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Notes on installing the PDC-1 PEAK Reading Adapter in
the MFJ-949E and MFJ-962D... (This document describes my 949E installation, but I also
installed a second PDC-1 in the 962D. Installation in the 962D is somewhat easier, since I
didn't have to move any tuner components. My aluminum box fit nicely between the variable
inductor and one of the capacitors.)
HI-RES Communications, Inc. offers the PDC-1 peak reading adapter for the Collins 312B-X wattmeter,
and according to Chuck Hawely, KE9UW, the adapter will work on any average reading wattmeter, so I
decided to give it a try on the MFJ-949E. After reading the literature that came with the kit,
I was concerned that components would have to be changed to get it to work with the MFJ, but such was
not the case. The internal meter resistance of the 400-084A dual-meter used in the MFJ-949E and
962D is well within the adjustment range of the PDC-1.
Components that come with the PDC-1 are for use with internal meter resistances between 1.18K and 680
ohms. The PDC-1 is adjustable within this range. (The reason the wattmeter internal meter
resistance is important is because the PDC-1 must duplicate, or mimic, this internal meter resistance
when the PDC-1 is switched inline.)
Since the PDC-1 is designed for the Collins wattmeter, the circuit is looking for 6.3 volts AC.
(The PDC-1 has a diode and 470-uF-filter capacitor for rectifying and smoothing the AC input.
The circuit would normally supply about 8.9 volts DC to the LM358 OP-AMP.) Since the MFJ-949E
uses 13.8 volts for the meter lamp, I decided to use this voltage and run it through an LM317T
adjustable voltage regulator, and set it to approximately 9 volts.
The LM317T normally uses a variable resistor to set the operating voltage, but I wanted to eliminate
the POT. This is easy to do. I set the variable resistor for 9 volts DC, and measured the
resistance. I replaced the POT with a fixed resistor that was close to the value of the
variable resistor setting.
(CAUTION: The Radio Shack packaging that comes with the LM317T incorrectly labels the pins. The
correct pin-out is as follows: Pin 1, which is clearly shown on the IC with a small dot, is the
"adjustment" pin, and NOT the input. Pin 2 is the output, and pin 3 is the input. Placing
the IC in the circuit backwards will not damage the IC. It just won't regulate at the set
voltage.)
I followed standard suggestions on adding capacitors to the input and output of the regulator IC, and
made sure it worked properly before connecting the PDC-1. (With 9 volts DC being supplied to
the PDC-1, you can replace the rectifying diode [D1] with a jumper wire.) To insure some
additional RFI protection, I installed the regulator and PDC-1 in a small aluminum box.
Since the MFJ-949E is a little short on internal space, I had to move the dummy-load resistor and the
balun to the back panel of the tuner. Care must be taken when mounting the 50-ohm resister,
because it has to be mounted low enough that it clears the top cover. To do this, I had to cut
one corner off a coax connector. This allowed me to lower the ceramic standoff on the positive
side of the load resistor. There was not a problem with mounting the groundside of the resistor
or the balun.
That was all that I had to change in the tuner to give me enough room to install the small aluminum
box containing the regulator and the PDC-1.
After some thought, I decided not to modify any of the existing switching arrangements on the
MFJ-949E. I wanted to keep it simple, and I wanted the MFJ-949E to act as designed with the
power switch off, so I wired the PDC-1 to come on when power is applied. Normally, the power
switch only turns on the meter lamp. It's important to note that I not only supply 13.8 volts
to the regulator IC, but I also us this voltage to turn on a small relay. I use the relay to
change the input connection for the selection of peak or average readings. (The output of the
PDC-1 can be left connected to the plus side of the meter along with the average reading connection
with no side effects.)
73,
Joe ( AJ8MH )
aj8mh@arrl.net
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