Try another tube. Maybe heater to cathode leakage

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-------- Original message --------

From: "thoyer via groups.io" <thoyer1@...>

Date: 2/16/25 5:07 PM (GMT-08:00)

To: [email protected]

Subject: Re: [HallicraftersRadios] SX-28A Hum

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Hi Jacques,

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Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area.

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Narrowing it down……….

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I appreciate all of the discussion going on – thank you everyone. We’ll get to the bottom of this one way or another.

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Tom

W3TA

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Hi again Tom,

Maybe some will find the following approach brutal, but here is what I will try with that SX-28A.

Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis.

Power-up the set again.

If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s).

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BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced.

Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within.

OR there is shorted turns in the winding of the primary on one side.

OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other.

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I just hope that this makes sense…

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Yes, hum is present – at the same level – regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot.

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CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner.

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Tom