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Re: [Amps] FW:  negative screen current

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Subject: Re: [Amps] FW:  negative screen current
From: "Paul Christensen" <>
Date: Tue, 31 Dec 2013 07:57:03 -0500
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" Tightening up the screw completely fixed the problem. I later added a secondary ground strap from
the board to the chassis for peace of mind."

Same issue here with my first Alpha 70V. A metering ground connection was dependent on maintaining good surface conductivity between the front panel and chassis. Since the entire front panel un-plugs form the chassis, adding a strap between the two pieces wasn't a viable option for serviceability. However, a 15-pin Molex connector that's used for the panel disconnect had one remaining position open. A dedicated ground wire was added to the open position for the completion of the ground return. The negative grid reading problem was solved.

Over the years, I've encountered several instances where stabilized equipment operation could have been better accomplished by completing a ground return that's separate from a panel to chassis connection. Examples include SO-239 connectors on the Elecraft K3, and Ameritron QSK-5. And, how many times have we seen key and headphone jacks that rely on panel connectivity without a dedicated return? Many of those issues were cleared up over the years with dedicated ground wires.

Then, there's the Alpha 77 series that utilize the chassis for the filament return, resulting in 10A of chassis current (20A for the 77Sx). Several years ago, I posted links here on AMPS of pics that showed a mod to balance the filament by using an RFC and dedicated filament return back to the power transformer. That same mod had a secondary benefit of lowering filament voltage within the 8877 specification as utility voltage here in the U.S. has been on a steady, but subtle increase over the decades.

Paul, W9AC
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