My MARK III stopped transmitting in AM please Help

[ratpatrol]

I just got my Mark III..everything was fine and up and running well...Yesterday, I keyed the mic in the AM mode and the power was going to the antenna, The "on the air" light was lit and people on the on the channel could hear me "key up", but there is no modulation...When I switch to USB or LSB...I have modulation...Its the radio of my dreams and I finally have one. I called a technician who makes house calls on radio equipment coming Monday. are there any suggestions or advice from this forum.? I greatly appreciate it it so much in advance..........JR

[2600]

I wrote a response to this thinking that you meant there's no AM carrier when you key the mike.

In your other post you said they hear you key, but don't get any audio. For now we'll assume that you have a 'dead' carrier coming out of the radio when you key the mike, but no mike audio is heard at the other end.


Always a good idea to have a wattmeter in line with the radio and your antenna so you can see if the radio really is transmitting at all.

Time to apply the calibrated eyeball. You need to check a 3.3k 1/2-Watt resistor R51 to see if it's burned.

This resistor supplies power to the mike amplifier. It burns up when the brown plastic of the AM/LSB/USB mode-select switch breaks down. This causes shorts across separate circuits.




When this happens, it's possible to bypass the mode selector and 'hot-wire' the transmitter for AM only.

Or you can replace the selector. Good news is the replacement won't be made of brown plastic, but from white ceramic. Doesn't break down with age like the plastic will do.

Bad news is that the part isn't cheap and the labor to change it out is significant. This is one repair task where accuracy counts.

But have a look at R51 and see if I'm right.

73