Calvin Vai help needed bias adjust

Archaide · Post by **Archaide** » 12 Jan 2023, 14:50

Hello everybody.

Please excuse my english. It is not my motherlanguage.

I already build a few pedals from pcbguitarmania and other sites. The only time i had to adjust something was when i build a phaser. In that case it was done by ear.

I`m not sure what to do with the three trimpots on the calvin vai.
How should that sound? Are there some voltages i should measure somewhere? Nothing about the pots in the building docs for that pedal.

Would someone be so kind and can advise on this?

I also cand find the general pedal buildung guide.pdf on the pcbguitarmania site.

Please excuse if these are dumb questions. I`m still learning.

Thanks in advance.

Archaide

mauman · Post by **mauman** » 12 Jan 2023, 16:35

Welcome! All questions are good questions.

Each trim pot on this PCB is connected to one transistor (2N5457). These transistors have three leads: drain, gate and source. In the schematic, the drain is at the top, gate in the middle, and source at the bottom. On the transistor, the leg sequence is drain-source-gate from left to right looking down at the PCB.

Use each pot to adjust (bias) the voltage at the drain (left) leg of each transistor to 1/2 the supply voltage. Set your meter to DC volts, put the black meter lead on a ground and the red meter lead on the drain of a transistor, and adjust the trim pot. For a 9V supply, adjust each drain to 4.5V. It's OK to go a little higher or lower than 4.5V if it sounds better to you, maybe 4 to 6 volts. You can do final adjustments by ear if you like.

Archaide · Post by **Archaide** » 12 Jan 2023, 17:13

Thank you very much mauman.
That is tremendously helpful and should definitely be part of the building guide.
I`ll try that as soon as i finding time for this. Can take a week or two.
But thank you nevertheless.

Archaide · Post by **Archaide** » 29 Jan 2023, 14:08

Hi all.

I'm coming closer to use the adjustment advice up there.
In the Moment i am a bit confused regarding the LED. If i place the board in a way you can See in the picture the led is not alligned to the board and need extra wiring.

: Pot to board alignment

On the other side the markings for the pots suggest this Position.

So, what is wrong? I guess the hole for the led in the enclosure is misplaced.

Is there something i miss here?

Kind regards,
Archaide

mauman · Post by **mauman** » 29 Jan 2023, 15:52

I think the hole for the LED should be at the place marked "LED" on the template, just above the foot switch. The hole you drilled is actually at the center point of the enclosure; unfortunately that place is marked with a small circle that looks just like a drilling hole.

However, it's not a problem. Just put the LED in the hole you made, and bend the legs to fit the PCB location. If the legs are too short, you can solder on two short pieces of wire and run those to the PCB holes. Be sure to insulate any bare parts of the LED legs with heat shrink or tape to avoid shorts.

Calvin-Vai-Drilling-Template.pdf: Calvin Vai Drill Template; (30.35 KiB) Downloaded 27 times

Archaide · Post by **Archaide** » 29 Jan 2023, 15:57

Thanks.
That's what i thought. I got the enclosure predrilled from Musikding.
Also the hole for presence was only 6mm instead of 8mm.

Archaide · Post by **Archaide** » 31 Jan 2023, 16:42

Hello eberybody.

Sorry to disturb, but i need help with the build.
The signal from the guitar travels well through the bypass to the amp. When the Pedal is off. The signal drops to nearly unhearable with a lot of noise when switching the Pedal on.
I managed to adjust the bias to around 4.5 volts on all 4 transistors. The signal from input goes to the switch and from there to the board. Outsignal from board via switch to output Jack. LED ist working. I hear a change in the noise from the AMP when using the knobs.
I'm lost. Maybe i don't see the big mistake right in Front of my eyes. Can someone please advice?
Something i can measure with a multimeter?

Thanks in advance.
Archaide