Son of Ben Troubleshooting

CanadianDave

Active member
Hey,

This Benson build is driving me nuts.

I’m getting no output, and also, very low voltage at my transistors.

Without transistors in the socket, I’m getting 9v at the socket.
Once I have transistors in, I’m measuring 1.6v to .4v

I found a faulty electrolytic, and reflowed my solder, but nothing has worked.

Any ideas?
 

CanadianDave

Active member
All of the soldering was touched up. There's electrical tape under the pots to avoid shorts.

The 10K resister that is positioned "wonkey" was replaced as a test, because that's where the voltage was dropping so significantly. I also swapped out the 22nf capacitors under the tone controls with no luck either.

There's also a visible bad solder under that treble pot. That's where there optional 10uf capacitor was, and I removed it while troubleshooting.

IMG_1279.jpg
IMG_1276.jpg
 

CanadianDave

Active member
I had a few issues.

One is that a few capacitors were blown. Not 100% certain why.

I pulled out the jfets and traced the voltage leading from the power supply through to the jfet to find the faulty ones.

I also had bad jfets too. The ones from small bear took care of the job.
 

zgrav

Well-known member
even though the tape under the pots is probably OK for insulation, I would bend them up a bit away from the board while you are trying to get it working to eliminate one possible cause.
 

Joshdrain

New member
I had this identical issue which is documented on another thread. I ordered J201's from three different places and the first two batches were fakes. With no other change other than new J201's I went from max just above 1v on the drain to 6v and easily dialed them down to 4v. Now it sounds great. I would find another place to source your transistors.
 

Chuck D. Bones

Circuit Wizard
Would it be possible for you to measure Idss & Vp on some of the "fake" J201s? To satisfy my curiosity as to whether they are just enough out of spec to not work or if they are way out of spec.

I regret not buying more legit J201s from SB when I had the chance. The 3 I did get are in my Corduroy and working well. I picked up 50 of the PF5102s. They are hotter then J201s, but should work in the M800, Pineapple, Stinger & Tantric. The Deofol & Benson would require resistor tweaks to use them.
 
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moonlightpedalbuilds

Well-known member
I got my J201s from china via ebay. Marked as Fairchild components. Trim1 set to 2k ohms and Q2 and Q3 Drain to 4.0v. Sounds good. I also mounted the smd version using PedalPCB Sot23 Adapter. MMBFJ201 came from Tayda. I tested both on....

242D080F-5A2F-44E6-8772-6C8B5E935581.jpeg
Both of them are read as NPN transistors. I have other JFets that are read as NJFET.

Question is, did I get fake ones on both MMBFJ201 and J201?
 

Chuck D. Bones

Circuit Wizard
If they were NPN transistors, you would not be able to achieve 4V on the drains and it would either sound like crap, or you'd get no sound at all. Based on the evidence presented, you got real J201s, or something close enough. If you want to be sure, measure Vp & Idss.
 

Chuck D. Bones

Circuit Wizard
I have the 2019 model of the $17 Chinese everything tester. It identifies JFETs correctly, but does not measure Vp or Idss. It finds an operating point and reports Vgs and Id for that operating point. It has never misidentified a JFET and I've tested a few dozen.
 
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