[AMRadio] MFJ-815B Watt-Meter with AM

Jim candela jcandela at prodigy.net
Fri Feb 9 20:47:51 EST 2007


   I recently bought a used MFJ-815B wattmeter. The price was decent, so I
took it home, and checked it out. The reviews on the net (eham) were often
giving it a 1 out of 5. I think they were overly harsh. Mine works, and
seems to agree with my Byrd 43 on 80 meters when I used a 50 and 500 watts
slug. My dummy load is pretty good (50 ohms resistive), but I still showed
1.3:1 SWR. There is an adjustment capacitor in the MFJ that nulled the
bridge to 1:1 just fine.

  Then I ran some AM, and switched to PEP mode. A voice modulated AM signal
with peaks upward to 100% (as seen on scope) only showed about 2X PEP power
above the carrier. I recall years ago using a Mirage wattmeter that had a
PEP function, and it would read 4X the carrier with 100% modulation via sine
wave. The big difference between the MFJ and the old Mirage was that the
Mirage had active electronics within, and the MFJ is all passive. It's hard
to do a peak detect and hold without transistors or OP-amps.

  Then I looked carefully at the circuit card within the MFJ, and there on
the silkscreen was several missing resistors, and a missing transistor. It
looked like the MFJ once had active circuitry powered from the 12 volt meter
light power. Does anybody know about this, and why MFJ may have created this
design, and then left these parts out of the meter? Maybe those parts were
used in a more expensive model?

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