SOLVED Son of Ben - Can't Bias Trim 1 to recommended level

apierz

Member
Problem: While trying to bias trim pot 1 on a Son of Ben PCB, the desired resistance was not within the trim pot range even with the trim pot at either extreme.

Solution: The jumper between the drain of Q1 and the cathode of D100 was not properly attached. I removed the pcb from the case and soldered a wire connection between these two points. Once properly attached, I was able to get the desired value of 1.67k (though it was very finicky!).


OP:
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I am trying to bias Trim 1 on my Son of Ben build following these instructions:

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.

However, when I set Trim 1 to fully counter clockwise and measure across R5 is get about 8.5k ohms and when I set it to fully clockwise I get about 4.8k ohms so it doesn't seem possible for me to adjust down to 1.67k.


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