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Radio receives but won't transmit

Old 06-18-2003, 09:41 AM
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Default Radio receives but won't transmit

I have an ICOM M45 that receives OK but won't transmit. As soon as you key the mike, the power cuts off. Then if I turn the radio switch off wait a few minutes and turn the radio switch on again, it receives-then if I key the mike it cuts out again. Any comments would be appreciated.
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Old 06-18-2003, 10:38 AM
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Default Radio receives but won't transmit

My first guess would be there's nothing wrong with your radio. I think it's the power coming into the radio. When you key the radio, your battery voltage is dropping low enough to shut it down, or you have a bad connection somewhere which is letting enough current through to run it in receive which doesn't take much and not enough to run it in transmit which takes alot more. Check your connections! One way to test it is to hook it direct to your battery temporarily to see if it does it then. If it doesn't shut down, then you know it's in your wiring. It won't hurt the radio to key it for a few seconds without the antenna to test it.

Clint
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Old 06-18-2003, 03:52 PM
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Default Radio receives but won't transmit

Good suggestions Clint, but I would be reluctant to key the Transmit button without the antenna load.
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Old 06-18-2003, 04:46 PM
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Default Radio receives but won't transmit

It's up to you, but I've been working on radios for years and it won't hurt your radio to key it for a couple of seconds without the antenna. Long enough for you to see if it shuts down. What happens when you key the radio without the antenna is that the power is reflected back into the radio and will overheat the final IC, however this isn't instantaneous, it takes a while. If you don't want to do it, you can also just try keying it in Low power (1 watt) with the antenna connected. If it doesn't shut down then this is another indication that your voltage is dropping off coming into the radio when you're drawing the higher current in Hi power, which could be due to a bad connection, etc. If it doesn't shut the radio down in low power then it's probably not the radio.

Clint
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Old 06-18-2003, 06:47 PM
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Default Radio receives but won't transmit

I had a similar problem with a Ray VHF. With engine off, would only X-mit on 1 W. 25 W , radio would shut down. Triple checked all connections and voltage to radio was 12.1 (box in 23' hardtop)When the radio was keyed in 25W mode, voltage to radio would drop to 11.8 V. Current was no more than 3 amps.
Removed radio connected to test equipment. Radio would shut down (25W X-mit) when the supply voltage was reduced to 11.9 (radio specs. 11.6-14.5) Bottom line was , radio did not operate to specs and I had some marginal batteries.
FYI, the batteries started the single 225EFI (current sensitive, not voltage sensitive) but the radio would not X-mit.
Replaced 2 batteries ( upgraded from size 24 to 27) radio works fine. FYI, the voltage meters built into the radios "can be in accurate" Heck of a way to bring marginal batteries to my attention.
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Old 06-19-2003, 07:54 PM
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Default Radio receives but won't transmit

Elwood,
DO NOT by any means xmit with your radio with the antenna disconnected.....it is one of the best ways to blow the final transistors in your radio and if the finals blow, throw the radio away because the transistors cost more than the radio is worth.
Some of the better radios have a SWR protection circuit built in them.......when the SWR is high the radio will either shut down or some will xmit in low power.

When you say "the radio cuts out", do you mean that the power shuts off and the lights go out?
If that's the case sounds like a circuit breaker is tripping so by the time the radio is turned again the breaker is reset.

As suggested above try connecting direct to battery just for a test.

DO NOT XMIT WITHOUT ANTENNA CONNECTED.


.............Booby
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