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Coax from telescopic aerial to circuit board in DAB radio
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Author:  dabber [ 09 Jun 2017, 07:16 ]
Post subject:  Coax from telescopic aerial to circuit board in DAB radio

In making changes to the mains input of my Tesco DAB111V portable radio I unsoldered and resoldered the coax cable from the telescopic aerial to the circuit board, to get better access. Now, strong FM stations are just OK, but I get “No DAB station”. I assume I have weakened the signal strength in reconnecting the aerial inappropriately. To what should the outer sheath of its coax cable be connected?

Author:  Peter W. [ 14 Jun 2017, 06:38 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

dabber wrote:
In making changes to the mains input of my Tesco DAB111V portable radio I unsoldered and resoldered the coax cable from the telescopic aerial to the circuit board, to get better access. Now, strong FM stations are just OK, but I get “No DAB station”. I assume I have weakened the signal strength in reconnecting the aerial inappropriately. To what should the outer sheath of its coax cable be connected?



Look for the schematic and follow that. Also, make sure you created no solder-bridges or blobs with the re-solder.

Usually the shield goes to a large (relatively) pad on the circuit board - but not always. Hence the suggestion that you find a schematic.

Author:  Geek [ 15 Jun 2017, 02:57 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

dabber wrote:
....Tesco DAB111V portable radio....


*looks up*

Quote:
Price bracket £25 - £30


You've probably already spent more in labour than replacing it.

But look for a shorted coax.

Cheers!

Author:  dabber [ 16 Jun 2017, 15:25 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

Peter W. wrote:
dabber wrote:
In making changes to the mains input of my Tesco DAB111V portable radio I unsoldered and resoldered the coax cable from the telescopic aerial to the circuit board, to get better access. Now, strong FM stations are just OK, but I get “No DAB station”. I assume I have weakened the signal strength in reconnecting the aerial inappropriately. To what should the outer sheath of its coax cable be connected?



Look for the schematic and follow that. Also, make sure you created no solder-bridges or blobs with the re-solder.

Usually the shield goes to a large (relatively) pad on the circuit board - but not always. Hence the suggestion that you find a schematic.


Thanks Peter W. Do you know how I could obtain the schematic of the Tesco DAB111V portable radio? I've searched the web without success.

At the point where the coax to the circuit board joins to the telescopic aerial, what should happen to the shield? The aerial only seems to have one connection, which is presumably to the inner wire of the coax?

Author:  M. Gregg [ 17 Jun 2017, 00:50 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

Why didn't you take a photo before you took it out?

I always take a photo of anything before I strip it? It takes seconds and can save hours of heart ache.
The number of times I have stopped myself just going ahead and got the camera...

Make sure the coax hasn't got a semiconductor layer between the shield and the inner core.
Probably a partial short between inner and outer conductor.

Regards
M. Gregg

Author:  dabber [ 17 Jun 2017, 07:35 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

Thanks M Gregg! I know I should have taken that photo beforehand. But it’s too late. Now I can only send what it looks like afterwards:
Image
1. Overview showing coax connecting telescopic aerial to circuit board.
Image
2. Close-up of connection with aerial: central wire of coax soldered to aerial tag; shield not connected (should it be?).
Image
3. Close-up of connection with circuit board: central wire of coax soldered to (the correct?) point on circuit board; shield not connected (should it be?).
Any thoughts would be very valuable, please.

Author:  M. Gregg [ 19 Jun 2017, 13:07 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

You know,

Its almost impossible to tell without other information, however in the last photo there are three solder connections.
What is on the middle solder pad? is it a broken piece of wire?<<<if it is that's probably the aerial wire.
The Aerial wire could even be an ordinary wire without coax shield because it just goes to the rod aerial (which is just a strait piece of rod/wire) so the shield is to reduce interference but it won't stop it working.

Whats happening with the large black wire? is it disconnected or have you soldered that as well..
These are just a guess..so best of luck.

Here is a wild guess<<and its at your own risk.
The middle connection is the aerial the top pad is the screen and if the black wire is disconnected its to the top pad.

Or the aerial is the top connection and the shield to the middle.
The black wire could be ground but it's hard to tell you could test the black wire continuity to the battery negative.
That's about as much as I can guess..and its a maybe.


Regards
M. Gregg

Author:  dabber [ 20 Jun 2017, 18:02 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

Hi M.Gregg and MANY thanks! Your opening paragraph was spot-on. The “broken piece of wire” was the remains of the aerial wire. When I reconnected it to the core of the coax fed from the telescopic aerial everything was back to normal. I connected the coax shield to the third solder pad, which is ground. At the aerial end I left the shield unconnected. The large black wire connects the 2 circuit boards’ grounds together.

BRILLIANT – thanks!

Author:  M. Gregg [ 21 Jun 2017, 14:54 ]
Post subject:  Re: Coax from telescopic aerial to circuit board in DAB radi

:up:

Regards
M. Gregg

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