Heathkit HM-102 RF Power Meter
Introduced: 1970
Produced until 1981
Replaces Previous Model(s): AM-2, HM-11, HM-15
Price when introduced: $24.95 and later rose to $29.95 in 1971.
Assembly Manual Introduction
The Heathkit Model HM-102 RF Power Meter is a reliable instrument for measuring the RF power output and the SWR (VSWR) of an amateur radio station or other similar types of transmitting systems. The Power Meter can be permanently installed in the transmission line without affecting the transmitting and receiving capabilities of your station.
The Power Meter aids proper transmitter tuning as the meter indicates maximum forward power when the transmitter is tuned for maximum output. At the SWR setting, the meter measures the mismatch between the transmission line connected to the input terminal and the load connected to the output terminal. This information is of great value in making antenna adjustments and in determining the allowable frequency working range of the antenna.
This unit is designed for use only with a transmission line [impedance] of 50Ω nominal characteristic impedance. It has negligible insertion loss. The indicating meter is mounted in a small cabinet for use at the operating position. For convenience, the detector portion may be removed and positioned up to six feet away with the supplied cable. No external power is required.
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I think this little box is a great tool to have in the shack. Sure there are newer, faster, smaller, better units out there but this one is from an era where you built things. The things you built then either worked or they didn't. If built correctly, you could make them work very well and have saved a lot of money versus buying something that supposedly did the same thing for more money built by a company whose quality you know nothing about assembled by people you'll never know.
Kit building was a great way to learn how to follow instructions if nothing else. If it broke or didn't work, you know who did it. With a little troubleshooting you were the one to repair it and put it back into service.
My best friend, N1XIO bought me one a few years ago as a Christmas present. I was excited to have something new in my Heathkit collection and wanted to get it fixed up ASAP. I did connect it to a transceiver and set the power to about 50 watts into a dummy load with this inline. The meter barely moved while in the 200 and less in the 2000 watt range. Same thing in SWR mode in the forward power setting mode. So I prepared a parts list based on the assembly manual and ordered parts. Alas, life gets in the way and it sat on the shelf until now.
October 16, 2022
I already disassembled the unit on Thursday the 12th to get an idea of what caused the low meter reading. After removing the remote sensor unit from the main meter case I saw rust. Not something anyone wants or expects to see. It was mostly up front but there was plenty on the bolts/screws I had a bit of a time getting loosened. After disassembly I took the two main meter case pieces outside to scrub them with a wire brush and some fine sandpaper being careful not to get any chemicals on the front face. The paint on the bottom of the case was flakey and the cleaning agent softened it up. I decided not to remove the paint and let whatever paint from the bottom remain if it stuck. Most did.
After the rust was cleaned out and the parts rinsed, I dried them thoroughly and left them in the sun for several minutes. Once dry I sprayed a couple of light coats of semi-gloss black onto the inside of the piece with the knobs to seal it up. After that dried I brought them in and began to work on the circuit board.
All resistors were removed knowing all were way out of tolerance due to the presence of moisture inside at some point. As expected each resistor was way up in value. The 68Ω 1 watt resistor showed 77Ω. The 22KΩ was 24.7K. These were Allen Bradley type 5% tolerance 1/2-watt carbon composition resistors.
So while the resistors were out I checked the capacitors. A couple had to be made one-legged for an accurate reading and all were close. I measured out some new ones and they read similarly off value so I left the originals.
Next came the Germanium diodes. The two in the SWR circuit path were good showing a .386 volt drop. Typical of Germanium signal diodes of the time. Later versions of Germanium types like the 1N60 are about .6 of a volt drop. Not as sensitive but work with some modifications to the circuit they are substituted for. The third diode I checked, another Germanium 1N295 like the other two failed. It was problematic. It would not register as a diode on either the diode tester or the multi-function tester. A basic digital multi-meter showed a higher resistance in one direction and a lower one in the opposite direction but no diode function. Since this was in the RF Power circuit for measuring watts output, it didn't need to be the same as the others. Still I was disappointed as the 1N295 just can't be found anywhere so I had to substitute a Hughes 1N60P. The calibration step will take care of any reduction in sensitivity.
The current state of the project is the circuit board has been reassembled and awaiting to be installed into the case. The sensitivity control to set the forward power needle in SWR mode has been cleaned with DeoxIt D5 then cleaned with CRC QD Electronics Cleaner. D5 isn't recommended for carbon track controls like linear volume controls. DeoxIt Fader will be put in the control after testing if there is no skipping going on in the sweep of the control. The Calibration switch on the board was disassembled and cleaned, relubricated with D5 liquid and reassembled. The push/pull switch on the sensitivity control was cleaned out and checked for continuity. It reads no resistance. It better with those silver plated contacts.
The control was reinstalled onto the case front and the rotary switch for the mode was cleaned with D5 spray. The silver plated contacts and contact rail shine again and are protected with a drop of Fader. This control is now installed onto the front case half and ready for wiring.
Speaking of wiring....I don't have any 5 conductor cable. I'm thinking about using RJ12 phone or CAT 5e network cabling but the solid core wires don't accept being flexed around very much before breaking from fatigue. I'm looking into alternatives with 16AWG stranded wire but is rather expensive compared to 18AWG stranded. Maybe I'll stick with that as the tiny ferrite beads must fit over the ends of the wires on the circuit board end.
So that's where I'm at right now. Hopefully I'll find the cable I need soon and get this ready to go when I get the Kenwood TS-450S/AT repaired finally.
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November 2, 2022
I found some CAT 5e cable with 24AWG stranded wire. A buddy gave me some extra cable he had to see if I could use it. Both were flexible and after putting my 6ft piece in I found the meter worked well. After getting it calibrated on one radio I went to another to corroborate the configuration. With the same results I declared the project a success and closed.
That's it for tonight.
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