Carvin Legacy 212 problem

[guitardog]

A few questions:

1) When you switched to the clean channel, the noise started. When you switch back to the lead channel, does the noise go away again?
2) What are the DC voltages on Q5 (E,B,C)?
3) Do you have a scope?

The static is always present, with or with out a guitar connected, and with all controls at zero. The only time the static goes away is if a cable is plugged into either effects loop jack. If i patch through the loop, the static is there. Voltages are 40 (E) 40 (B) 150 (C). I do not have a scope.

[The Dude]

I'm thinking we rounded. What is the voltage base to emitter?

[guitardog]

I'm thinking we rounded. What is the voltage base to emitter?

Emitter to base is .4. Emitter to collector is 40.

[The Dude]

I might try unsoldering one side of C66 and see if the noise goes away.

[guitardog]

I might try unsoldering one side of C66 and see if the noise goes away.

Ok, I'll do that tomorrow when I get home. Just to summarize, I removed Q5 and the noise went away So I replaced it and C66 and C67. It worked for about 10 minutes then the npise returned.

Thanks.

[Enzo]

Removing Q5 stops the noise. That tells us Q5 COULD be noisy, but all it really tells us is that the noise is coming from Q5 or before. And by "Q5" we include R29, 82, 83, 65.

My approach would be to get the noise going, then short across R21. NOise stop or continue? SHort across R83, kill noise or not? I can't short across R82, but that 4.7 MEG resistor is a very high value, and it gets me suspicion.

[m1989jmp]

The latest: I replaced Q5, C66 and C67 today. I put it all back together, and powered it up. I let it warm up for about a minute, then flipped it off standby. No noise. All controls at zero. I grabbed a guitar and started playing on the lead channel. It sounded good. I pushed the button to try the clean channel, there was a slight "pop" sound and bam, the static was back. So now I'm pretty much back where I started. Defective transistor from the factory? I did get 2 of them in the event that happened.

It's weird that the issue started again when relay was engaged. Mick mentioned earlier that relay contacts can go bad too.

Try this just to make sure relay is fine. Switch to clean channel and bypass relay pins 9 and 13 with alligator clips, preferably from R8 to R20 just to bypass a few solder joints too.

Switch to lead channel and bypass from volume 1 wiper to R20.

[guitardog]

Removing Q5 stops the noise. That tells us Q5 COULD be noisy, but all it really tells us is that the noise is coming from Q5 or before. And by "Q5" we include R29, 82, 83, 65.

My approach would be to get the noise going, then short across R21. NOise stop or continue? Short across R83, kill noise or not? I can't short across R82, but that 4.7 MEG resistor is a very high value, and it gets me suspicion.

Ok, here is what I found. First, I want to point out that I had some trouble locating R82 and R83 as Carvin printed the labels under some of the components. I checked color bands to verify, and what I suspect to be R83 checked out as 1.0M per schematic, but R82 by color bands is 4.2M. Yellow, Red, Green, Gold. I can't find a 4.2 any where on the schematic, so I am a little confused about that. On to results. Shorting across R21 stops the noise but creates a hum. I was able to short across R82 and it canceled the noise, as did R83. I accidentally shorted between the same side as 82 and 83, noise gone. I attempted to short between the other ends and when I touched the lead of 82, there was a noticeable change in the static and it seemed like I could here the probe touching the lead. When I did short the other side of the 2, I just got low hum. I probed R82 with a chopstick and there seemed to be some change in the noise. I shorted between R8 and R20 with no change. There was also no changed between volume wiper 1 to R20.

[guitardog]

It's weird that the issue started again when relay was engaged. Mick mentioned earlier that relay contacts can go bad too.

Try this just to make sure relay is fine. Switch to clean channel and bypass relay pins 9 and 13 with alligator clips, preferably from R8 to R20 just to bypass a few solder joints too.

Switch to lead channel and bypass from volume 1 wiper to R20.

Ok, here is what I found. First, I want to point out that I had some trouble locating R82 and R83 as Carvin printed the labels under some of the components. I checked color bands to verify, and what I suspect to be R83 checked out as 1.0M per schematic, but R82 by color bands is 4.2M. Yellow, Red, Green, Gold. I can't find a 4.2 any where on the schematic, so I am a little confused about that. On to results. Shorting across R21 stops the noise but creates a hum. I was able to short across R82 and it canceled the noise, as did R83. I accidentally shorted between the same side as 82 and 83, noise gone. I attempted to short between the other ends and when I touched the lead of 82, there was a noticeable change in the static and it seemed like I could here the probe touching the lead. When I did short the other side of the 2, I just got low hum. I probed R82 with a chopstick and there seemed to be some change in the noise. I shorted between R8 and R20 with no change. There was also no changed between volume wiper 1 to R20.

[guitardog]

I might try unsoldering one side of C66 and see if the noise goes away.

I did not unsolder C66 yet because I wanted to try some of the other suggestions first. I will post what I found after this. I did short C66 to ground and the noise goes away.

Ok, here is what I found. First, I want to point out that I had some trouble locating R82 and R83 as Carvin printed the labels under some of the components. I checked color bands to verify, and what I suspect to be R83 checked out as 1.0M per schematic, but R82 by color bands is 4.2M. Yellow, Red, Green, Gold. I can't find a 4.2 any where on the schematic, so I am a little confused about that. On to results. Shorting across R21 stops the noise but creates a hum. I was able to short across R82 and it canceled the noise, as did R83. I accidentally shorted between the same side as 82 and 83, noise gone. I attempted to short between the other ends and when I touched the lead of 82, there was a noticeable change in the static and it seemed like I could here the probe touching the lead. When I did short the other side of the 2, I just got low hum. I probed R82 with a chopstick and there seemed to be some change in the noise. I shorted between R8 and R20 with no change. There was also no changed between volume wiper 1 to R20.

[Enzo]

4.2? Perhaps that red band started life as violet and has aged. In any case, the difference between 4.2 meg and 4.7 meg is irrelevant.

OK, now unsolder on end and lift C66, does the noise stop? Oh wait, shorting across R21 stopped the noise. That means the noise is at least that far back, nothing to do with the Q5 circuit. The fact it makes hum doesn't matter, we are chasing noise. The hum is from your test wires.

Ground the left end of R20?

If that stops the noise, we are back to the channel relay. SO does changing channels have any effect on the noise? Does VOLUME 1 adjust the noise? On the other channel, does shorting R8 kill the noise?

[guitardog]

Grounding the left end of R20 on each channel does nothing. Grounding the right end causes hum both channels. Yes, shorting across R21 stopped the noise on both channels. None of the controls has any effect on the noise. Changing channels doesn't seem to have any definitive effect on the noise. The static is very random and occasionally it pops and hisses for no apparent reason.

[Enzo]

Look up top there, see B4 and B5, with 20db printed nearby? I see those are the send and return to the reverb circuit. Does turning the reverb control up and down affect the noise? Shorting across the reverb control SHOULD have the same effect as shorting R21. Does it?

[guitardog]

Yep, I see it. Be aware that I disconnected the reverb tank and wires to eliminate them as a cause. With that being said, adjusting the reverb control has no effect on the noise.

[guitardog]

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?