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

[mattysaurus]

Okay! Q1 test was interesting. Tested Pin 3 again, same method. with the clean channel engaged the reading was -46V (not mV). But when I switched it to dirty my multimeter couldn't get a reading. It kept switching between numbers, all the way up to about 190mV, hitting 0.L and climbing back up with about 30-40mV jumps.

[Enzo]

OK, I am not quite sure what we are doing...

But this: A JFET is ON until turned OFF by a voltage at the gate. If you pick a JFET from your parts bin and measure resistance source to drain, you will get a low resistance. A voltage to the gate turns it off so the resistance goes high.

So if the gate of Q1 gets no voltage, then it remains on, and of course the controls in that channel remain working. Q1 turns ON the gain channel path. Q2 works the other way, it shunts the signal; path in the clean channel. Lacking gate voltage, it stays on shorting the signal to ground. SO see why we worry about switching voltage? No voltage at those gates causes the problem you describe.

Now if the gate of one of the JFETs shorts to the source/drain, then that will stick the part, and that can drag down the gate control so both JFETs are affected,. SO it is possible for a bad Q1 to affect Q2, and vice versa.

About testing them. If you test with power off, they will alwys measure as ON, as in seem to be shorted. You must do these test with power on.

[mattysaurus]

So jumping back on to this, is the weird Q1 issue I described above indicative of something?

[The Dude]

You need to test the FET's to see if they are switching. Measure the resistance from source to drain while in clean mode and again in dirty mode. In clean mode, the resistance should be high. In dirty mode, the resistance should be low. It may be quicker/easier, at this point, to just replace the FET's.

[mattysaurus]

I’m confused. Aren’t Q1 and Q2 the FETs? The J111, right? I’ve already tested them and posted the findings.

[Enzo]

I don't recall what you did, but I explained how they work because the ONLY tests that mean anything are done in a live circuit.

By measuring resistance source to drain across Q1, or Q2, while it is running, and as we switch channels, we discover if the resistance of teh things is chaging or not.


Never think up reasons not to check something.

[mattysaurus]

I’m not thinking up reasons not to check something, I’m just testing what The Dude tells me to test and reporting the readings. He told me to test the voltages on the FET when I switch from dirty to clean, and I found that Q1 was fine on the clean but the readings weren’t stable on the dirty channel.

[g1]

How about the same test for Q2 then.

[mattysaurus]

Check back a little earlier on this thread. Q2 was stable and switching between -184mV and 1V (or mV? It’s late, noted it on the post though) on clean vs dirty.

I ordered some J111’s. I’ll let you know guys know how replacement goes. Hopefully this fixes the problem... I read another thread about a guy with a Champion 110 with the same issue and replacing the J111 fixed it.

[g1]

Not sure if you have it all apart yet, if not, and you want to try something, clip off Q2 close to the board.
You're going to replace it anyway. Now with Q2 missing, see if the clean channel works right.

[mattysaurus]

Okay, so Q2 was the one with the correct (well, at least more correct voltage switching). I removed Q1 and the volume pot on clean works, but the volume is very very soft, basically non existent even at 10.

J111 should be here next week. Hopefully that’ll fix it.

[g1]

Clip out Q2.

[mattysaurus]

Yep. Should’ve listened to you. Took out Q2 and it’s now loud and switching correctly. Will probably just replace both when the new parts come.

[g1]

I just wanted to make sure there wasn't also another problem that could be dealt with while you waited for the parts.
With no Q1 in there, I assume when you switch to gain ch., it just acts like clean ch.? Or how does it act? Just curious, agree with replacing both.

[mattysaurus]

It acted sorta like a clean plus channel. If you upped the gain it got a little dirty, but not nearly as much as it was with Q1 in.

I tested it for about 10 minutes earlier with both fets in and had no issues (aside from switching). I think retouching all the solder points with fresh solder was a good idea.