BSIAB2 Questions and solutions - Ground point PCB and Trimmer to adjust Q5

[Jquijano]

Recently built a BSIAB2 pedal designed by General Guitar Gadgets. I have to problems to troubleshoot:

1) the Ground PCB terminals were damaged so i soldered the three (3) Ground together and used a jumper cable to continue the PCB circuit path to C105. Pedal does work but i believe it needs to have a better ground. How do you tackle this problem?

2) How do i adjust Q5 to 4 volts? Should i place my Digital Multimeter red probe in one of the legs of Q5 and the black to ground?

[antonis]

1) I don'g think C105 needs to be grounded..



RED path C105, BLACK path GND

2) Red probe on Q5 Drain (the leg connected to 10k trimpot) and the Black probe to GND..

[ElectricDruid]

1) I don't think C105 needs to be grounded..

I think they might mean C3a, which is marked "105" - the value.

That ground path, the black trace on the diagram, needs to be connected all the way around. With the power off, you can test it for continuity. Stick the black probe on ground, and then test each of the points where another component meets it (the black circles along the route). You should have continuity to each one.

[Jquijano]

1) I don'g think C105 needs to be grounded..



RED path C105, BLACK path GND

2) Red probe on Q5 Drain (the leg connected to 10k trimpot) and the Black probe to GND..

I managed to set it to 6.3 volts. It doesnt go down to less than 4.3 volts dc. Here i am posting a pictures of all the grounds soldered together with jumper wire to next connection in pcb

[antonis]

I managed to set it to 6.3 volts. It doesnt go down to less than 4.3 volts dc.

Considering +9V supply, you get a maximum voltage drop of 4.7V across 100k Drain resistor (trimpot at its max value)..
That calls for V_GS of -263mV..
By lowering R15 (5k6) Source resistor value, you lower V_GS resulting into raising Drain-Source channel current, hence lowering Drain voltage..

P.S.
I'd replace R16 (100k trimmer) with a resistor of 22k - 27k and R15 with a 5k multitrurn trimmer..
(succeeding Tonestack doesn't like to be driven from a high impedance, like Q5 Drain resistor..)

[Jquijano]

Well i soldered the trim pot that came with the kit. Its weird it doesnt dial the 4 volts and only reaches to 4.3 volts.

[antonis]

Which trimpot..??
(and of which value?)

P.S.
Anyway, Q5 Drain between 4.5 and 5.5V should be fine..

[Jquijano]

Which trimpot..??
(and of which value?)

P.S.
Anyway, Q5 Drain between 4.5 and 5.5V should be fine..

Which trimpot..??
(and of which value?)

P.S.
Anyway, Q5 Drain between 4.5 and 5.5V should be fine..

Thanks. What about the common ground? Should i leave the wires soldered to the jumper wire and wrap them with 3M electric tape?

[antonis]

I'd use heat shrink tube instead of electrical tape and secure the wires junction on solder side of PCB via hot glue..

[Jquijano]

I'd use heat shrink tube instead of electrical tape and secure the wires junction on solder side of PCB via hot glue..

Thanks for your input. Ill try it in such a way.

[Jquijano]

Which trimpot..??
(and of which value?)

P.S.
Anyway, Q5 Drain between 4.5 and 5.5V should be fine..

I dialed it to 6.19V so i can get more crunch of the pedal. Should i lower it to 5.5 Volts?

[antonis]

I dialed it to 6.19V so i can get more crunch of the pedal. Should i lower it to 5.5 Volts?

Trim it to whatever Drain level you like..

In theory, ideal Q5 bias should be obtained for Drain voltage sitting at (Vcc + V_Source) / 2 (plus a little bit higher to factor in the undecoupled Source resistor) but this strongly depends on particular JFET linearity, so your ear should be the best judge..

[Jquijano]

I dialed it to 6.19V so i can get more crunch of the pedal. Should i lower it to 5.5 Volts?

Trim it to whatever Drain level you like..

In theory, ideal Q5 bias should be obtained for Drain voltage sitting at (Vcc + V_Source) / 2 (plus a little bit higher to factor in the undecoupled Source resistor) but this strongly depends on particular JFET linearity, so your ear should be the best judge..

9v/2 = 4.5 Volts. It lacks drive at that setting.

[antonis]

Maybe..
(it strongly depends on Drain resistor value..)

e.g. for R16 set at 82k (say), "independent" Treble drive is downgraded to 50% and Bass drive to 35%..
Tonestack "combined" drive ability goes down to 25%..

You need pretty lower Drain resistor value (hence larger current)(*) to obtain both "ideal"(for symmetrical output swing) Drain bias and drive immune to Tonestack equivalent impedance..

(*) of course, Source resistor should be scaled down accordingly to Drain resistor..

P.S.
If you like they way pedal sounds with Q5 Drain at 6.19V, let it be and don't mess up with those %$#*&^ JFETs anymore..