Adding The Workman MD-4 To Other Radios
Adding the MD-4 to radios with internal analog meters is more difficult than the Magnum 257 and 357DX. Although the following amplifier circuit works with the Magnum 257/357dx radios, its not necessary unless you desire the bar graph meter to also work. Adding the MD-4 external meter to the radios internal meter signal adds a load to the circuit. Both meters would share the signal increasing the current draw on the circuit.
The internal meter adjustments on most radios won’t calibrate with both meters in line. In addition, adjusting the MD-4 meter control also changes the internal radiometer calibration. The solution is to add an op-amp meter amplifier circuit. You can add a 2.5mm jack Mouser Part # 161-1650 to the back of the radio to accept the MD-4 plug. Run a wire from the tip connection of the jack to the output of the meter amplifier circuit.
Connect a wire from the ground to the ring terminal of the 2.5mm jack. The circuit below won’t effect the internal meter calibration even when the MD-4 meter control is adjusted. Once the meter is connected, either inject a signal to the radio or find a station reading “S”9 on the internal meter and adjust the MD-4 to the same signal.
The meter scale of the MD-4 is not as accurate as I would like, but the meter amplifier circuit somewhat compensates for the upper dB readings. Readings from “S” 3-7 are a little generous, but it’s not bad. It appears the meter scale should have the “S”9 signal approximately in the center with the other reading proportionately spaced on the scale.
However, the large meter is great and can be seen many feet away. The 220uf capacitor on the input of the meter amplifier is optional. Its purpose is to hold the peak reading and make a more smooth meter movement. Without the capacitor, the meter is more active with the speech in AM and SSB. The sideband “S” readings show their true peak readings with the capacitor in place. Many radios have a quad op-amp in the AGC and meter circuit with one section not used.
Instead of adding the circuit below, the unused op-amp can be used by adding the component values from the schematic below. The Magnum Delta Force is such a radio that has a spare op-amp. The (+) input is pin 12, the (-) input is pin 13, and the output is pin 14. The red and black meter lamp wires can be hooked up to either an internal 13.8 Volt source, an external power source, or vehicle dimmer control.