Carvin Legacy 212 problem

[Enzo]

Yes, the 180k R20 is the equivalent of that 3.3M resistor in Fender amps.

Shorting R21 kills the noise, so it seems to be coming from that far back. Looking back, the circuit branches two ways, one through R20, the other through B5. Grounding the far end of R20 does not kill the noise, so I look up the B5 route and come to the reverb control. Turning that control to zero does not kill the noise, so that tells me the reverb circuit is innocent. That leaves the pot. We could remove the pot to see.

I would also be looking at the copper traces on teh board. From the top of R21, follow every branch and curve of that coper trace and look to see if any contamination is on teh board or anything suspect.

[guitardog]

So the signal path (common to both channels) passes through pin 9 of the relay and on to B4 and R20. Is that correct? And then the wet signal which is controlled by the reverb pot is mixed back in on the other side of R20? To recap, shorting across R20 causes a load hum as does grounding the right side of R20. Grounding the left side (coming from the relay) has no effect. I'll get back on it tomorrow after work. Thanks for your help!

[Enzo]

ALmost, pin 9 is the input side of the relay and only has the LEAD 1 channel. Pin 9 carries only CLEAN 2 channel. The relay common is pin 13. But yes, reverb wet returns to the right side of R20.

[m1989jmp]

I think you are on to something though. Am I reading the schematic correctly in that B4 is the "dry" signal to the reverb and B5 is the "wet" or return?

How about pulling V4 (reverb driver tube) out? Just to eliminate the associated circuit. I know you said all tubes are new, but there can be something funky around that tube.

[guitardog]

Sorry Enzo, I meant pin 13. I was thinking about pin 9, so that's what I typed. Ok, when I got home I removed V4 and the noise persists with no tube in V4. I removed V1, then V2 individually and the noise is still there. I revisited removing V3 and V5, this time individually. With either removed, the noise is gone but I only have output from the lead channel. So before I do anything else, I am going to study the schematic some more and see what you all think. It's got me perplexed! Thanks again all.

[Enzo]

Whether I am on the right track ultimately, did you follow my thinking to isolate the problem by progressive probing?

V5 is the phase inverter, and is WAAAAY after the noisy area we have been exploring, and yes pulling it would kill noise from anywhere in the amp other than the power tubes. V3 - one half is a buffer feeding the phase inverter, and pulling it should also kill sound from anaywher before the power amp. It too is way after the noisy area.

[guitardog]

That's what I thought. So why do I have input with V3 or V5 removed. If I'm reading the schematic correctly, there should be no signal from either channel with V3 or V5 removed. I thought maybe I was overlooking a circuit that was sneaking around either. The last place that I last left off probing was R20. Which shorting across made a loud hum, as did shorting the right side to ground. Shorting the left side to ground did nothing.

[Enzo]

Hum is just your clip wires acting as an antenna. We were trying to find the source of your noise. Whatever kills the noise is a clue - ignoring that the TEST may cause new noises like hum when performed.

With V5 removed, I see no path for signal to reach a speaker.

Are we sure what we are calling V5 in the amp is the same tube called V5 in the schematic?

Is the sound out the speaker with the tube pulled full sounding or thin and tinny? (That would indicate crosstalk)

[guitardog]

It is thin and tinny.

[Enzo]

Then it is probably crosstalk, rather than a direct path. That means the sensitive grids of hte power stage are picking up signal radiated from the preamp circuits. Normally you don;t hear it because it is overwhelmed by the real signals.

[guitardog]

That makes sense.....that avenue of troubleshooting has caveats. So now what?

[Enzo]

I am still focused on that R20/R21 junction. SHorting the resistor to ground kills it, the far end of the series resistor does nothing. So we still want to narrow it down.

R21 itself could be noisy.

I tend to doubt it, but R20 COULD be making some noise. The far end of it doesn't kill the noise, but it is possible.

The path through B5 over to the reverb pot could be the issue.

It is also possible the problem is fooling us and is in R83. SHorting R21 puts C66 across R83, shunting noise like a bypass cap.

SO at this point I might be lifting things. I would unsolder and lift one end of R20. Noise stop or continue?
I would unsolder one end and lift C66. Does that kill the noise?

Lifting C66 tells us the problem is on the R21 side or the R83 side.. Lifting R20 assures us the problem is not IN R20 nor coming through it.

And again we need to carefully explore the entire path of copper traces on the board from R21/20 through B5 to the reverb control, just to see if anything is funny. And I would remove the reverb control just to see.

There is no obvious source of your noise, at least not to me. So this approach may look like a million tiny little steps, but that is what happens when it isn't just a noisy tube or something simple.

[m1989jmp]

That makes sense.....that avenue of troubleshooting has caveats. So now what?

Be careful with the solder pads when you start to desolder components.

Don't pull on anything too hard and don't overheat them, or you will lift pads for sure. Take your time and use solder wick.

I'm sure you know this already, but I'm just putting it out there anyway.

[m1989jmp]

That makes sense.....that avenue of troubleshooting has caveats. So now what?

Also, I would try to definitely eliminate the reverb circuit as a source of problems.

I know you said reverb pot doesn't affect the noise, but try grounding reverb pot wiper and R39 junction in case C36 is leaky or the reverb pot is defective.

[guitardog]

Thanks guys. I have a busy few days ahead of me, so it will take a while to get back to it.