VHF not transmitting

[Bodach na mara]

I discovered this problem on the Clyde Scuttlebut Cruise. Basically, the set is a Compact SX-25 and it receives fine. It is transmitting a carrier only. The collected wisdom of the assembled experts was to try it with another aerial, which I have now done. No joy, I still get a carrier, but no modulation.

I went into the place where I bought it and was told it was too old for an economic repair (it is only 9 years old!) Has anyone any clues as to what is the problem, and whether it is really unrepairable?

Oh, I forgot. I tested the aerials using a resistance meter. The new one reads about 2 ohms across the central pin to screen. The old one reads about 300 ohms there.

 

[ThereAndBack]

Sounds much more like a microphone fault to me. The TX button is activating the TX circuitry but the mic isn't picking up your voice.

Certainly worth trying another mic before spending money.

Alan

 

[MM5AHO]

My suggestion too. You said a carrier, so rule out antenna (providing carrier is a strong signal. The listening boat can give an approximation by listening to your carrier. If it fully quietens their receiver, strong signal).
No modulation means either that part of the transmitter is faulty, or no audio to it. That could be the mic. Check for broken wires in the mic lead, either the plug end or mic end.
Try another mic if one available. Know anyone with same unit? (to borrow their mic).
Good luck

 

[irl2605]

Hi Guys. Hope you dont mind me jumping in. But what should the resistance across an antenna be. I have a similar problem (weak signal)and am trying to rule out the antenna before I send the Icom401 unit for repair. I have fitted a new Glomex 1 mtr.
fair winds Iain

 

[graham]

Im a complete non expert here but have you checked the voltage at the set?

 

[irl2605]

Yes. Plenty off juice and everything else works ok. I think myself its the set but just trying to rule everything else out. thanks anyway ..

 

[Bodach na mara]

Thanks for the suggestions folks. The voltage is OK and I did test continuity of the wiring from the plug to the microphone. All seems normal, but a resistance test will not show up a mike transducer that is not working and I do not have an oscilloscope. I will try to borrow a mike to try this component.

In answer to 260_spirit, my second aerial (on the mizzen mast) is a Glomex 1 metre and it is the one that shows about 2 ohms between the central pin of the plug and the screen

 

[VicS]

[ QUOTE ]
Hi Guys. Hope you dont mind me jumping in. But what should the resistance across an antenna be

[/ QUOTE ] Low usually. You really cannot test an aerial with a multimeter although if you can disconnect the cable from the aerial you can test for continuity of both centre conductor and sheath and also for the absence of any short between them.

 

[fireball]

There are 3 connections between the Mike and the set .... all 3 have to be connected for it to work ..... That would be my first port of call (with the same set myself - albeit off the boat now - but had the same problem when running it up as a spare)

 

[William_H]

I repaired a friends radio a while back with similar symptomms it was the microphone insert itself. This was virtually a little speaker used as a mike if yours is the same it should have a resistance of 200 to 500 ohms and click when you put the multimeter across it. This type of mic is about 3cms in diameter. I replaced it with a speaker out of a cheap set of headphones and that seemed adequate.

Of course some radios may use a capacitive mic which is generally very tiny ie 1cm in diameter and this type can not be checked for resistance.
Antennas can be of 2 different designs the short 17inch ones are 1/4 wavelength and usually have no connection centre to outer for DC. While the longer type have a coil or similar providing a connection to ground for DC.

good luck olewill

 

[Bilgediver]

Hi Guys. Hope you dont mind me jumping in. But what should the resistance across an antenna be. I have a similar problem (weak signal)and am trying to rule out the antenna
****************************************************

If you have a similar problem of tranmitting a carrier but no voice then concentrate on checking the mike and its connections. Could be a broken lead in the mike plug, damaged mike insert or even a loose connection between the plug and socket on the radio.

Sometimes I have had to squeeze the cheap tin socket inserts in the plug to make them tighter on the pins in the radio..The navigators compass is a good tool for this..

 

[maxi77]

Can you try the set with some one elses mike, many of the plug and sockets are standard CB radio ones.

 

[Bodach na mara]

The general concensus seems to be that the mike is the probable cause of the problem. Thanks for all the advice. I have arranged to borrow a mike tomorrow from an ICOM set which I hope fits. If it doesn't, I will follow up maxi77's suggestion and try to get hold of a CB radio one. I will anyway take the mike unit to bits and try to find any fault in it. It is a pity I no longer have access to an oscilloscope, as that would have made the checking easier.

 

[irl2605]

Thanks Ken & Vic.. I will test those things when I get down to the boat tomorrow.. Happy Sailing .. Iain /forums/images/graemlins/cool.gif