Greenbean FET concern

[aefpv]

Just made the Greenbean (TS inspired clone). I was rocking before boxing and wondered if I have a problem. With the clipping toggle to the FET, I am getting no clipping. It is essentially unity sound. The pots all work and effect the signal appropriately. All power measurements match those given by Madbean.

I made a few substitutions due to availability. The BAT41 was changed to a 1N34a. Transistors are MPSA18.  The voltage across the 1n34a was 0.04 v.

With this in mind, is the FET supposed to clip (overdrive) the signal? Is the 1n34a or 2n7000 bad or is something else amiss?

[mauman]

The FET clipping (switch right, lugs 2 & 3 shorted) should sound somewhere in between the LED setting (center position) and the silicon diode setting (switch left, lugs 1 & 2 shorted.)  If the FET switch setting sounds clean, the switch may not be connecting the FETs.  Check that the FET switch setting does short toggle switch lugs 2 and 3.  If so, verify that op amp pin 1 is being shorted to Q3 drain and Q4 source, and that op amp pin 2 is shorting to D5 anode and drain of Q4.  If all those connections are being made, then you're down to Q3, Q4 and/or D5.  Make sure each of them is pointed in the right direction. 

[aefpv]

Great suggestions. Continuity was seen at all the suggested points and toggle switch. Am I correct to think that I now have an issue with a a FET or diode? Thanks again

[mauman]

Agreed.  I'd start replacing them one at a time. 

[BryGuy]

I would start with Q4. If you have a diode tester on your multimeter might be a good to check you transistors before plugging back into the circuit

[aefpv]

As an update and for advice:

To date, I have check the following:

1.  Continuity between the toggle (and toggle lugs), IC (pins 1 and 2), diodes, 2N7000 and LEDs.  All connections were correct. 
2.  Voltages have all been verified from IC, 2N7000s, and MPSA18's as correct (based on Madbean voltages given). 
3.  Audio-probed the circuit, including the clipping section, and I have sound in the appropriate places.
4. Reflowed the clipping section of the circuit.
5. Removed LEDs to determine if sound was only going through the LEDs when toggled to run through the FETs.  With LEDs removed, I still obtain clipping with the toggle in LED and diode (1n914) settings (curious finding).  The clean sound is still present with the FET toggle setting.  In other words, no change.



I am going to try the following to further detect the problem. 

1.  Remove Q4 (2n7000).  If no change, then proceed with Q3 and D5 (1n34a).
2.  Remove D3 and D4 (1n914). 
3.  Lastly, remove the toggle switch.

What do you all think?  Good plan of attack or am I wasting my time?  Thanks again for the help.  Cheers.

[mauman]

With the switch in the center position and the LEDs removed, you're still getting clipping from R7 (a bit) and the Drive pot (if it's set anywhere above CCW.)  So it's OK to hear some clipping with LEDs removed.

I would say remove and replace Q4/Q3, then D5.  If any are in the wrong direction, that will also affect your clipping.   MOSFET transistors like 2N7000 are sensitive to ESD (static discharge), it's possible to blow one up by handling them without a static strap or by packaging them in non-metallic plastic bags.  If you have spares, ground yourself briefly before handling and you should be fine.   If D5 is Ge, the filament inside the glass can crack and go open if you bend the leads too close to the glass.  No need to remove the Si diodes.

[aefpv]

I think I have figured it out.  Thanks for all of your help!

It looks like Q4 (2n7000) was bad.  I guess I'll be making another order to Stompbox parts! I got the original 2n7000 from Tayda, so I don't know if it was bad from the factory or I destroyed it with my soldering skills.

To figure it out, I removed the LEDs, but the problem was still there.  Next, I removed Q4 and I was able to get clipping and a volume boost.  I placed sockets for Q4.  When Q4 was placed, the problem returned.

[mauman]

Congrats!  BryGuy called it right.  You can run it without Q4 in the mean time, it will just clip very asymmetrically on the FET setting.

[BryGuy]

Congrats!  BryGuy called it right.  You can run it without Q4 in the mean time, it will just clip very asymmetrically on the FET setting.

Lol just seemed like the best path forward. Trouble shooting is a valuable skill for sure.