[AMRadio] Ultrasonic feedback

Peter A Markavage manualman at juno.com
Tue Feb 3 15:16:34 EST 2009

Pulling the clipper tube just removes the diode action. It doesn't
eliminate the pass-through connections. Does this Apache modulator still
have two adjustable controls (one labeled Gain on the front panel and one
labeled Level, which was behind the key jack)? If Level control (adjusts
audio drive to the 12BY7) is set to high,  any clockwise adjustment
(beyond a few degrees above the off point)  of the front panel control
can set the second half of the 12AU7 and 12BY7/EL34s into oscillation. Of
course, a major modulator mod that might have been done (parts pulled,
new parts, changed wiring, etc.) may have turned these two controls into
different functions from what they were originally intended to be.

Pete, wa2cwa

On Tue, 3 Feb 2009 11:11:56 -0800 (PST) John King <k5pgw at yahoo.com>
> There is NO Clipper circuit in this Apache. The clipper tube was 
> removed and the tube socket is empty. This seems to be what many did 
> as a modification. I did have the redrawn schematic showing the mod 
> but have misplaced it. Thanks for the assistance. 73, john, k5PGW 
> --- On Tue, 2/3/09, Peter A Markavage <manualman at juno.com> wrote:
> > From: Peter A Markavage <manualman at juno.com>
> > Subject: Re: [AMRadio] Ultrasonic feedback
> > To: amradio at mailman.qth.net
> > Date: Tuesday, February 3, 2009, 8:54 AM
> > The front panel "gain" control is for the speech
> > clipping gain. If you
> > turn it up too high, depending on the setting of the
> > "real" audio gain
> > control which is located behind the key jack, the modulator
> > will squeal.
> > This is barring any other electrical or component issues.
> > 
> > Pete, wa2cwa
> > http://www.manualman.com

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