How to improve the MFJ-9030 Receiver (Rev-D, MARCH 13, 2002):



The MFJ-9030 is a great little QRP transceiver. Within a month of getting mine on the air I had worked about 20 states and a dozen countries including a ZL1 on 30 meters! The design concept of all these little MFJ-90XX's is clever to the point of being a classic. One unfortunate aspect of the implementation of the 30 meter version, however, is the choice of 6.000MHz as the i.f., which is exactly equal to that of a powerful North American shortwave broadcast transmitter, located on an island republic in the Caribbean. During winter months in the evening this strong signal can leak in, through several paths, to the i.f. amplifier, interfering with amateur communications. The QRM shows up as a low amplitude high frequency beat note with a.m., no matter where you tune the radio. Even though seldom bothersome beyond a mild annoyance, the knowledge of its existence and origin motivated me to improve the jam-resistance of this great little rig. Following MFJ's suggestion of adding a  6MHz trap at the receiver input did not do much for it. The following information is a summary of how to re-crystal your MFJ-9030 and eliminate this problem.

The idea is to change the i.f. from 6000kHz to 5990kHz by using inexpensive easily obtainable crystal units. Checking Mouser catalog #597 on page 50, there are crystals at 5990.400kHz made by ECS International selling for $.84 or $1.12 depending on the case size. I ordered 8 of the part number 520-HCU599-S (the $1.12 ones). Next remove all six 6.000MHz crystals from your MFJ-9030. You will have to unscrew all of the screws which hold in the printed wiring board to get to the solder side of the board and don't forget to unscrew the final ampifier transistor's heatsink before removing the pwb/ front panel assembly from its anodized chassis. Use solder-wick to remove the solder around the leads of the four crystals in the filter, the BFO crystal and the transmitter oscillator crystal. You can locate them with MFJ's manual. Bend the leads up with a screwdriver blade so you can remove the crystals with a pair of small pliers. The other part of the modification is to remove capacitors C43 and C47. Then replace C48, C44 and C45 with 100pF units (Radio Shack 272-123, 2 for $.49). This may be done by clipping all of the leads from these cap's and soldering the new ones in from the solder side.

The final step is to align the unit per MFJ's instructions (field alignment procedure). I found that the VFO was REALLY sensitive to rotation of the slug tuned inductor setting its frequency so you should probably leave that for last, after you have a working i.f. and BFO. The way the rig is set up, this is an independent adjustment and only calibrates the dial anyway. You want to make sure the BFO frequency is above the peak of the i.f. response so that when you tune the radio upwards in frequency across a signal, the c.w. pitch increases.

With the above modification, the beat note of the QRM from the broadcast station comes out at 10kHz rather than within the communications audio spectrum. Additional bypassing of the switched Vcc line going to the MC1350 i.f. amplifier also helps. Also replacing L10 with a good quality shielded inductor reduces pickup--in fact this is the most sensitive place in the rig where the desired signal is lowest. L10 can act as a ferrite antenna and insert a 6MHz signal at this point!!  The ECS crystals from Mouser apparently have a higher L/C ratio than the stock MFJ crystals and this accounts for the need to change the coupling capacitors C48, C44 and C45. You could also use the 6144MHz crystals at Radio Shack-- I think the VFO could be pulled far enough, but check that frequency for VOA's signal strength at your QTH.  This type of crystal filter is described in "QRP Classics" from ARRL and seems to have come from Seymour B. Cohn, in "Direct- Coupled- Resonator Filters," Proceedings of the IRE, February, 1957, p. 187.

73, Paul, AA1LL

Mason, NH