[Boatanchors] Morrow Modulation Question

[Carl]

What happens to the plate and screen voltage at those peaks?

Ive not owned a Morrow but would probably isolate the audio section and 
place a resistive load of the correct value across the modulator 
secondary and with no DC. Use resistors that are non inductive at audio 
frequencies.

The RF stage load to calculate the resistor is Ep divided  by Ip.

Is the modulation xfmr designed for DC voltage on the secondary?

Carl
KM1H



----- Original Message ----- 
From: "Jeff Anderson" <jca1955 at sbcglobal.net>
To: "Boatanchors" <boatanchors at mailman.qth.net>
Sent: Wednesday, March 11, 2009 12:36 PM
Subject: [Boatanchors] Morrow Modulation Question


> I'm troubleshooting a Morrow MB-565 transmitter (very similar,
> electrically, to the MB-560).  I can't get more than about 50%
> modulation in AM mode.  I've checked the voltages and resistances, and
> everything seems to be fine.
>
> Plate voltage is about 550 VDC.  Modulation seems to be limited by the
> modulator (push-pull).  That is, as I turn the Mic Gain up I see the
> input at the grids of the push-pull tubes start to clip (when this
> happens when the grids become forward-biased -- that is, the grids are
> biased at -44 vdc, and the input voltage peaks are high enough to 
> raise
> the grids to 0 volts).  But even with this large input signal to the
> modulator, my max modulation is only about 50%.
>
> Is this typical for Morrow transmitters?
>
> One possibility, of course, is that there's something wrong with the
> modulation transformer, but I don't know how to verify this.  I have
> measured the primary and secondary resistances -- both measure about 
> 110
> ohms, but I've nothing to compare this to, so I don't know if this is 
> in
> the ballpark or wildly out of spec.
>
> Thanks for any help you can provide!
>
> - Jeff, K6JCA
>
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