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Old 10th October 2006, 09:29 PM   #1
Bengali is offline Bengali  United States
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Default Tube Amp Hum

Hi,

I would greatly appreciate your suggestions on the hum issue I'm having. I have a Jolida 502b. I'm getting hum out of the woofers, both channels.

I'm using the JD-50 remote volume unit so I have bypassed the internal balance and volume pot. A Mogami 2534 cable directly from the pcb input out to the rca jack.

Another problem with the hum is it greatly increases in volume when I have the JD-50 disconnected, since there is no volume pot connected to the input, the hum is just keeps getting amplified as the tubes warm up. I have to power down like within a few seconds. I don't have any source connected at this time.

All the components on the power supply and main board have been replaced with quality new parts. Only old original parts are the 3 transformers.

I'm at a dead end. I have tried reflowing all solder points, checked the components, wiring,etc... It plays fine other than the hum which is very audible when the JD-50 volume is midway.

Just hope you could shed some light before I have to take it to a pro and burn more money.

Thanks.
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Old 10th October 2006, 09:39 PM   #2
zobsky is offline zobsky  India
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what happens if you short out the inputs?

does the hum increase with increasing volume?

a crude way to trouble shoot this (if you know what you're doing) is to systematically disconnect/desolder (or maybe even short out, depending on the stage) the different stages starting form the input and listen for the hum.

i'm sure someone here will chime in with a better suggestion
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Old 11th October 2006, 06:12 AM   #3
Bengali is offline Bengali  United States
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It's a steady hum with the volume turned all the way down, the hum does increase and there is higher pitch noise riding on it when the volume pot is midway. This is with the remote volume unit.

What I find strange is with no physical connection to the inputs of the pcb, the hum noise will increases pretty quickly, I'm sure to deafing levels if I don't power down the unit. so the volume pot is keeping the noise from blowing out my speakers.

I'm not sure what affect shorting the inputs would have. With an open, it's major hum, with some resistance, it's reduced.

Thanks.
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Old 12th October 2006, 03:47 AM   #4
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Deffective rectifiers at the power supply?

Lack of good isolation at the RCA input connectors? (must be NO contact to chassis at those points... also check if the inside cable shield makes good contact to the "outside" body of the RCA connector)

Circuit ground path to chassis interrupted?

Just to start....

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Ricardo
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