"Both channels the hum is the same so I made the same mistake on both"

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Posted by Goneill (A) on April 27, 2008 at 19:58:09

In Reply to: Ella, Implemented most of Graham's mods and it hums posted by OzMikeH on April 26, 2008 at 00:50:32:

That's what I used to believe! The only bit in common is the PS so if both channels hum or buzz then it must be there. Seems obvious.

But recently I had a couple of issues. In one there was a hiss/buzz from both channels. Turned out I'd broken the link on one of the 1K resistors (pin 5 on the power tubes). On the other there was a hum, again both channels, after I removed the Zobel networks from the speaker outputs. I re-soldered one of the speaker post to ground wires and it went away.

So don't think that noise from both channels means the problem is in a common area. Both channels affect each other.

For a hum RHone is right - the most likely is a grounding issue. Measure resistances everywhere you can think of, starting with the outside of the RCA inputs to the ground pin on the IEC power plug. If it's above about 0.2R on either channel then you should trace through and find out why. You're looking for any value that seems higher than it should be or is different between the channels. Since you've been working on the PS board check the connection between that board and the chassis - it grounds using one (and only one!) of the screws that hold it down.

For checking the neg.bias you need the the power on, but not necessarily with the power tubes in, and measure the two outputs from the PS board. One should be around -30V and the other around -60V.

Remember when checking with the power on to be very careful. And remember when checking with the power off to discharge the capacitors before touching anything.

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Follow Ups Full Thread

Topic - Ella, Implemented most of Graham's mods and it hums - OzMikeH 00:50:32 04/26/08 (9)

"Both channels the hum is the same so I made the same mistake on both" - Goneill 19:58:09 04/27/08 (6)

RE: "Both channels the hum is the same so I made the same mistake on both" - OzMikeH 00:58:39 04/28/08 (5)

Sorry...don't know about the Signature (Brian!) - Goneill 20:57:46 04/28/08 (4)

Hum almost gone now - OzMikeH 02:54:58 05/03/08 (3)

RE: Hum almost gone now - Goneill 04:10:35 05/03/08 (2)

RE: Hum almost gone now - OzMikeH 18:27:24 05/03/08 (1)

RE: Hum almost gone now - mccormick 12:36:10 05/07/08 (0)

RE: Ella, Implemented most of Graham's mods and it hums - RHone 10:15:54 04/27/08 (1)

RE: Ella, Implemented most of Graham's mods and it hums - OzMikeH 06:37:45 06/25/08 (0)

Follow Ups

"Both channels the hum is the same so I made the same mistake on both" - Goneill 19:58:09 04/27/08 (6)

RE: "Both channels the hum is the same so I made the same mistake on both" - OzMikeH 00:58:39 04/28/08 (5)

Sorry...don't know about the Signature (Brian!) - Goneill 20:57:46 04/28/08 (4)

Hum almost gone now - OzMikeH 02:54:58 05/03/08 (3)

RE: Hum almost gone now - Goneill 04:10:35 05/03/08 (2)

RE: Hum almost gone now - OzMikeH 18:27:24 05/03/08 (1)

RE: Hum almost gone now - mccormick 12:36:10 05/07/08 (0)

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