What's up with the clean channel on my Fender Champion 30?

[mattysaurus]

All the solder connections *should* be good since I reflowed each with fresh solder. With the amp off, I confirmed continuity between R56 and d7/d8. I also confirmed that d7 and d8 and operating correctly.

Hmm, I'm guessing the next step is to check Q1 and Q2?

[The Dude]

Yep. Also check the gates of the FET's and see if the voltage there tracks with channel switching.

[mattysaurus]

Yep. Also check the gates of the FET's and see if the voltage there tracks with channel switching.

Okay, so I haven't done a voltage check yet, but here are the results of the ohm reading. What I understood from the spec sheet is that with the J111 the resistance one polarity should be high, with the other direction being low. On Q1, which goes to the drive channel, I get about 8.5 ohms resistance from source to each gate, and 0.L in the reverse direction. On Q2 however, which goes to the clean channel, the resistance is about 13.5 both ways. Ground on Q2 still shows continuity.

I dare to ask, but could this be it?

[The Dude]

I'm not sure what you're talking about there. A FET has a gate, source, and drain. Below is the datasheet for a pinout. Can you clarify your readings and let us know which pins you are measuring? It would also be helpful if you could measure gate voltages for both clean and dirty and post them. Also, when the amp is in dirty mode, the fet's should be near shorted source to drain and much higher resistance in clean mode. You can measure the resistance from source to drain on the fets in each mode and see if they are switching. That said, they won't switch if the switching voltage isn't getting there, so check that first.

https://www.onsemi.com/pub/Collateral/J111-D.PDF

[mattysaurus]

Okay, so before I test that, I retested everything and pins 1 and 2 of Q2 are both shorting to ground, but only when the heatsink for IC u5 touches the chassis. When I hold the board in the air there's no continuity. It would normally touch the chassis as it's screwed in. Does that have any relevance?

[The Dude]

That is normal as one leg of Q2 is ground and it's a jfet.

[mattysaurus]

Ugh okay, to clarify. I've retested a half dozen times now, and Pin 1 and 2 both shorting to ground no matter whether I'm lifting the pcb up or letting it rest on the chassis. You're saying that's normal that two pins short to have continuity with ground?

[The Dude]

I edited my post above to respond to your edit.

[mattysaurus]

I edited my post above to respond to your edit.

Okay, believe it or not I'm actually getting better at reading this schematic. Thank you again for your patience. So you're saying the next step is to check the voltage at Q1 and Q2? Which pin should I be checking? I'm guessing Pin 3 on Q2, but what about Q1?

Update: on Pin 3 of Q2, I'm getting about -184 on Clean and 1.0 on Drive. That seems... off.

[The Dude]

Yes. Check the gates (pin 3) and see if it tracks when you switch from clean to dirty.

[mattysaurus]

Yes. Check the gates (pin 3) and see if it tracks when you switch from clean to dirty.

Check out my update above.

I tried testing points 11 and 12, but they came back with 0 on both. In both cases, I set my multimeter to ACV, black probe to ground, red to the closest capacitor with continuity. Am I doing something wrong?

[The Dude]

Check out my update above.

I tried testing points 11 and 12, but they came back with 0 on both. In both cases, I set my multimeter to ACV, black probe to ground, red to the closest capacitor with continuity. Am I doing something wrong?

No. Not test points 11 & 12 and not AC. Check the DC voltage on the gates (pin 3) of the FET's both clean and dirty and see if they are being "told" to switch. We know the switching voltage is at TP18. I want to verify that it's getting all the way to the FET's.

[mattysaurus]

No. Not test points 11 & 12 and not AC. Check the DC voltage on the gates (pin 3) of the FET's both clean and dirty and see if they are being "told" to switch. We know the switching voltage is at TP18. I want to verify that it's getting all the way to the FET's.

Just copying this from above (sorry, I'll stop editing posts).

"Update: on Pin 3 of Q2, I'm getting about -184 on Clean and 1.0 on Drive. That seems... off."
I got these results with black probe to ground, red probe to closest pin with continuity, and multimeter set to DCV.

Just for future reference, should I not test points 11 and 12 with ACV? I thought since the test point was VAC it'd be the correct way to do it.

[The Dude]

Yes, those test points are AC. We aren't concerned about that right now. We're looking for a switching issue. And, it's probably 184 millivolts- not volts.

[mattysaurus]

Yes, those test points are AC. We aren't concerned about that right now. We're looking for a switching issue. And, it's probably 184 millivolts- not volts.

You are correct again. On the clean channel the reading is -185 mV and on dirty the reading is 1mV. Is that good or bad? I’m wagering a guess that it’s good because the voltages are switching?