"Peak Power" Modification for the
OHR WM-1/WM-2 QRP Wattmeter

Larry East, W1HUE
Tucson, Arizona
(contact author)

This article appeared in the January 1995 issue of The QRP Quarterly.

After several months of procrastination, I finally ordered an OHR WM-1 QRP Wattmeter kit (the fact it was on sale helped!). It went together like a breeze; now I don't have to use a DVM, RF probe and calculator to determine the output power of my QRP rigs! In case you are not familiar with this great little gadget, it is a direct reading Wattmeter available as either a kit or fully assembled from Oak Hills Research (and some dealers). It has three power ranges: 100 milliwatts, 1 Watt and 10Watts full scale – perfect for QRP work! My one complaint; it does not have a "peak" mode for use with SSB. (Well, none my other Watt/SWR meters do either, for that mater.) Noting that there was an unused operational amplifier (U2B) available in the WM1, It occurred to me that it should be possible to use it to implement a "peak reading" circuit.

After some experimenting, I came up with the simple modification shown in Figure 1. The circuit works as follows: The DC output of U2A charges a capacitor through a 1N34A diode, and U2B is now used as a high input impedance voltage follower. A one megohm resistor supplies the (primary) discharge path for the capacitor. The discharge path is to the output of U2A rather than ground so that the voltage drop across the diode (only a few millivolts when the capacitor is filly charged) will not affect the final output of U2B. A SPDT switch connects the meter to the output of U2A for "normal" power readings, or to the output of U2B for "peak" power readings. In the "peak" mode, the meter will follow the peak average power over short time intervals and slowly return to zero when the source of RF is removed. The "averaging" time is determined by a variety of factors; the mechanical characteristics of the meter, the resistance of the diode and output impedance of U2A (both nonlinear functions of current) and the RC time constant at the input of U2B. The time for the meter reading to return to zero is determined by the RC time constant of the 4.7µF capacitor and one megohm resistor at the input of U2B.

The ability of the meter to indicate "peak average power" is heavily influenced by the value of the storage capacitor at the input of U2B. In order to determine the proper capacitance, I ran a series of tests by recording short one syllable words and playing them back through the microphone input of my TenTec 505 connected to a 52A dummy load through the WM1. A calibrated oscilloscope was used to monitor the transmitter output. I adjusted the recorder playback level so that the WM-1 read three Watts output in "peak" mode with a storage capacitor value of 4.7µF. The 'scope indicated a maximum peak-to-peak RF voltage of approximately 36V, corresponding to a peak power level of approximately three Watts [Power=(Vpp)2/(8R)]. I then used clip leads to connect various other capacitors to determine the effect on the meter reading. Typical test results are shown in Figure 2. These tests indicated that at least 3µF is required for the meter to properly indicate SSB "peak" power. For values greater than 10µF the meter reading actually decreased slightly, probably due to the RC time constant of the diode series resistance and the storage capacitor. I settled on a capacitor value of 4.7µF, but any value in the range 3.3 - 6.8µF will work. All tests were made using a 1N34A diode; other diode types might yield different results.

Modifying the output circuit of the WM-1/WM-2 as shown in Figure 1 is not difficult, although it does require that some minor surgery be performed on the PC board. I also made two other simple WM-1 modifications which you might find useful: I added a "power on" indicator and provisions for external DC power.

I found that many small LED's will operate satisfactorily at 1 to 2mA, and installed one on the front panel of the WM1 connected through a 4.7K resistor to pin 4 of rotary switch S2. Since a maximum of 4mA is drawn by the WM1 during operation, you don't want to use a current hungry pilot lamp that will quickly kill the internal 9V battery!

In order to allow use of an external source of DC power, I added a 2.5mm DC power jack and SPDT switch on the back panel above the battery holder. I used an LM78L09 voltage regulator, as shown in Figure 3, to allow any source of DC between about 11V and 24V capable of supplying at least 10mA to be used. The voltage regulator and two bypass capacitors were mounted "spider web" style on the back of the power jack.

Figure 3.  External power input circuit.

The "peak power" modification can be made as follows:

1. Unsolder the wires from the RF connectors on the rear panel.
    
2. Remove the meter from the front panel.
    
3. Remove the five screws holding the PC board, and carefully remove the board. If you want more room to work, you can also remove the rotary switches from the from panel and disconnect the wire from the battery holder.
    
4. Find the solder pad on the bottom of the PC board to which pin1 of U2A, R10, C5 and two pins of R8 are soldered. Refer to Figure 4 and the pictorial drawings in the WM1 manual for help.

   

   Figure 4.  Partial bottom view of WM-1 PC board showing modifications to be made.  NOTE:  Board layout may vary due to production changes and does not apply to the WM-2.

5. With a small sharp knife (X-Acto, etc.), carefully cut a slot in this solder pad to isolate R8 from the other components, as shown in Figure 4. Use an ohmmeter to make sure it is isolated.
    
6. Solder a 2.5 inch piece of small gauge insulated wire between the part of this pad to which the two pins of R8 are connected and the center pin of a miniature SPDT switch. Be careful not to create any solder bridges on the PC board.
    
7. Find the pad to which pin 5 of U2B is connected; carefully cut the PC trace to the adjacent pad as shown in Figure 4. Again, check your work with an Ohm meter.
    
8. Solder a 1N34A diode and one megohm resistor (on the bottom side of the board) between pin 1 of U2A and (the now isolated) pin 5 of U2-B. Make sure the components do not touch any other solder pads and that the cathode end of the diode is connected to pin 5 of U2-B.
    
9. Solder a small 4.7µF capacitor between pin 5 of U2B and a convenient grounding point (I used a small molded tantalum). Things will be pretty congested at pin 5 of U2B, so be extra careful not to create any solder bridges. Also make sure that the capacitor will clear the bottom of the cabinet when the PC board is reinstalled and that its wires do not touch any other solder pads.
    
10. Solder a 2.5 inch length of insulated wire to pin 6 (or 7) of U2B and one of the outer pins of the SPDT switch used in step 6 above.
     
11. Solder another 2.5 inch length of insulated wire between pin 1 of U2A and the other outside pin of the SPDT switch.
     
12. Drill a mounting hole for the SPDT switch on the front panel between the two rotary switches and below the lettering just below the meter. If you want to add an LED "on" indicator, drill a mounting hole for it below the hole for the SPDT switch. Also drill the appropriate mounting holes on the rear panel if you are adding an external power connector and switch.
     
13. Remount the front panel rotary switches, if removed, and mount the LED if you are adding one. Reinstall the circuit board and mount the SPDT switch in the hole on the front panel.
     
14. If you installed an LED, connect its anode through a 4.7K resistor to pin 4 of S2. A convenient grounding point for the cathode is the small tab on the frame of S1.
     
15. If you are adding an external power connector, install it, a SPDT switch, LM78L09, etc. on the rear panel (see Figure 3).
     
16. Reinstall the meter (watch the polarity).

You now have a peak reading QRP wattmeter! This modification does not affect meter calibration, but you should go through the calibration procedure in the manual anyway since you might have moved one of the pots while handling the PC board. Check the difference in meter readings when switching between "normal" and "peak" while operating SSB – you will be amazed!
 

———— Copyright © 2005, 2012 by Larry East, W1HUE ————


Page last updated on June 17, 2012