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Morrow MB 560, 565 modulation

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Author Topic: Morrow MB 560, 565 modulation  (Read 2428 times)
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RF in the shack

« on: March 05, 2009, 08:39:05 PM »

My buddy Jeff, K6JCA, has a Morrow MB 560 he's been using lately.  He can only squeeze about 50% modulation out if it.  Before he really tears into it, he's wondering if that's about par for the course for those little transmitters.

Anyone know anything about this transmitter?

« Reply #1 on: March 06, 2009, 04:27:08 PM »

Thanks, Jon, for posting my problem to the forum -- as of today I'm now registered with AMfone, and I thought I'd add some more information...

Here's what happened...while transmitting on AM last weekend with my Morrow MB-565, I heard arcing and smelled smoke.  Afterwords, I put the radio on the workbench and discovered a burned-out resistor (the cathode-bias resistor in the modulator).  I repaired it and did some testing, and discovered that I could only get about 45% peak modulation (as verified both on a 'scope as well as with a peak-power meter (peak power was only two-times (roughly) carrier power).

This seems to be pretty light-weight modulation, but the problem is...I don't know if it's correct for the Morrow, or not. (I should have been paying more attention to my wattmeter before the smoke appeared.  Oh well!)  The manual states that the radio should give 100% modulation, but I wonder if this is really true, and given the small size of the modulation transformer, the 40-50% that I'm seeing may be more realistic -- but I just don't know.

Does anyone know how much modulation I should be able to get out of a Morrow transmitter?  Does 40 to 50%, max, sound about right?

Also -- does anyone know what the resistance readings should be for the modulation transformer?  I'm measuring (roughly) 110 ohms for both the primary and the secondary, implying that the turns ratio is 1:1.

Thanks for any help!

- Jeff, K6JCA

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« Reply #2 on: March 10, 2009, 08:00:45 PM »

Hi Jeff, (john),

Welcome to this great AM Forun,  Jeff.

The answer from me is,  "No".

But W6WUH has Morrow twins on the air.  He prob knows from experience,  and perhaps other data.

73  GL  SRI to hear of the ensmokement.   Vic
« Reply #3 on: March 11, 2009, 08:19:33 PM »

Thanks for the information, Vic.  I've sent an email to Larry via his email address that's listed on QRZ.com (hopefully it's a valid one).

Best regards,

- Jeff, k6jca

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