Motorola MICOR Exciter Bias Removal

Modification of the Motorola® MICOR Exciter

By Kevin K. Custer W3KKC

Concept:
1 - Remove bias voltage from audio input of MICOR exciter
2 - Ensure proper keying when the PL encoder is removed or absent.

Description:
1 - Most Motorola® MICOR exciters, for mobiles, and some stations, exhibit a dc voltage intended to run the microphone amplifier that exists in the hand microphone. This voltage is not necessary when using an external audio source, such as a repeater controller or radio interface to drive the exciter. This voltage is provided by a low-value resistor from the exciters voltage source to the audio input line. This resistor also determines the audio input impedance, 330 ohms in high-band, 560 ohms in UHF Station models. This is a rather low impedance, making driving the exciter difficult for some equipment.

2 - When the OEM PL encoder is removed - the path for keying the exciter is also removed. There is a jumper that straps (enables) the exciter's keying voltage when the encode PL deck is removed or absent. Since you may be uncertain of where your exciter may have come from, this modification will ensure proper keying if you are not using the original equipment PL encoder deck.

Benefits:

1. Removes DC bias voltage and raises audio input impedance, and increases its sensitivity. Some repeater controllers cannot drive low impedances very well. Many (not all) op-amp circuits used in repeater controllers can only drive loads in the 1 - 2k ohm range without distortion. Lifting the bias resistor will drastically raise the input impedance and distortion will not result because of an extremely low load impedance.

2. Some controllers have a large coupling capacitor in the output stage. If the bias is not removed, there could be a period of no audio while the capacitor is charging. This could result in chopped ID's and missed syllables during the first part of the transmission.

3. Proper keying will result when the PL deck is removed.

Modification:
To eliminate the DC bias on the MIC audio line, remove R401, (R407 some models). This is 560 ohms in UHF Stations, and is 330 ohms in high-band models - - both Mobile or Station.

There are two options of modification to allow proper keying of the exciter, depending on your use of the original PL encode deck.

1 - If you never plan on using the MICOR PL encoder, determine that jumper wire JU405 (JU402 some models) exists between pins 8 and 10 on the long interconnect pins (P902).

2 - This method uses a semiconductor replacement for the jumper wire. This is especially handy for those who on occasion find it necessary to remove the PL encoder board for any reason. Instead of placing a jumper on the exciter board, install a silicon diode like a 1N914 or 1N4148 - being sure of its polarity. With the diode installed, it will make no difference if the PL encoder board is installed or not. The banded end of the diode will go toward P902 pin 8.

Have a high band exciter and don't know where these are? See the image below.
R401 is at the top of the image near the 19 long interconnect pins.