Strange VHF problem

[Babylon]

I have a Simrad RT64 VHF (a few years old) which is behaving quite strangely.

With the antenna properly connected, it can only make and receive transmissions at very short range (ie from a marina berth to the marina office) but not further afield. I have checked that it is not the antenna that is the problem, by re-testing it with the emergency antenna rigged and connected - and got exactly the same problem.

With the antenna DIS-connected however, it receives transmissions from further afield (eg coastguard, VTS, other vessels)! I haven't however tried to transmit from it in this state, as I'm worried it might damage the unit further.

In both of the above tests, I also used my handheld in parallel - which worked perfectly despite being operated from down in the cabin.

Is this a common problem, what might be causing it? Can the radio be properly tested and repaired? (I don't want to buy a new DSC VHF if I can avoid it.)

 

[oldsaltoz]

Does this model have a selector for low (1 Watt) and High (5 Watt) TX?

 

[Thistle]

Did you connect the emergency antenna direct to the set or to a socket on deck? If it was the latter, it is possible that the cable from the deck socket to the set is faulty. In any case, I think I would want to make sure that all the connections, on the set and at deck level, were completely clean. It doesn't seem to take much dirt or corrosion to upset signals at these frequencies.

 

[Babylon]

Yes, can select between 1w and 25w, but that should of course only effect transmissions - its reception that's a big part of the problem. In this regard, I tried calling CG for a radio check on both high and low power, but did not hear any reply, either on this VHF or my handheld.

 

[Babylon]

In originally trying to source the problem, I replaced the entire VHF antenna (which was getting pretty old as it was and was rusting at its base), and in so doing got rid of the deck-socket by running the coax cable through a deck gland.

Re-testing the VHF with a completely separate (emergency) antenna hoisted up a halyard,but the same problems continued - thereby entirely eliminating the antenna/coax end of things - ie it is almost certain that problem lies with the radio unit itself.

 

[Thistle]

Coax cable, particularly in old installations, has the endearing habit of allowing water to run down through the air channels in the cable. This can corrode connectors or, if you are unlucky, get into the set itself and cause damage. Worth checking for corrosion on the connector on the set before opening it all up.

(It happens at home too: at different times I've lost a TV and a VCR to this problem.)

 

[wotayottie]

from what you say you have tried a radio check with the marina but nothing else other than listening. If I remember correctly the RD68 I had is equipped with protection circuits so you cant fry the tranmission output. Check with Simrad to see if I'm right and if so do your radio checks with the cg. Dont bother trying to tranmit without aerial - pointless.

Assuming the problem is still there, then it still sounds like aerial since by disconnecting it the only bit you are changing is from the aerial socket outwards on the radio. It is possible, if a bit unlikley , that both aerials are duff.

Presumably you made the new aerial plug yourself so thats the first thing to check - its so easy to get a few strands of the outer braid short circuiting the plug. So remake it with care and try again.

Then if you still have problem either borrow a vhf set from a pal or buy one of the adapter plugs that allow you to connect your hand held to the boat aerial and try again. That at least should allow you to firmly establish whether the problem is aerials or radio.

If the problem does lie with the radio then its likely to be best to junk it. The RD68 was a very early DSC set, never a very good one and is likely tobe expensive to repair.