Notebook Series - Adding the PDC-1 Peak Reading Adapter to the MFJ-998, MFJ-949E and the MFJ-962D Tuners
[ Marquette, MI - April 2012 ] Notes on installing the PDC-1 Peak Reading Adapter in
the MFJ-998 1.5KW Auto-Tuner...
MFJ has been making tuners for a long time, but it's only been in the last few years they've offered an active peak-reading circuit design and dumped their "switch in a capacitor" design. However, as of April 2012, the MFJ-998 1.5KW auto-tuner's analog meter still has not been upgraded. In fact, the old "switch in a capacitor" gimmick isn't an option, either. Hence, HI-RES Communications to the rescue once again with their PDC-1 Peak Reading Adapter.
Nothing has changed to the HI-RES Communications PDC-1 since I was first introduced to the board in 2001. It was designed for use in the Collins 312B-4/5 station console, so it looks for 6.3 volts AC. I write about it in detail below in my original article, so I won't cover it again.
Even though the manual says the PDC-1 will work with any voltage between 6 and 13.8 vdc, I feed 13.8 vdc to an LM317T regulator and adjust it for around 9 volts. This is also covered in the article below. You may want to save time and components, and just use 13.8 volts for your project, but I can't verify proper operation. It should work fine.
Adding the PDC-1 to the MFJ-998 is easy. First, temporarily remove the aluminum shield that's between the main circuit board and the front panel. It's held in place by two screws.
I elected to mount the PDC-1 circuit board to the back of the Control and Display board by adding a solder lug under one of the mounting screws, and simply soldering the PDC-1 to the lug. This connection doubles as the ground for the PDC-1. 13.8 volts and the "forward" voltage for the meter also come from the Control and Display board.
Before mounting the PDC-1, connect 13.8 volts. Unsolder the "forward" voltage lead from the Control and Display board (which runs to the meter), and solder it to the output of the PDC-1. Solder a new wire from the "forward" voltage point on the Control and Display board and run it to the input of the PDC-1.
Now, mount the board, but be sure you leave clearance for the aluminum shield, which will be reinstalled after the PDC-1 is adjusted. Also, make sure you have clearance between the PDC-1 and the MFJ-998 Control and Display board.
Connect your rig and dummy load. Transmit a low power CW carrier and set your PDC-1 so the MFJ-998 analog meter reads the power level showing on the MFJ-998 digital display. Next, adjust the hold time of the PDC-1 to whatever you're comfortable with. After the adjustments are made, remove power and reinstall the aluminum shield.
This installation doesn't allow for switching the analog meter between average and peak. I found no need to switch between the two readings, but if you do, you can simply add a switch to accomplish the task.
Important. Since acquiring an amplifier, one issue I've noticed needs to be addressed. When the tuner "METER RANGE" is set to "AUTO" and you're using SSB, the meter will not automatically switch to the high power scale when using high power. This is because the MFJ power sense circuit does not read voice peaks. If you change the menu to "METER RANGE 3000", the meter will read SSB voice peaks as expected. You may find this inconvenient, but I haven't. Remember, the digital display will read low power just fine.
I haven't experienced any RFI issues with the PDC-1 while running 500 watts or less through the tuner. However, additional filtering or shielding may be needed with higher power levels. I have always used a snap-on choke on the power leads to the MFJ-998.
For more information, search for HI-RES Communications and/or PDC-1.
ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH
[ Austin, TX - September 2001 ] 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
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 680 and 1.18K 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: In the past, the Radio Shack packaging that comes with the LM317T incorrectly labeled the pins. The correct pin-out is as follows: Pin 1, which is clearly shown on the IC with a small dot (sometimes), 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. Please see LM317T diagram.)
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 ground side 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.)
ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH