Batboard

Hello...

I have a M1225 on a pwer supply used as a base model radio in a police station. When the user transmits, any receiving radio hears a loud HUM. The wires on the back side are slightly frayed/cracking.

Would this be the cause for the loud hum?

Any ideas/suggestions?

Thank you

That really could be caused by several diffrent things

A hum doesn't come from frayed D.C. power wires, but it should be fixed. If it shorts, it will blow the fuse or even worse blow the power supply. The first and easiest thing to try is swapping out the mic. The next easiest thing to try is swapping out the power supply if possible. A spare mic is something most people have, a spare power supply? Probably not.

If the power supply is old or has been run in 'hot' conditions i.e. no ventilation, then I'd start looking at the supply's filter capacitors. If it's a conventional linear supply similar to an Astron, usually there's one large computer grade filter that sits right across the bridge rectifier's output that could be dried out/low capacitance, but it's more likely a 100-300uF cap that filters the control section of the supply. The control sections frequently use only a single diode rectifier, half wave, so the filter sees ripple at 60 hz rather than the full wave bridge's output(120 hz) feeding the computer grade cap. If you have one, watch the supply's output with a scope under transmit load, and you'll see any ripple if it's present.

Does it have the black hand mic, desk mic, or?

There are known 'hum' problems with the black hand mic, and some desk mics.

Hello Cduda, a couple of things come to mind on the same track with the other replies. Is the power supply big enough (output amperage) for the mobil radio? We did that at our Road & Bridge offices and had to cut the power output on the radios down to 5 watts so we could use them on ElCheapo power supplies. Also the two button desk Mic had given us problems similar to what you describe in proximity to computers and large copy machines. A hand mic cured that problem.Good luck and 73 de Jeff/N0SIH