SOLVED Son of Ben not quite working

[DAJE]

Works fine in bypass, LED does what it's meant to, etc.

But when on, it only works very slightly. If I push the volume knob up full, I can hear it through the amp, and it sounds... very quiet, and slightly distorted. Not at all usable. Really voltage-starved is how I'd describe it.

I left C5 and C10 out but put sockets in, so I can try it with them in. I haven't tried that yet because it's quite late here. Didn't change anything else.

The transistors are MMBFJ201s that are soldered to cheapo adaptor boards. I tested them before putting them in, and I pulled them out and retested them: they are definitely in spec and working properly. I believe I have them properly biased, as I read the thread here about biasing the Benson / Son of Ben.

I've built a bunch of working pedals, my soldering isn't perfect but it's decent. I cleaned up the far side of the board, but no pics because I'd put it back together again before I decided to take pics.

Suggestions?

 

[BuddytheReow]

Did you bias the j201s to the correct voltages? Test the drain pins with your DMM, black to ground. They should be 4v per the schematic for Q2 and Q3. Once that's done dial in the trimmer for Q1 until you get the sweet spot based on your ear. I'm thinking it should be around 4-5v too? Let us know how that goes.

If you don't dial in the correct voltage the FETs wont work properly

 

[temol]

First of all - make sure that pinout for those adapters is right. It's not obvious.
You need DSG. From what I remember, those cheap green adapters have different pinout - something like SGD.
If that's the case, you just have to cross the legs of the adapter.

 

[fig]

You are correct @temol. I can just make out 2 3 1 on the adapter board, which for a J201 would be
S G D.

 

[fig]

Once that is corrected, here is a link to CDBs biasing procedure;

Trimpot Adjustment - Son of Ben

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...

forum.pedalpcb.com

 

[DAJE]

What you're not seeing there is that I soldered the MMBFJ201s to the adaptor boards SLANTED. The pinout is correct.

Whatever mistake I made, it wasn't that one.

 

[temol]

Then you should check drain voltages. If the drain voltages are correct then use the audio probe to trace the signal. You can probe the signal at drain of each transistor. Every stage (each transistor) boosts the signal so it should be fairly easy to narrow area of the problem.
C5 and C10 increase gain of the stage. With both removed the overall gain is lower about 12dB or so.. (according to ltspice simulation).

 

[Boba7]

What you're not seeing there is that I soldered the MMBFJ201s to the adaptor boards SLANTED. The pinout is correct.

Whatever mistake I made, it wasn't that one.

Did you check the continuity between the smd transistor lugs and the pads on the little adaptor boards?

Also voltages would be good to have yes

 

[DAJE]

Did you check the continuity between the smd transistor lugs and the pads on the little adaptor boards?

Also voltages would be good to have yes

I have a Peak Atlas transistor tester, and all three transistors read as normal and within spec. I keep saying that, and everyone keeps saying "yes, but did you check the transistors?" It isn't the transistors.

I'm fairly sure it's the Drive pot, and maybe the Volume pot. I did try again and sweeping the pots kind of worked, but mostly just crackled. I don't test pots because they usually just work, but I should. I don't have another A1M but I do have a spare B1M pot that I can swap in, if needed.

I don't have any contact cleaner and it's hard lockdown here now, so I can't easily pick some up. Might have something I can use, will have to check.

 

[benny_profane]

The question is about the transistor biasing. Regardless if the parts are installed correctly and within published specs, the circuit will not work properly unless biased correctly. Can you please measure the drain voltages for each transistor?

 

[DAJE]

parts are installed correctly and within published specs, the circuit will not work properly unless biased correctly. Can you please measure the drain voltages for each transistor?

Pedal powered by 9V adaptor, tested and actually putting out 9V.

Multimeter set to 20V.

Q1 set to 1.63V, which is pretty close to where it should be, I believe.
Q2 set to 3.99V
Q3 set to 4.07V

I thought that's where they were set but rechecking them showed they were way off. Not sure what I did wrong, but hopefully I have improved things. Testing again now...

 

[Boba7]

I have a Peak Atlas transistor tester, and all three transistors read as normal and within spec. I keep saying that, and everyone keeps saying "yes, but did you check the transistors?" It isn't the transistors.

Just making sure and trying to help, relax man. I did ask about the voltages too but you chose to ignore that.
Oh well, good luck with your build then.

 

[benny_profane]

Q1 set to 1.63V, which is pretty close to where it should be, I believe.

Try adjusting this to about 6V and adjust from there. See if you get better results.

 

[DAJE]

Just making sure and trying to help, relax man. I did ask about the voltages too but you chose to ignore that.
Oh well, good luck with your build then.

Sorry, was getting a bit frustrated.

 

[DAJE]

Try adjusting this to about 6V and adjust from there. See if you get better results.

Will do, the pedal is now working but it's pretty mild. Sounds good, though.

 

[DAJE]

OK, success. I think the problem was that when I biased the transistors last night, I biased Q2 and Q3 to .4V, not 4V. Moral of the story: decimal places matter. My fault, and it was in fact the transistors - though not the adapter boards and not the pinout.

Thanks to all for your suggestions and advice.

 

[Boba7]

Great, good job!