0V on D1. Can you measure in both octave/non octave mode?

There's something wrong with D2, because it should have something like maybe a tenth of a volt across it being biased that way.

-When i play i hear some pop's, like popcorns . As more bass i play, more pop's
i.e. E sixth string -> lot of pop's. E fisrt string - some few pop's

Could well be a bias problem, maybe an electro cap backwards. Could you measure the voltage on both pins of the electro caps and verify that they are getting more + voltage on the + pin?

-when i full clockwise the volume knob, no sound.. :/

Where in the rotation does it quit, and does it stop abruptly or fade out?

-when turn on the octave switch, all is ok( no more pop's).

That is also indicative of a possible bias/electrolytic capacitor problem.

This will not be as simple to solve as some of the bugs we've handled, as there is no single voltage measurement that definitely says "ahah, this is what is wrong". YOu may also be having some parasitic oscillation causing the volume knob thing. Give us the more data and we'll see.

[Gabriel Hendrix]

"... but that could be measurement error"

ok, now i had measured up with more accuracy.

Q1
B - 0.12V
E - 0.013V

Q2
C - 6.9V

Q4
B-0.42V

All others values are same.

Diodes

non-octave
                       D1    D2     D3   D4
K                      0.0    3.6   0.0    0.0
A                      0.0    3.8   0.0    0.0

octave

                   D1    D2  D3  D4
K                 2.6    2.6  0.0  0.0
A                 2.8    2.8  0.0  0.0

Could you measure the voltage on both pins of the electro caps and verify that they are getting more + voltage on the + pin?

All is ok.

"Where in the rotation does it quit, and does it stop abruptly or fade out?"

Stop abruptly


thanxs once again

[R.G.]

ok, now i had measured up with more accuracy.

Q1
B - 0.12V
E - 0.013V

Unless you have an NPN germanium transistor in there  - which I doubt - then Q1 is definitely messed up. That base should be up at 0.4 to 0.6V. Check the usual - bad soldering, shorts, wrong component values, transistor pinout, etc.

Diode measurements now look OK.

See what you can find at Q1.