Quiescent noise barely heard in headphones, with speaker is pronounced

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KA1J
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Quiescent noise barely heard in headphones, with speaker is pronounced

Post by KA1J » Sat Mar 09, 2019 12:20 am

A question for any techs that have come across this before. Any suggestions are very much appreciated.

I hear a background noise from my amp which sounds like a crackly buzzing (not 60 Hz hum), which has frequent clicky somewhat loud bursts which are obvious in a quiet room. The audio appears perfect, no distortion in the clean channel and I have re-tubed the amp to see if that cleared it up (it did not). The clicks are never as loud as a played note. the sounds come and go, sometimes lasting a few seconds, usually about a second. They seem to build up and suddenly end. There is an underlying hum but again, it doesn't remind me of a 60 cycle hum.

I have been planning on using my LCR meter to check for capacitor/resistor issues and to lube/clean all the potentiometers with Deoxit. However, tonight I tried using headphones and used the speaker mute switch to play headphones only. Wala, there is almost perfect silence except for the sounds from the instrument. Reverb and all the contour settings are almost clean as a bell with the headphones only. Listening to the headphones for a while I can detect the tiniest bit of what I hear in the speaker, it is nowhere near as pronounced.

As these sounds are only pronounced when the speaker is engaged & I can't imagine it's the speaker, I feel it is an issue accentuated in the final circuit.

Any suggestions of something in particular I ought to check out?

Thank you.

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domct203
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by domct203 » Sat Mar 09, 2019 8:35 am

It would help to know which Recto this is, and how you are getting the headphone signal.
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KA1J
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by KA1J » Sat Mar 09, 2019 1:10 pm

Hi Dom,

Ugh, I posted this in the wrong forum.

Apologies, my mistake, I meant to post it in the F-series forum. It's an F-50. There is a switch in the back to mute the speaker and the headphone jack is near-by.

I know some of the Mesa line do not have this option, it's one of the reasons I chose this particular amp, I can practice & not wake the GF.

silentbob
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by silentbob » Tue Mar 12, 2019 8:39 pm

I had a similar issue with my Road King and was fortunate that there is a good tech (electrical engineering professor) within reasonable driving distance. He replaced some of cheap carbon film resistors with higher quality components and it has worked like a champ since then.

KA1J
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by KA1J » Tue Mar 12, 2019 10:25 pm

Thank you for that reply.

Sunday I removed the head and used deoxit on all the pots and every electrical connection including the parallel connectors inside. The difference in the smoothness of the pots was remarkable, as expected, but there was no change in these noises.

I removed both 6L6 tubes and listened through the headphones and the noises were there still. For fun I exchanged the Mesa Svetlana 6L6 tubes with a matched pair of Sylvania 6L6 and there was no improvement. I replaced each of the 12AX7 with Telefunken NOS and then Sylvania 5751 Gold Brand tubes and the noises remain.

Seeing as the noises exist irrespective to the tubes, pots and connector cleaning, the issue has to be from an internal component(s). You mentioned the carbon film resistors; I looked carefully at all the components for any sign of heat/stress, anything showing change and all looked perfect. I do find if I crank up the gain and leave the master volume low there is a minimum of noise. A small amount of gain and a large amount of master brings on the noise.

I'm not sure what this means in terms of where to look, but the master gain circuit seems likely. Do you recall where in the circuit the resistors were replaced? I should look in that area on this amp and see if I find anything.

Thank you again.

silentbob
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by silentbob » Wed Mar 13, 2019 7:48 pm

The tech had my amp for a few weeks and had it back together before I picked it up. I haven't taken it apart since then to look for changes.

KA1J
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by KA1J » Wed Mar 13, 2019 9:13 pm

I do thank you for the reply. I'll have to do what I can to check the resistors, there's a flock of them in there but it's only a project.

Cheers!

silentbob
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Re: Quiescent noise barely heard in headphones, with speaker is pronounced

Post by silentbob » Wed Mar 13, 2019 11:16 pm

They weren't damaged or out of spec, just noisy.

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