New Metro JTM45 build - Hum without Brightness cap

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ghancq
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New Metro JTM45 build - Hum without Brightness cap

Post by ghancq » Mon Nov 14, 2022 3:40 am

Hello all, New first time amp builder here. I finished my Metro JTM45 and had some issues along the way but everything seems to be working well for the most part. But the High Treble channel is really tinny, to the point it is hard on the ears. I of course tried EQ and that didn't quite tame the treble as I hoped it would so I did some reading and came across several posts that suggested removing the Brightness Cap on the Volume pot. That cleaned things up nicely however now when I turn the volume below "2" on the HT volume channel or even all the way to "0" I get a fairly loud hum at ~120 cycles, putting the brightness cap back cleans it up.

I traced the circuit and doubled checked all resistors and caps (caps for shorts) in the circuit and all looked Ok. The Amp is not in a cabinet so there is no shielding but I suspect this is not a shielding issue since it does it when the volume is at zero. The Normal Volume channel is clean through the entire sweep of the pot. Also I am using the brass Mojo CTS pots.

Is this normal or do I have an issue somewhere?

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