No Sound coming out of my Cobalt Drive

[GregDS]

Hello there,
that's it I give up... Solid state troubleshooting is not my forte and after trying signal-tracing with my oscilloscope and getting results I can hardly interpret, I turn to you folks.
I built the Cobalt Drive a couple of months ago and no sound is coming out of it, no scratchy pots, no funny sound, just none at all.
I'm really at loss troubleshooting it. With a tube amp, it's sort of easy, you trace the signal, in voltage amplitude and when you get less than expected, you're close. But with solid state I dont really look where to look as I dont know what behavior I should expect from these components.
I check for continuity where there shouldn't be. I know the PCB looks really dirty, but it's flux, not solder all over the board.
Can you folks help me out?

Thanks a ton!

 

[GregDS]

On the collectors of Q5 and Q6 I'm getting around 7.5V, around the same I was getting with the J201, IC1-6 gives me 4.85V. pretty much the same results I was getting before the swap...
You mention matching Vp, is there a way to measure this easily?

 

[A2K]

I think you have Q2 & Q3 switched in your measurements above. Take another look at page 1 of the build doc.

Based on these measurements, it's a fair bet that your JFETs are bogus. We can do a quick test on them out of the circuit.

Remove one of the JFETs, then measure resistance from D to S. You should get <200Ω and it should read the same in both directions. If that reads correctly (I'm betting it doesn't), then use the DIODE measurement function on your DMM to measure the forward voltage from G (red lead) to S (black lead). You should get around 0.6V.
I think you already know the pinout, but here it is just in case.

View attachment 6135

I am preparing to build the Cobalt Drive and am trying to determine if the J201's I have are fake/out of spec (I don't even remember where I got them from...possibly a local electronics store). I did the tests outlined above and got readings of around 500-600 ohms for resistance (same in both directions) and around 0.7V for forward voltage. So a few questions:

1) Does this indicate fake/out of spec J201's?
2) Can I still use them in the Cobalt Drive?

 

[Chuck D. Bones]

1) Could be out of spec, or possibly your DMM is putting a little too much current into the JFET.
2) They will probably work in a Cobalt, but you want to match the Vp as close as possible for each pair.

Vp is easily measured with a DMM and a 9V power supply (or battery). Connect JFET Drain to +9V with a 10K resistor in series. The resistor is there to protect the JFET in case you hook it up wrong. Connect Gate to the minus side of the power supply or battery. Set DMM to measure voltage. Connect DMM from Gate to Source (+ lead on GATE). The DMM will read Vp. Vp will be a negative number. The closer you match Vp, the better. Match Q1 to Q3. Match Q2 to Q4.

 

[A2K]

1) Could be out of spec, or possibly your DMM is putting a little too much current into the JFET.
2) They will probably work in a Cobalt, but you want to match the Vp as close as possible for each pair.

Vp is easily measured with a DMM and a 9V power supply (or battery). Connect JFET Drain to +9V with a 10K resistor in series. The resistor is there to protect the JFET in case you hook it up wrong. Connect Gate to the minus side of the power supply or battery. Set DMM to measure voltage. Connect DMM from Gate to Source (+ lead on GATE). The DMM will read Vp. Vp will be a negative number. The closer you match Vp, the better. Match Q1 to Q3. Match Q2 to Q4.

Thanks very much for the response and clear instructions on how to measure Vp. I was able to find a pair measuring -1.273 and -12.75 and another pair measuring -1.200 (both) so I should be able to use these for Q1/Q3 and Q2/Q4.

Just curious - is this Vp matching always required/recommended for J201's? Or does it vary from circuit to circuit? I am also planning on building the Super 64 Overdrive, but those have trimmers for biasing. Do those trimmers allow for proper matching/biasing in that circuit?

 

[Chuck D. Bones]

Looks like you lucked out and found two good matches (you slipped a decimal point there). Vp spec for J201 is -0.3V to -1.5V. Yours are pretty "hot," but should work ok in the Cobalt. If I was building it, I'd bump R6 & R7 up to 6.8K with those JFETs.

The Blues Driver circuit requires JFET matching for proper operation because the JFETs are configured as a differential amplifier. Most other OD pedals are not that config and do not require matching. For the most part, JFETs are not plug-n-play. Some circuits are more forgiving than others. Trimmers will accommodate some variation in JFET parameters, but are no guarantee that every JFET will work. Professional pedal builders have to test and select their JFETs for most circuits to obtain the desired operation.

 

[A2K]

Looks like you luckout out and found two good matches (you slipped a decimal point there). Vp spec for J201 is -0.3V to -1.5V. Yours are pretty "hot," but should work ok in the Cobalt. If I was building it, I'd bump R6 & R7 up to 6.8K with those JFETs.

The Blues Driver circuit requires JFET matching for proper operation because the JFETs are configured as a differential amplifier. Most other OD pedals are not that config and do not require matching. For the most part, JFETs are not plug-n-play. Some circuits are more forgiving than others. Trimmers will accommodate some variation in JFET parameters, but are no guarantee that every JFET will work. Professional pedal builders have to test and select their JFETs for most circuits to obtain the desired operation.

Awesome, thanks for the info and advice (and you're right, I meant -1.275, not -12.75...)

 

[Forjo]

Did you ever solve this? I have the exact same problem. Checking the J201s and ordered the SOT versions.