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MFJ-9020 Technical Questions

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I just bought a used MFJ-9020 QRP CW transceiver, and I have some
questions.

            I’ll do my best—I no longer work for MFJ and it’s been quite awhile!   

1.  WWV at 10.000 MHz….

Bleed through was never a common complaint with the 9020, but your suggestion of installing a suck-out trap on the receiver antenna line is a good one. We’ve used that strategy with 9030s. These rigs use a 6-MHz IF and bleed-through from Radio Havana bothered some users—especially if they used long-wire or multi-band antennas.   

2.      When I first start to transmit, the sidetone appears to overwhelm the
AGC.

Confirm that the AGC threshold voltage is set for the correct value (I believe the manual suggests both a value and measurement test point). Also, make sure there’s enough headroom on the radio’s voltage regulator (supply voltage should be >1.5V above your supply voltage). You can set a faster AGC attack time by reducing the value of the 2.2 uF cap or changing the value of the 680K resistor across it. Alternatively, on the QRP-ACRI web site, check out a modification I made to the “Cub” AGC. I haven’t tried adapting that change to the 9020, but it might give you the same “seamless” break-in performance. Finally, if receiver sensitivity is low, the attack problem will be much more obvious because the AGC voltage will be resting below threshold.  

3.      Is there any way to reduce the VFO drift?

I never could eliminate warm-up drift (lasts about 5-10 minutes and shifts about 250 Hz) with this VFO circuit, but had good luck controlling both long-term and thermal drift through inductor and capacitor selection. I’m not sure what MFJ is providing for fixed capacitors in the VFO tank these days. If there are two NPO polystyrene caps in there, you might try replacing one with an NPO monolithic. If there are two NPO monolithics, replace one with a NPO polystyrene. More importantly, check the manufacturer of the VFO coil.  I designed the radio using a Coilcraft “Slot-TEN” 10-mm inductor and it worked well. If the coil manufacturer has changed, (either to Toko or an imported “no-name”), the thermal characteristics will be entirely different and the old temperature compensation technique will not work (something RF engineers know and purchasing agents don’t).    

4.  Can you suggest ways to improve sensitivity?

 I’m not sure what to suggest. The radio’s somewhat “different” AGC set-up requires higher volume control settings than most rigs (the volume control is really an attenuator on the AF amp output). However, normal atmospheric background noise should predominate over the receiver’s electrical noise in the speaker.      

            Hope this has been helpful. As noted, nearly 15 years have passed since I designed the rig!  

73

Rick  K1BQT                

Article details
Article ID: 104
Company: MFJ
Date added: 2017-01-12 15:03:39
Views: 40

 
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