[Boatanchors] Morrow Modulation Question

[Jeff Anderson]

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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