BSIAB2 trimpot not affecting Q5

[daleykd]

Hey guys...

I've now attempted 5 different BSIAB2's.  Every single one has had some nasty problem or another.  Out of the 5, so far, I only have two that I considered complete-worthy.  The first, I completely scrapped.

I've been working on my last two for the last three days, and am stumped.

Board 1:
The trimpot is completely minned, and the resistance is reading at ~90k ohm.  However, the Q5 D voltage is reading at 3.5 DCV.  I know that's really close, but I'm curious to know why I have to be completely minned on the trimpot to get this?  I would have thought my trimpot would be 40-50%.
Also, I get a nasty feedback squeal when I dime the Drive pot and mute my strings.  My other two BSIAB's didn't do that.
However, other than that, the pedal sounds pretty good.  I just want to be able to dime the drive without worrying that it'll squeal.

Board 2:
I haven't even completely soldered this board in, yet.  It's only hooked up to 9V and GND.
At one point, it worked great.  But then I think I blew a diode and have had hell ever since.
Right now, I have the trimpot completely minned, reading at ~90k ohm, and no matter what, the Q5 D is 8.5 DCV.  I have looked over this board what seems like 100 times trying to find a bad trace or a short, but I can't seem to find one.  Any thoughts on what might cause this?

Bonus question: When I have Q1, Q2, and Q5 socketed (Q4 and Q3 don't seem to affect this equation), the resistance of the trimpot always reads 06.5 (DMM set to 200k ohm).  However, if I unsocket any of those three, the resistance reads at ~90k ohm.  What is the logic/reason for this?  Also, if I keep Q5 socketed, but unsocket Q1 or Q2, the DMM SLOWLY climbs to 90k ohm instead of an instance read.

Thanks in advance, all.  I really want to finish these last two BSIAB2's, and never look back.
Kyle

Debugging Info
GGG BSIAB2 Layout, slightly tweaked in EAGLE to use my bigger trimpots
Printed my own PCB
No substitutions

[dwmorrin]

The trimpot is completely minned, and the resistance is reading at ~90k ohm.  However, the Q5 D voltage is reading at 3.5 DCV.  I know that's really close, but I'm curious to know why I have to be completely minned on the trimpot to get this?  I would have thought my trimpot would be 40-50%.

A guess: JFETs have wide variations, thus needing a trim pot in the first place.  Your particular FET needs the max resistance, but if it works it works.  Add a fixed resistor in series to get more wiggle room out of your pot.

Also, I get a nasty feedback squeal when I dime the Drive pot and mute my strings.  My other two BSIAB's didn't do that.
However, other than that, the pedal sounds pretty good.  I just want to be able to dime the drive without worrying that it'll squeal.

I would start with comparing one that doesn't squeal and seeing if there's anything obvious.  I would use a test signal and scope and check the gain of each stage, and look for signs of unwanted oscillations.  A more forceful approach might be to try clipping in a small bypass cap (to limit high frequency response) at various points until it goes away.

Board 2
I haven't even completely soldered this board in, yet.  It's only hooked up to 9V and GND.
At one point, it worked great.  But then I think I blew a diode and have had hell ever since.
Right now, I have the trimpot completely minned, reading at ~90k ohm, and no matter what, the Q5 D is 8.5 DCV.  I have looked over this board what seems like 100 times trying to find a bad trace or a short, but I can't seem to find one.  Any thoughts on what might cause this?

You think you blew a diode?  The reverse polarity protector diode?  Did it fail open or short?  Is 8.5Vdc = the power supply?  Either way, Q5 is not conducting, so pull it and test it out of circuit.  Maybe you blew a FET too.

Bonus question: When I have Q1, Q2, and Q5 socketed (Q4 and Q3 don't seem to affect this equation), the resistance of the trimpot always reads 06.5 (DMM set to 200k ohm).  However, if I unsocket any of those three, the resistance reads at ~90k ohm.  What is the logic/reason for this?  Also, if I keep Q5 socketed, but unsocket Q1 or Q2, the DMM SLOWLY climbs to 90k ohm instead of an instance read.

Q5 un-socketed means R16 is open on one side = good measurement.
Slowly rising resistance measurement is from capacitor interaction.  Caps will take some time to charge up and will mess with the meter reading until they are charged.
Why 6k5 when all transistors are in?  Somehow you're drawing enough current to fool the meter.
The meter outputs a small, constant voltage, and measures the current that it gets back.  Ohms law, R=V/I, is computed via an analog or digital method, and the "resistance" is displayed.  Erroneous current = erroneous resistance.  That's why it's important to lift the leg of a resistor, in circuit, if you want an accurate resistance measurement.

[daleykd]

Quick update:
Yeah, I had blown the reverse polarity diode.  When checking DCV, I wasn't getting any reading on the other side of it (I had at one point).  I replaced the diode, and suddenly, the rest of my board had voltages.

On Board 2, I had a bad solder point for a cap touching ground.  I was checking voltages and noticed I was getting 8.5DCV on ground.  Once I fixed that solder point, the trimpot did its job.

When I get some patience, I'll go thru and slowly troubleshoot Board 1.  But as of now, 3/4 of my BSIAB2's are complete.

Hooray!

[Jquijano]

How to you measure the needed 4 volts at Q5? Digital Multimeter black probe on ground and positive in which leg of the transistor?

[antonis]

https://www.diystompboxes.com/smfforum/index.php?topic=131474.0