[Boatanchors] Morrow Modulation Question

[Jeff Anderson]

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