[Boatanchors] Morrow Modulation Question

Jeff Anderson jca1955 at sbcglobal.net
Thu Mar 12 07:38:10 EDT 2009 

Hi Mac,

My MB-565 manual has a voltage chart.  If you (or anyone else on the 
list) need one, I can try scanning the page for you and sending it in an 
email.  Let me know!

For plate modulation, I agree regarding 1120 volts, but the Morrow uses 
both plate AND screen modulation (per the manual) -- maybe this allows 
one to have less peak plate voltage?

I wouldn't mind find a junker MB-565 or MB-560 myself

Thanks for your help!

- Jeff

(By the way -- if you want to check out my Morrows, check out my blog 
at:  http://k6jca.blogspot.com/)


D C *Mac* Macdonald wrote:
> If my understanding of modulation theory has not
> totally gone away since tech school, to get 100%
> modulation under these circumstances, you'd need
> to get a peak positive voltage on the high side of
> the modulation transformer secondary of 1120 Volts!
>
> What are the plate, screen, and bias voltages of
> the 6AU5GT modulator tubes?  Is the bias output
> voltage of your power supply between -67 and -75V?
>  
> Plate voltage on the 6AU5GT tubes should be virtually
> the same as plate voltage on the 6146.
>  
> Unfortunately, there is no voltage chart in my MB-565
> manual.
>  
> My MB-565 has a broken drive mechanism for the VFO
> and I have no idea where I might find a replacement
> other than finding a total junker MB-565,
>  
>
> 73 - Mac, K2GKK/5
> Oklahoma City, OK
>  
>
>  
> > Date: Wed, 11 Mar 2009 16:36:38 -0700
> > From: jca1955 at sbcglobal.net
> > To: macklinbob at msn.com; boatanchors at mailman.qth.net
> > Subject: Re: [Boatanchors] Morrow Modulation Question
> >
> > Hi Bob,
> >
> > Just ran a test...the 6146 plate voltage is 560 volts and the plate
> > current is 120 mA in xmit. This results in an equivalent resistance of
> > 4.7K ohms.
> >
> > I've just disconnected the plate cap from the 6146 and connected a 4.5K
> > ohm power resistor from the mod. transformer secondary to ground.
> > Measuring (via a voltage divider, because my scope can't handle the
> > voltages), I see the same peak-to-peak swing across this power resistor
> > as I see when the tube is in-circuit.
> >
> > So...it seems either:
> >
> > 1. The transformer is bad (shorted turns, etc.)
> > 2. The Morrow manual is wrong (they state 100% modulation is 
> achievable).
> > 3. Something else is going on.
> >
> > Thoughts on what to do next, anyone?
> >
> > Has anyone looked at a Morrow's waveform in AM? Can you get 100%
> > modulation?
> >
> > Does anyone have a Morrow transmitter that they can quickly pop the
> > cover off of and make a resistance measurement across the transformer
> > primary & secondary windings (I'll tell you where).
> >
> > Thanks!
> >
> > - Jeff, k6jca
> >
> > http://k6jca.blogspot.com/
> >
> >
> > Bob Macklin wrote:
> > > 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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