UHF trasmitting problem

Submitted: Friday, Mar 12, 2010 at 16:09
ThreadID: 76806 Views:4220 Replies:10 FollowUps:10
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Howdy all - I have a GME TX3400 which up until a few weeks ago was working fine but all of a sudden I am only transmitting no further than 50 to 100 metres but can still receive clearly from 5 to 6 k's away. All buttons seem to be set properly and aerial is in good condition. Any suggestions ?

Thanks Gazz
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Reply By: SteveL - Friday, Mar 12, 2010 at 16:28

Friday, Mar 12, 2010 at 16:28
If you can receive OK but can't transmit very far, it's probably a blown rf power module.
AnswerID: 408578

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 17:02

Friday, Mar 12, 2010 at 17:02
Thanks Steve - I thought it may have been something simple or perhaps something stupid that I did with the buttons !

Cheers
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FollowupID: 678514

Reply By: Bryan (WA) - Friday, Mar 12, 2010 at 16:55

Friday, Mar 12, 2010 at 16:55
HI Gazz,
Mine did the same thing earlier this year.
The RF PWR module is blown, you can purchase a new one and solder it in. you will also need to re-tune the pwr variable resistor as the replacement pwr modules are not direct replacements so the voltage level to the trigger in the pwer module is not correct.
I was lucky and didnt have to replace 2 of the surface mount resistors that GME say need replacing when re-tuning.
I have all the info from GME I can email to you if you like - cct diagram, what to do, etc.
If your in Perth I can give you a hand.


regards
Bryan
AnswerID: 408582

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 17:06

Friday, Mar 12, 2010 at 17:06
Thanks Bryan - looks like it's going to the cb doctor coz I'm not that good with electronic stuff ! If I was as good at electronics as I am at drinking beer I would be building launch pads for NASA !

Cheers Gazz
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FollowupID: 678515

Reply By: Bryan (WA) - Friday, Mar 12, 2010 at 17:02

Friday, Mar 12, 2010 at 17:02
sorry forgot to mention the reason the output pwr module blows is due to high SWR and the output power being reflected back into the module.
you will get this occur with a stuffed antenna, transmitting with no antenna, bad connector/coax.

I replaced my antenna as well as I found my original antenna was broken internally where the SWR tuning coil is.
if you can borrow a UHF SWR/PWR meter you need to check if its all ok otherwise you will repair the unit, only to have it blow the module again... :-(
AnswerID: 408583

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 17:09

Friday, Mar 12, 2010 at 17:09
Thanks again - I will get the shop to check everything out.
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FollowupID: 678516

Reply By: MEMBER - Darian, SA - Friday, Mar 12, 2010 at 17:53

Friday, Mar 12, 2010 at 17:53
Ah yes - a sad tale for me too, re the comments above - I blew my RF module in a relatively new CB by not paying attention to what I was doing with antenna connections - if the signal has nowhere to go, it comes back and cooks the radios gizzards (sorry, re the technical speak :-o).......... had the same result - great reception and only about 50m of transmission ! Took it to the CB doctor - was an expensive repair - lesson learned (but ouch) - suggest you have the doc fully check your antenna setup.
AnswerID: 408589

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 18:05

Friday, Mar 12, 2010 at 18:05
Thanks Darian - the complete unit including aerial was supplied and installed by a large and reputable company that specialises in communications only and is about 4 years old so I will take it back to them for repairs - it's only money !
I could get it fixed 300 times and it would still be cheaper than my wife's shoe collection !!

Cheers
Gazz
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FollowupID: 678521

Reply By: Member - Fred B (NT) - Friday, Mar 12, 2010 at 18:01

Friday, Mar 12, 2010 at 18:01
Gazz,
before you get too carried away, there are a couple of things you can check first....
It could be that the cable connection to the radio end, or to the antenna end has come loose or has a poor solder/dry joint. It's not always a power module that is blown. If you cable and connections are good, and antenna hasn't cracked or broken, then you can suspect the power module. But I would look really carefully first. It could save you some money...
regards
Fred B
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AnswerID: 408592

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 18:07

Friday, Mar 12, 2010 at 18:07
Thanks Fred - will check out everything

Cheers
Gazz
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FollowupID: 678522

Follow Up By: Member - Bucky - Saturday, Mar 13, 2010 at 07:33

Saturday, Mar 13, 2010 at 07:33
Fred
That's a great call...
Had the same problem myself short range, ect....

Turned out to be just one tiny strand of earth from the shield was shorting out..
Another thing to watch out for is to make sure that you are powerign the unit directly to the battery. Put in a fuse at the batterty end, and earth it back to the same battery, weather it be the main or the secondary.

They love a good power source. I had a lot of trouble with my original UHF till I worked that out.. Tried everything else, in the mean time.. Der me,.. but it's a learning curve, and I never once thought it couold be lack of good direct power ..


Cheres
Bucky
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FollowupID: 678567

Follow Up By: hazo - Saturday, Mar 13, 2010 at 13:45

Saturday, Mar 13, 2010 at 13:45
I too would eliminate a faulty antennae first, my son in law has just had exactly the same problem on his boat, a new antennae fixed it!
Not saying its not the transmit module but if you have access to a nother aerial try that first, as if it is the module you cannot damage it further and it may save considerable expense.

Cheers
Brian
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FollowupID: 678593

Reply By: Bryan (WA) - Friday, Mar 12, 2010 at 19:11

Friday, Mar 12, 2010 at 19:11
Gazza,
If you can receive transmissions fine - as in you can hear the repeaters, but you cant access them when you normally could in the same locations, it wont be cabling or connectors. if your lucky it could be the variable resistor that sets the bias voltage to the tx Module is out and has be be adjusted, but without the gear to test it, you wont know..
A dry solder join in the connector or antenna - and internal in the antenna where the tuning coil is (for ground plain independent antennas) will kill the SWR and cause the TX power to be reflected into the radio and the RX will be fine still.

depending on how much they want to charge you to check and fix, it might be cheaper to buy a new unit and new antenna setup with new coax......

Bryan
AnswerID: 408600

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 21:18

Friday, Mar 12, 2010 at 21:18
Thanks Bryan you've been a great help ! I will have a chat to the cb gurus and make a decision after that.

All the best

Gazz
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FollowupID: 678548

Reply By: Member - Brian A (NSW) - Friday, Mar 12, 2010 at 19:16

Friday, Mar 12, 2010 at 19:16
Gazz,

I have the same uhf, that had the same problem. Metro Comms at Banksia in Sydney removed the unit, replaced the faulty module, reinstalled in vehicle, check transmission strength, all for $135. Excellent & Friendly service.

My radio had exactly the same symptoms - received well and couldn't transmit.

All good now.

Brian
AnswerID: 408602

Follow Up By: Outback Gazz - Friday, Mar 12, 2010 at 21:24

Friday, Mar 12, 2010 at 21:24
Howdy Brian - I will be taking my ute into the experts and getting them to fix the problem as I am no electronic wizz ! Was good to get an idea on costing - thanks ! Wish my wife could only receive and not transmit !!


Cheers

Gazz
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FollowupID: 678550

Reply By: Stu & "Bob" - Friday, Mar 12, 2010 at 21:44

Friday, Mar 12, 2010 at 21:44
It could also be as simple as a broken centre conductor in your co-ax cable.

Easy to check with a multimeter.

.
AnswerID: 408623

Follow Up By: SteveL - Friday, Mar 12, 2010 at 22:35

Friday, Mar 12, 2010 at 22:35
Broken centre conductor = no reception as well as no transmit
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FollowupID: 678556

Reply By: vk1dx - Saturday, Mar 13, 2010 at 08:42

Saturday, Mar 13, 2010 at 08:42
No transmit can also be caused by a crimped or sharply bent antenna cable. If there is any extra cable hanging around tidy it up to a nice circular loop, as you would a hose. But not too tight a loop as a very tight loop actually puts an electronic short circuit onto the antenna system and the signal will not get out. Technically it can be called an RF loop, trap and other names.

We include BEFORE every trip a test of the radios AND their antenna systems before leaving. Do it with the pre-trek service. And by someone who knows (has formal training and skills) what they are doing.

Bazz - Take it to the radio doctor. May be worth a "service" / tidy up anyway.

Phil
AnswerID: 408636

Reply By: Member - Jack & Lorraine B (QL - Saturday, Mar 13, 2010 at 18:41

Saturday, Mar 13, 2010 at 18:41
Another problem that can cause loss of output is water in the coax or the fittings, causing short circuits.

Do NOT transmit when your fittings are under water!!!!!!

It pays to check the antenna cable every now and then .
Disconnect from antenna and radio first.
then put the antenna back on and test on the radio end.
use an ohm meter to check resitance between core and shield.
If it is not ZERO then it wont work well!
both readings should be the same and ZERO.

If you have one of the better rigs it will show you an SWR reading, that should be below 3. Any higher is not much fun and can damage the radio as the power goes back into the final stage!

The same applies to all radio transmitters !!
It can get very expensive with the high powered transmitters on HF!

Have fun out there and look after the radio , the antenna and the cable in between.

Check that you have a good earth on the outer core of the antenna coax termination.

AnswerID: 408694

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