Carvin Legacy 212 problem

[guitardog]

I forgot about that. You are correct. Shorting across R21 did eliminate the noise and produced hum. I'll change it, but I don't think I have any 33K on hand so it may be a few days. Thanks for the help

[guitardog]

I will do that too Enzo. I just reaffirmed that shorting across R21 did eliminate noise, and in doing so there were some other noises being generated as I probed around that resistor. I'll probably have to order some 33K so it will be a few days. Thanks for the help!

[The Dude]

The value isn't that critical. Got a 27K or couple of 15K's you can put in series? Also check that the ground side of R21 is indeed ground. You may have a poor ground connection. Maybe even run a clip lead to ground to make sure you have a good connection there.

[guitardog]

I will have to see what I have. I will check the ground as well.

[Enzo]

Don't worry about the hum. At this point our procedure is not to fix the amp, it is to find the problem. The problem we have is noise. We are fighting to eliminate all the various sources of noise that might be - we need to narrow it down to the smallest area. SO we short things out in the signal path. If it kills the noise, then the noise is coming in either AT or BEFORE that short. The fact the short ads hum or something is irrelevant. All we care about in the test was whether it killed the noise or not.

[g1]

The way the threads run on here gets confusing so I don't see posts right away.

At the upper right near the top of the page is a tab marked 'display'. Make sure it is set to 'linear'.

[guitardog]

Thanks g1!

[guitardog]

I was able to dig up a 33k resistor to replace R21. I cleaned around R20 and R21 with alcohol. No change. the noise is still present. I am thinking that the hum produced when shorting across 21 is loud enough to mask the noise. This thing makes my head hurt~! LOL

[The Dude]

......I am thinking that the hum produced when shorting across 21 is loud enough to mask the noise.......

To eliminate that possibility, I'll go back to my earlier suggestion of unsoldering one end of C66 instead of shorting R21. It may give us better information.

[guitardog]

I'll try to do that in the morning. Thanks

[guitardog]

Update. I lifted one side of C66. Noise still present.

[m1989jmp]

Update. I lifted one side of C66. Noise still present.

Concentrate your efforts now on the circuit surrounding Q5.

Resistors connected to high voltage (like R29) often can go bad. Try lifting R29 followed by R82 and R83. Make sure if you replace the mentioned resistors that your solder joints are solid.

[guitardog]

Ok, will do. Am I correct in reading the schematic that Q5 is being used as an amplifier in this circuit? To recap, I already removed and replaced Q5 and when I 1st powered up the amp the noise was gone. I did have a signal to the speakers but it was very weak with Q5 removed. So looking at the schematic, am I correct that the signal path with Q5 removed was routed through R83 and R65?

[Enzo]

No, R83 and R65 both go to ground. The signal path is not through ground. With Q5 removed, you are hearing crosstalk - the later stages are picking up signal from the earlier stages.

[guitardog]

Thanks Enzo. I was wondering how I was getting a signal without a complete circuit.