Brown Betty trimpot/gain knob stuck between high gain and really high gain

Kroars

Kroars

Well-known member
So, built two more Brown Betty’s one works great one works ok…. All components tested prior to install (except for IC’s).

Problem: On one of the two the trimpot doesn’t affect gain much at all. Almost like it’s always full CW even when turning. I can’t get that low gain level when the Gain knob is turned full CCW like I can with the others I’ve built. The only real difference I’ve found is that on the good pedal the pins on IC’s are standard 9v on pin 8 and close to 4.5 on others. On the bad one they’re all the same except for pin 7 on IC2 which reads 3.4v.

Also on C22 the good one goes from 4.498 to 4.489 and the bad one goes from 4.555 to 4.407 when adjusting trimpot. Seems like a bit of a difference.

IC’s were changed from good one to bad one -same issue. Also replaced trimpot -same issue.

Anyone have an idea what to check next?

Thank you!!
 

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Kroars said:
... Almost like it’s always full CW even when turning. I can’t get that low gain level when the Gain knob is turned full CCW like I can with the others I’ve built.
Click to expand...


Maybe the trimpot is faulty, and if so it could be stuck wide open, no matter where you turn it.


I've been lucky and haven't had any faulty components yet, but there seems to be a surprising number of people here on the forum that have brand new parts (caps, pots, "legit" ICs, etc) that are faulty from the factory.


However, Pin 7 of IC2 suggests the problem may lie elsewhere. Have you checked all the resistor values leading to/away-from pin 7 IC2?
 
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Feral Feline said:
Maybe the trimpot is faulty, and if so it could be stuck wide open, no matter where you turn it.


I've been lucky and haven't had any faulty components yet, but there seems to be a surprising number of people here on the forum that have brand new parts (caps, pots, "legit" ICs, etc) that are faulty from the factory.


However, Pin 7 of IC2 suggests the problem may lie elsewhere. Have you checked all the resistor values leading to/away-from pin 7 IC2?
Click to expand...
I had replaced the trimpot already, no luck. Im getting back from WA tomorrow and will double check values. I eyeballed everything with the other board that works perfectly and everything seemed the same.
 
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It looks like you have 1k instead of 10k for R14. That is in series with the trimmer. That could explain the issue with the trimmer not behaving.

EDIT: I may have the colours wrong as it’s hard to tell if that 4th stripe is brown or red.
 
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JamieJ said:
It looks like you have 1k instead of 10k for R14. That is in series with the trimmer. That could explain the issue with the trimmer not behaving.

EDIT: I may have the colours wrong as it’s hard to tell if that 4th stripe is brown or red.
Click to expand...
It’s a 10k. I appreciate the help!
 
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music6000 said:
Do you have the clipping leds in the right way

Continuity test on trimmer to matching circles :
View attachment 15108
Click to expand...
Continuity looks good. I suppose it’s a possibility I put the led’s in the wrong way. I’m fairly certain they’re correct, but it’s a possibility. Based on the issues I brought up with weird voltage only in IC2 and trimpot seemingly stuck between mid-hi gain -would you think pulling them out and putting new ones in would be the next reasonable step?

Update:

Played with it a bit more since arriving home late last night. It almost seems now like it’s the Gain knob (pot) that doesn’t do much. I can at least hear the gain hiss climb when turning the trimpot CW. The actual effect just stays at high gain. Funny, I noticed the Gain potentiometer seems to have less of an affect on the effect than the trimpot. Although putting a DMM on the gain pot it seems to work great.
 
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With all knobs @ 12.00 O'clock & Gain knob at minimum there should be medium gain & as you increase clockwise you should hear a obvious rise in Volume & more Sustain all the way up to Max. rotation.
You used Cusack pots for Volume & Gain, ARE the Pot values right, Volume A50K , Gain A1M?
If the Clipping led's have a flat machined at the base, the pair should be facing opposite each other on each side!
 
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Kroars said:
...I suppose it’s a possibility I put the led’s in the wrong way.
Click to expand...

So long as the clipping LEDs within each set are anti-parallel to each, it won't matter:

>|
|<

👆
is the same as
👇
|<
>|

Whether going to VDD, or in the loop of IC2.2



The other clipping diodes in IC3.2's loop, D1–4, match the PCB's silkscreen, so no problem there so long as the soldering is good.


I know, I'm probably not telling you anything you don't already know...
 
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music6000 said:
With all knobs @ 12.00 O'clock & Gain knob at minimum there should be medium gain & as you increase clockwise you should hear a obvious rise in Volume & more Sustain all the way up to Max. rotation.
You used Cusack pots for Volume & Gain, ARE the Pot values right, Volume A50K , Gain A1M?
If the Clipping led's have a flat machined at the base, the pair should be facing opposite each other on each side!
Click to expand...
They are. Like I mentioned, I put a DMM on the pots and they read the same between the working one and the one in question.
 
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music6000 said:
Can you confirm R3, R7, R14, R101 are all 10k = Brown, Black, Black, Red, Brown ?
Click to expand...
Yep. Individually tested prior to installation as well. Both boards were in front of me, both exactly the same. One works great, on the other the gain knob doesn’t seem to do anything at all. LED’s got significantly brighter when turning the trimpot CW, but nothing with the gain knob.
 
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Kroars said:
Yep. Individually tested prior to installation as well. Both boards were in front of me, both exactly the same. One works great, on the other the gain knob doesn’t seem to do anything at all. LED’s got significantly brighter when turning the trimpot CW, but nothing with the gain knob.
Click to expand...

Test For Continuity with matching Circles, If this is Good then you may have a dud Pot!!!
I hope you have a dud trace between Gain pot Legs 2 & 3!

Gain Pot Continuity Test.jpg
 
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music6000 said:
Test For Continuity with matching Circles, If this is Good then you may have a dud Pot!!!
I hope you have a dud trace between Gain pot Legs 2 & 3!

View attachment 15170
Click to expand...
Ended up changing pot, because why not. Same issue. I have continuity between all points you outlined EXCEPT for anything between the pin 7 of IC1 and neither of the blue circles on the clipping LED nor the 1st leg of the gain pot you have circled blue. I mean, I’ve had a damaged trace in one other board (out of over 300) you think that may be it?
 
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