Trimpot Adjustment - Son of Ben

[Jovi Bon Kenobi]

The build doc says

Is there a way to test trim 1 without removing is from the enclosure?
For Q2&3, is it the same as when biasing a fuzz face? One probe to ground and the other on the drain?

Thanks!

*Edit for future forum searchers* "son of ben" isn't searchable because the words are too long, short, or common. Ha. Here ya go..."Benson preamp"

 

[Mourguitars]

Think you could post pic's of the process in your other thread for us newbie's JBK ?

 

[Chuck D. Bones]

With the power off, connect Q1 drain to Vcc. Now you have the pot in parallel with 10K and nothing else. 2K in parallel with 10K = ... now let's not all raise our hands at once... 1.67K.

Unless otherwise specified, all voltages are measured w.r.t. ground.

After that, I'd give it a listen and tweak as req'd.

 

[Jovi Bon Kenobi]

Think you could post pic's of the process in your other thread for us newbie's JBK ?

Id like to keep that thread as basic as possible

 

[Jovi Bon Kenobi]

With the power off, connect Q1 drain to Vcc. Now you have the pot in parallel with 10K and nothing else. 2K in parallel with 10K = ... now let's not all raise our hands at once... 1.67K.

Unless otherwise specified, all voltages are measured w.r.t. ground.

After that, I'd give it a listen and tweak as req'd.

This is slightly out of my grasp of understanding. I've tried googling where the VCC function is on the multimeter but came up more confused.
What I've done is set my multimeter to the "v" mode then ran a probe to ground. Then measured the drain pin on Q2 and Q3. Both trimpots were then dialed to 4v. I still don't understand how to read trimpot 1 to 2k

 

[Jovi Bon Kenobi]

I should specify, I think I successfully biased Q2 & Q3 to 4v with the pedal plugged in to my power supply but not engaged. I'm at a loss on how get the trimpots value to 2k while it's in the enclosure. What I did do is just turn trim 1 until it sounded good. So, I guess I did the opposite of what you suggested. Anyway. It sounds GREAT!

 

[Chuck D. Bones]

At the end of the day, that's what matters!

FYI, here's more specific instructions on setting TRIM1 to 2K. Assume that you are viewing the component side of the board with the DRIVE & VOLUME pots at the top.
1) Disconnect power from pedal.
2) Install jumper (clip lead or tack solder) from Q1 drain (left lead) to D100 cathode (right lead). D100 cathode is the point in the schematic named Vcc.
3) Set meter for Ohms, 2K or 2,000 scale.
4) Connect meter one meter lead to one end of R5, connect the other meter lead to the other end of R5.
5) Adjust TRIM1 until meter reads 1.67K. You don't have to hit it dead-on, just get between 1.5K and 1.8K and you're close enough.
6) Disconnect meter & jumper lead.

 

[Robert]

For prototyping I just adjusted so the resistance measured from VCC (Cathode of 1N5817) to Q1 Drain was 12K.

12K total (Trim1 + R5) is what you're really shooting for (this was a fixed resistor in the original), the trimpot was added in the event that further adjustments were necessary.

 

[Jovi Bon Kenobi]

Ah. Ok I get it now. Thanks thanks for clarifying! ?

 

[Chuck D. Bones]

For prototyping I just adjusted so the resistance measured from VCC (Cathode of 1N5817) to Q1 Drain was 12K.

12K total (Trim1 + R5) is what you're really shooting for (this was a fixed resistor in the original), the trimpot was added in the event that further adjustments were necessary.

What about the sneak path from Vcc to ground thru all the FETs? Even if you dial the other two trimmers up to max resistance, they still influence the measurement. I guess it gets close enough, hmmmm?

 

[Jovi Bon Kenobi]

I just ran it through again with that method, Chuck.

I dialed it in to exactly 1.67k. Boy oh boy does it sound great now! Trim 1 is interesting. It goes from no sound to a broken crackling starved voltage sound to perfection. I can see it being tricky for people. Thanks for the excellent method.

 

[Robert]

What about the sneak path from Vcc to ground thru all the FETs? Even if you dial the other two trimmers up to max resistance, they still influence the measurement. I guess it gets close enough, hmmmm?

It worked fine for me, but if it hadn't I would have just tweaked it by ear. There's really no reason not to take the extra time and do it the way you suggested though... (or just measure it out of circuit before installing)

The original used a fixed 12K with no adjustment possible, which really baffles me considering the other two stages have trimmers.

I'd be curious to know what the VCC -> Drain measurement reads (with all FETs installed) after adjusting to 1.67K.

 

[Chuck D. Bones]

Had I know that Q1 was in a socket...
1) Remove Q1 from socket.
2) Measure from D100 cathode to the bottom end of R2.
3) Adjust TRIM1 for 2K Ohms on meter.

 

[Mourguitars]

Very cool...now i get a grasp on how its done, a picture is worth a thousand words !

Glad you got it JBK..

Mike

 

[CanadianDave]

This info is really helpful. The Benson is next up on my work table.

 

[cooder]

Excellent, just adjusted the populated pcb it this way, all good! Thanks for this explanation!

 

[cooder]

For what it's worth, I measured after adjusting trim 1 to 12kohm the drain measurements on Q1 as 5.1 V / Q 2 and 3 adjusted to 4 V each, input voltage 9.6 V.

 

[Vinnyallen]

Can someone explain busing q2 and q3? I ran a probe to ground and a probe to the drain on both with the pedal plugged in. They both read 9v and that value doesn’t change when I adjust the pot. What am I doing wrong? I successfully adjusted q1 to almost 1.67

 

[music6000]

They look like SILICONIX J201's by the Dot on Top, where were the sourced ?
Do they have Markings like this:

 

[Bobb Lobblaw]

Can someone explain busing q2 and q3? I ran a probe to ground and a probe to the drain on both with the pedal plugged in. They both read 9v and that value doesn’t change when I adjust the pot. What am I doing wrong? I successfully adjusted q1 to almost 1.67

Yes, Co-sign this request!
Please can some one explain this process as if you were talking to a sixth grader raised by wolves in a national forest?