The MC-60 desk microphone by Kenwood is a high quality item. This microphone also has a built in pre amplifier which can be powered either by batteries, or by power fed from the radio.
When the preamp is being run from the radio power supply you may leave the preamp permanently turned on. Of course, when using batteries, although the current drain is quite small, the batteries will last longer if you turn the preamp off when you have finished using the microphone. The switch for the preamp is located (not so conveniently) on the base of the microphone.
As a result of leaving the preamp on, when the batteries eventually fail, the preamp becomes inoperative.
LED Preamp Indicator
I have modified my MC-60 to include an LED indicator which illuminates on transmit when I have the preamp in the microphone turned on. By having a visual indicator, I know at a glance whether the preamp is active and whether or not to use the speech processor in the radio. As I derive power for the MC-60 from the radio, I completely removed the battery from the circuit, and made changes to the internal wiring as shown in the schematic by the red lines.
The modification simply consists of an LED and a 1K resistor wired across the +5v and PTT lines in the microphone. The resistor can be placed either side of the LED as it is merely a current limiter. Depending on the LED used, it may be necessary to alter the value slightly to suit.