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

[mattysaurus]

Hi folks!

Picked up a Fender Champion 30 over the weekend for a Jackson and it works, but... not really how it’s supposed to. The amp has two separate controls: a single volume pot for clean, and a separate drive and volume pot for overdrive. Currently, however, the volume pot for clean doesn’t work at all, but the drive and volume pots that should only affect the overdrive channel also control the clean channel. So basically, it's stuck in overdrive.

These mim’s are apparently kinda notorious for bad solder joints, and sure enough, there were about a half dozen on the pcb, mostly pot leads. I ended up retouching every point with fresh solder, and while that fixed some problems (less hum!), it didn't fix the channel switching. The LED turns on and off, but I have to assume whatever relay switches the drive has a short.

Unfortunately, I don’t have a ton of experience with schematics, but I have found at least one resistor (R69, one of the large white rectangular ones) that is reading 0 when it should be reading .3 on my multimeter, so at the very least that one has to go. I've been trying to teach myself to read the schematic, but it's been slow going. I believe it's possible the BA4560 might've gone bad, but I'm not even quite sure where to place my test points to confirm. I've included a picture of the part of the schematic that deals with the channel switching, any help would be greatly appreciated!

https://imgur.com/a/31INu07

[Jazz P Bass]

Here is the Champion 30 DSP service manual.

Champion 30 DSP .zip

Read Page 5 as it pertains to the signal flow.

[The Dude]

^^^^^^ That, and resistors don't short, so either your meter isn't zero'd properly or you're reading across another shorted component in regards to R69.

[Enzo]

The resistor is fine.

SO the switching circuit. OK, the schematic has test voltages, so what results do you get when you test those? The LED goes off and on? The the IC is working. But verify the voltage at pin 7.

The channel switching is made by Q1 and Q2. So follow R56 from IC pin 7, 1 meg. Pin 7 should toggle from -14 to +14 volts when you push the button. Does it? Now assuming it does, what does the other end of R56 do?

[The Dude]

Champion 30 DSP.pdf

I had a bit of a problem with the .zip file, so here it is unzipped.

[Enzo]

The snippet he posted was from the non-DSP version.

[mattysaurus]

The resistor is fine.

SO the switching circuit. OK, the schematic has test voltages, so what results do you get when you test those? The LED goes off and on? The the IC is working. But verify the voltage at pin 7.

The channel switching is made by Q1 and Q2. So follow R56 from IC pin 7, 1 meg. Pin 7 should toggle from -14 to +14 volts when you push the button. Does it? Now assuming it does, what does the other end of R56 do?

First off, thank you taking the time to look at this in depth. Second, I'm pretty amateur with this so please be patient with me as I ask a few questions out of an extreme abundance of caution.

First off, I need to have the amp powered on to get accurate measurements, correct? Second, so I need to touch pin 7 to r56 correct? Does it matter which pole touches which? I would assume the black probe touches the pin while the red probe touches the resistor?

Finally, you mention Q1 and Q2, but I do not see those on the schematic, at least on the snippet I posted. What are these referring to?

[Jazz P Bass]

Non DSP: fender_champion-30_sch.pdf

[Enzo]

No, you ground the black probe to the chassis, then the red probe is used to take readings. I am sure there are You Tube videos of tutorials on how to use a volt meter.

If you don't know how to use a meter, this will be rough going.

[The Dude]

......First off, I need to have the amp powered on to get accurate measurements, correct? Second, so I need to touch pin 7 to r56 correct? Does it matter which pole touches which? I would assume the black probe touches the pin while the red probe touches the resistor?......

Yes, the amp must be powered on. Test point 18 or IC U5B pin 7 is measured to ground. So, black probe on ground and red on pin 7. Switch from clean to drive channels and verify the voltages are approximately what is noted in the box for the test point. Be extremely careful your probe does not slip. For that reason, it's probably better to take the measurement from R56, which is tied to the IC pin and the same point electrically. (see post #8 for schematic)

Edit: <simulpost>

[mattysaurus]

Yes, the amp must be powered on. Test point 18 or IC U5B pin 7 is measured to ground. So, black probe on ground and red on pin 7. Switch from clean to drive channels and verify the voltages are approximately what is noted in the box for the test point. Be extremely careful your probe does not slip. For that reason, it's probably better to take the measurement from R56, which is tied to the IC pin and the same point electrically. (see post #8 for schematic)

Edit: <simulpost>

Got it. Does it matter which side of the resistor I touch it to?

Again, thank you for being patient with me. I'm treating this as a learning experience since I didn't spend much money on the amp, but I'd prefer not to screw it up.

[The Dude]

You'll want the side that connects to pin 7 of the IC. If you can't tell which side that is by looking, check for continuity to pin 7 with your meter and power off. Or, as I said, you can just check directly on the IC. Just be careful your probe doesn't slip and short pins.

[mattysaurus]

Alright! Now we're getting somewhere. Both of the voltages check out in the ball park. Switching from clean to overdrive I can also hear an increase in hum from the speakers. As mentioned before, the volume does nothing, but it does seem to work because it must be a little dirty and is causing a small amount of crackling when I move it a lot. The volume and drive pot on the overdrive channel continue to affect the clean channel.

[The Dude]

Check the solder connections on R56 and check that there is a connection (with a meter) from the resistor to both D7 & D8 cathodes. It's more likely a connection problem than both fets not swithcing off.

[mattysaurus]

Check the solder connections on R56 and check that there is a connection (with a meter) from the resistor to both D7 & D8 cathodes. It's more likely a connection problem than both fets not swithcing off.

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.