Son of Ben - Voltage/Trimpot Problem (Q3 randomly fluctuates)

[Sasan]

Hi, me again. I have two issues regarding the Son of Ben Pedal.

1) The last two weeks I noticed that my Son of Ben sometimes has "outages"...
While I was testing the pedal on another board , I faced another "outage" and noticed that the voltage of Q3 (Originally, set to Q1 ~5.2V, Q2&Q3 ~4.2V. ) randomly changes to almost 0-1.2V. Re-adjust and it sounds fine again...but that's a pita if I close the enclosure or play live...
How comes the voltage fluctuates this much without touching the trimpot?

2) In the 1:1 comparison to the original (only with all knobs at 12 o'clock), I noticed that SoB has way more gain and less mid & presence than the original.
Any idea on how to fix that?

Thanks in advance!

 

[Sasan]

noone?

 

[phi1]

I doubt the trimmer is actually moving, I’d guess it’s more likely you have another intermittent issue (cold joint or something shorting). Just a couple of thoughts of things that sometime come up, maybe you’re on top of these already...
- where did you source your j201s? Through hole type or smd?
- are your jfets socketed? I only view sockets as permanent on op amps. Transistors and other parts I always solder in before boxing so they can’t lose contact.
- is your dual pot (bass) insulated from touching the board?

Regarding comparison with the original, later Benson versions omitted a 10uF cap (noted in pedalpcb’s build docs), so you could check if your build matches the version of the benson you have.

 

[Mcknib]

Sounds like cap might be playing up obviously caps charge up and discharge so there's a variance and as @phi1 says unlikely the trimmer would randomly vary

You'd need someone with greater technical knowledge than me but basically R11 and C10 set the gain or q and also help keep it stable

There's some info in Mr Wamplers book here not too technical basically tells you what some of the components do page 15 to 24

https://drive.google.com/file/d/0B1sj33FDgVfUSDR3NFBwam54Uzg/view?usp=drivesdk

 

[Sasan]

I doubt the trimmer is actually moving, I’d guess it’s more likely you have another intermittent issue (cold joint or something shorting). Just a couple of thoughts of things that sometime come up, maybe you’re on top of these already...
- where did you source your j201s? Through hole type or smd?
- are your jfets socketed? I only view sockets as permanent on op amps. Transistors and other parts I always solder in before boxing so they can’t lose contact.
- is your dual pot (bass) insulated from touching the board?

Regarding comparison with the original, later Benson versions omitted a 10uF cap (noted in pedalpcb’s build docs), so you could check if your build matches the version of the benson you have.

Thanks for your reply.
Regarding your questions:
- I got the j201s from Musikding (European distri of PedalPCB), they normally have decent stuff (sometimes they fall for Chinese fakes though), but I think the JFETs are ok. I have no way to test them, though...
- They are not socketed. Well, they are smd and were soldered on a small board beforehand, I just put lugs on them and soldered them to the PCB.
- yes, I have those self-adhesive pads for potis...

Will definitely try to see how it sounds without the C10.
Have to re-solder everything and see if there was a cold joint...

Sounds like cap might be playing up obviously caps charge up and discharge so there's a variance and as @phi1 says unlikely the trimmer would randomly vary

You'd need someone with greater technical knowledge than me but basically R11 and C10 set the gain or q and also help keep it stable

There's some info in Mr Wamplers book here not too technical basically tells you what some of the components do page 15 to 24

https://drive.google.com/file/d/0B1sj33FDgVfUSDR3NFBwam54Uzg/view?usp=drivesdk

Thanks, man! Will try how it sounds without C10!

 

[phi1]

Since the jfets are Smd I think they are probably ok. The ones you have to look out for are through hole j201 and 2n5457.