Manual zz. Check idler wheel height adjustment. Replace idler wheel if it has a "hardened" appearance. Don't use tuner contact cleaner as it usually contains silicon lubricant.

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These devices are handy to monitor and adjust antennas for proper operation, but only give a relative indication of the power a transmitter or amplifier is outputting. The reason for this is the frequency sensitivity of the Monimatch-style pickup used to sample the forward and reflected RF energy. Since the sensitivity control has to be adjusted for each band or even band segment the meter cannot be accurately calibrated in RF power.

Some SWR meters, such as the Osker SWR, come with a sensitivity control with a graduated dial and a separate calibration table so the sensitivity can be set and the power read on the meter. They also featured RF wattmeters in their SB amateur radio console.

The later units featured a peak monitoring capability for reading PEP power and required power either from a battery or external power source. Table I lists the units and their features.

Page 1 of 7. Often a kit was reduced for a short period on special. The HM- paint scheme is the green and gray motif of the original SB amateur line as are the knobs and panel lettering, making it a match for use with any of the early SB transmitter receiver pairs or transceivers up to the SB Fig 2: HM Rear View.

An approximately six-foot, permanently wired, cable connects the sensor with the cabinet. Room is provided within the cabinet to stow the wire should you wish to leave the sensor module installed in the case.

The HM has two power ranges that measure watts and 2, watts full scale. It also measures SWR on a scale that goes from to The front panel has two controls as well as a large, but unlit, meter.

The second control is a potentiometer with a switch that is activated by pulling out on the knob. The rear panel Figure 2 is part of the removable sensor module. There are also two holes allowing access to two adjustments inside on the circuit board. These are marked with their schematic parts nomenclature C-4 and R A piece of insulated cardboard covers the holes between calibration periods. There is a space below the sensor module when it is installed in the cabinet.

This space is for stowing the cable connecting the module with the cabinet circuitry. Copyright , R. This encompasses nine pages in the manual. This is stuffed first with fixed resistors, capacitors and the three 1N diodes germanium 40V 50 ma.

Next the pre-wound toroid coil is mounted using an eyelet that fits through the toroid center hole and is soldered to the foil side of the PCB. Later the lead between the input and output coax connectors will pass through this eyelet and the center of the toroid. Once the board is completed the five-wire cable is attached to the board. A ferrite bead is placed on each lead before it is soldered to the board. Remote chassis assembly involves mounting the SO coax connectors and metal standoffs to hold the circuit board, mounting the circuit board and some simple wiring between the two coax connectors on the remote chassis.

The five-wire cable is then passed through a hole in the remote chassis cover, a Heyco strain relief is attached and the cover is assembled to the remote chassis. Cabinet assembly starts with the mounting of feet and a solder lug, followed by the front panel controls and meter. The other end of the five-wire cable from the remote sensor chassis assembly is attached to the cabinet using a cable clamp. The cabinet wiring involves wiring up the fivewire cable, the controls and the meter.

HM Calibration: alibration consists of first setting the mechanical zero adjust on the meter. A perfect null is the ideal outcome. Next the power circuit is calibrated. The HM- includes some internal circuitry to produce a frequency dependent calibration standard valid on the 40 meter band. When the sensor switch is set in the CAL position, the wattmeter reads accurately.

However this is only valid on meters. If your radio transmits on 40 meters it is a simple matter to apply a reasonable power into the meter, note the setting with the switch in the CAL position, move the switch to the NORM position and adjust the power calibrate trimpot for the identical meter reading.

This holds for all bands. Heathkit also offers another way to calibrate the meter if a meter transmitter is not available. HM Circuit Description: The HM circuit is straight forward, but it may seem more complex due to the way the schematic Figure 6 is drawn. These components look like Copyright , R. Page 3 of 7. On 40 meters this circuit produces about 7 volts DC when watts is present on the wire passing through L1.

Seven volts corresponds to watts on the non-linear scale of the meter assuming 9 volts is full scale. This circuit is just used for a calibration source and plays no purpose during day-to-day operation of the power meter. The additional components for detecting reflected power are D2, C2, R1 and C3.

The additional components for detecting forward power are D3, C5, R4 and C6. L1 inductively picks up energy from the flowing RF. It is reasonably frequency independent; but the addition of R2 across the coil reduces the Q of L1 further making it frequency independent across the HF bands. C12, C4 along with C1 and C13 provide a small voltage to the center tap of L1. This voltage corrects for capacitive anomalies in L1. It is in phase with one winding and out of phase with the other.

With no SWR reflected power on the line, C4 can be adjusted to null out the voltage appearing on the left half of L1. Each side of L1 is rectified by a diode D2 and D3 filtered by a capacitor C2 and C5 and decoupled by an RC network R1, C3 and R4, C6 The result is that forward power produces a relative voltage on terminal B and any reflected power produces a relative voltage on terminal C. These voltages are relatively frequency independent unlike those in the earlier SWR bridges.

In the watt position an additional resistor, R9 is placed between the meter and R8 further scaling the meter to respond to a full scale of watts.

The cabinet circuitry is rather straightforward. In the SWR position, when the SENsitivity control is pulled out the forward power is connected through the control to the meter and when the control is pushed in the reflected power is connected.

In the position the watt output from the sensor Pin F is connected to the meter and in the position the watt output from the sensor Pin G is connected to the meter. C15 thru C18 bypass any stray RF to ground. It is connected in the transmission line where you want to measure RF power. This is usually in the feed line to your antenna or dummy load.

Once installed the power can be read on the meter. In the position the meter is read on the top scale 2 KW FS and in the position the meter is read on the middle scale W FS. The control is then carefully pushed in so as not to change the pot setting and the SWR is read on the bottom scale. Physically it is identical in size and weight to the HF model. It differs in power capability, having full-scale ranges of 25 and watts. These ranges are more practical to the average VHF operator than the higher ranges.

UHF connectors are used and the sensor module may also be placed remotely to the main cabinet. Page 4 of 7 Copyright , R. The ferrite inductor, and the scaling resistors and capacitors have been selected for the higher frequency range and the lower power measuring levels.

A few bypass capacitors, not needed at the VHF range, are also absent. An over-temperature sensor lights a lamp to warn of excessive dissipation. The lamp is powered by a standard 9-volt battery NEDA The only function of the battery is to operate the lamp during an over-temperature condition. This is more a test bench instrument than an amateur accessory.

The HM was short lived appearing in and vanishing in early The circuit for the power meter part of the dummy load is identical to the HM with the exception of the forward SWR output which is not needed. Neither is the reflected SWR output except when nulling the bridge. The power circuit varies only in the scaling resistor used for 1 KW instead of 2 KW.

The watt scaling resistor remains the same. The HM HF model is similar. The HMA sports a brown motif to match the later Heathkit ham gear; otherwise it is unchanged. Each has two separate meters on the front panel. One indicates forward RF power and the other reflected power.

The meters also can measure average power. When using the PEP feature a source of power is required; however the regular readings will function without power. Also, the GRA adapter was mentioned in the early manuals. Heathkit HMA RF Wattmeter: With the color scheme change from the green and gray SB series of amateur gear to the brown color scheme of the later amateur gear, Heathkit updated the HM to the HMA which for all practical purposes is the same device in new clothes.

Some small circuit changes may have been made, but the schematics appear identical. Page 5 of 7. Thus the HM-9 power meter came into existence. In truth the kit contains the components to assemble the meter to cover either the HF band MHz , the six-meter band MHz or the two-meter band MHz.

When completed the meter works only on the chosen frequency range. Full scales of 5 watts and 50 watts can be front panel selected as can the SWR mode.


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