[Boatanchors] Morrow Modulation Question

[Bob Macklin]

If the modulator is saturating a 50% modulation it sounds to me as if the
modulation transformer impeadance is not correct.

You need test the modulation transformer with the final disconnected but
with a resistive load equal to what the final appears to be.

Bob Macklin
K5MYJ
Kent (Seattle), Wa,
"Real Radios Glow in the Dark"

----- Original Message ----- 
From: "Jeff Anderson" <jca1955 at sbcglobal.net>
To: "Carl" <km1h at jeremy.mv.com>
Cc: "Boatanchors" <boatanchors at mailman.qth.net>
Sent: Wednesday, March 11, 2009 2:03 PM
Subject: Re: [Boatanchors] Morrow Modulation Question


> Hi Carl,
>
> Thanks for the reply.  The PA plate and screen voltages also look
> clipped when the modulator input begins clipping (because the modulator
> grids start conducting) -- but this shouldn't be too surprising.  If the
> input signal clips, then the output will look clipped, too.  And
> increasing the mic gain further just results in broader "flat-topping".
>
> Don't know if the modulator xfrmr is designed for DC voltage on the
> secondary, but I assume it is (it's connected between B+ and the PA
platel).
>
> There's plenty of gain up to the input of the push-pull modulator (I can
> easily push the input into clipping as I increase the mic gain) -- it's
> just not swinging the B+ as much as I think it should.  In other words,
> the gain through the push-pull modulator stage seems a bit light.  The
> only reasons I can think that this might be are:
>
> 1.  Tube gm incorrect.  But I've verified the bias current.
> 2.  Tube(s) bad.  But I've exchanged both tubes with others -- no
> difference.
> 3.  Transformer bad.  Not verified.  Not sure how to verify.
> 4.  Transformer secondary load bad.  I've swapped 6146 PA tubes -- no
> difference.  Plate current is adjusted per manual, so I'm assuming the
> plate resistance is to spec.  Perhaps there's another parallel, "audio
> frequency range" resistance that's lowering the overall gain?
>
> Regarding the load on the modulation transformer secondary -- you're
> saying that it only sees the PA plate load, but not the parallel
> "Antenna" load, right?  (I'm assuming that's because the antenna &
> loading circuit appear as a very high impedance at audio frequencies,
> and thus are insignificant compared to the PA plate resistance.)
>
> (By the way -- a week or two ago I checked the modulator's voltages and
> performance -- voltages were correct (per manual), and operation was
> fairly symmetric (as I recall).  Just low gain.)
>
> Thanks again,
>
> - Jeff, k6jca
>
> http://k6jca.blogspot.com/
>
>
> Carl wrote:
> > 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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