Son of Ben - No Signal

[Eike]

Hey there,

I tried to build a Son of Ben. It’s my second attempt to build a pedal ever, so please excuse my lacking knowledge.
The first mistake I made was to put everything in the case without testing first. Dry through was working, LED worked but no wet signal. I got everything out of the box again and checked if any of the pots were touching anything. I messed up the top right film capacitor and am not sure if that’s causing any trouble.

I am a little lost here. Something I was noticing while trimming Q2 and Q3: I get values in the range of 0.4V. But it should be 4V, right?
Any ideas? Would really appreciate any help

 

[jcpst]

Is this touching? Hard to tell.



Also, where did you source those J201s? An authentic through-hole version is a pricey part.

 

[Barry]

Have you Biased the transistors? Solder is a mess, many of those joints look pretty cold.

 

[chris.knudson]

I have to second @jcpst. Where did you get your 201s? There are a lot of bad ones out there -- especially if you bought them from eBay or Amazon. You can get good J201s, but about the only places you can trust them from are Mouser or Digikey. Also, what is the purple plastic on the bottom of the B1M. If that's electrical tape, that's probably no good. Leads are pointy, and they will go right through electrical tape -- you really need a hard plastic to prevent accidental shorts. You're better off to use one of the dust shields from one of the other pots there.

I agree with @Barry that you have some suspect solder joints, but I don't see anything that would be causing your problem. My guess is that the problem is either a short on the B1M or a transistor issue -- either a biasing issue or a bad transistor.

 

[Eike]

Thank you all!
Yes, the purple plastic is electrical tape, I removed it. I tried to reflow the solder points which looked questionable, but still nothing.

I ordered it as a kit from musikding.de so I kind of expect the J201s to work. But I try to replace them.

 

[AceHyzer]

The solder joint on your positive power looks suspect. Also, after removing the electrical tape did you replace it with something to prevent the pot from shorting the points below it?

As Chris mentioned above, you can pull the dust shield off one of the pots that are hanging off the board and put it on that pot if you don’t have another dust shield. Since those two pots hang off the end of the board they won’t touch and therefore short any solder joints.

 

[peterzahn]

Hey Folks,
a friend just came to me with the same problem and today I had a detailed look on the pcb and the components.
With an oscilloscope I identified that the signal is not amplified by the first JFET.
The JFETs he got are marked with "JCJ 113". After a quick search I think that these types are J113 JFETs.
The datasheets of the J113 and J201 are not very good to compare. But the datasheet of J201 states that it is made for audio applications, J113 doesn't mention that.
Then, I did an LTSpice simulation of the first part with both JFET. The amplifier is working with J201 but not with J113.



As you can see the V(out) signals are very different. The signal is amplified by J201 but not by J113.

My friend got his kit from Das Musikding in germany.

What is your marking on the JFET?

 

[temol]

What's the voltage on the J113 drain?

 

[temol]

So.. it's possible that you have around 1V on a J113 drain, mayble less. With J201 you have around 5-6V.
Assuming you changed only transistor, without adjusting Rd (R13 or R12) or Rs.
That's because J113 has higher Vp and Idss. This 1V is way too low. You can change source resistance to 3k9 or 4k3 but don't expect miracles.

You can also try to bypass the Rs with a capacitor and resistor in series with capacitor.

Here's an example.


Green - J201, red - J113


And frequency response

 

[swelchy]

id say clip the suspect j201 and bite the bullet and get some j201 adapter boards and throw some legs on them or do some video watching and dive into laying some smd j201's down.. and reflow a lot of your joints.... insulate back of pots correctly

 

[peterzahn]

@temol thank you for the explanation.

Well, the way to go is to get the correct JFETs. On the PCB are SMD footprints for that part so it it easy to solder them on.

 

[peterzahn]

FYI: I ordered some MMBFJ201 from mouser, soldered them on, and it works!
These are SMD types, cost only 0.30€ in a pack of 10.
I have some left, if someone is interested, inside the EU I can ship them.