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Old 2nd Mar 2018, 5:54 pm   #41
crackle
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Default Re: Car radio aerial issue

If the coil is open circuit I would get that fixed before worrying about transistors.
The transistors have been proved to be working, they may be intermittent or on their last legs but that can be checked later.
The radio is going to just be an ornament without a working L3.

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Old 2nd Mar 2018, 9:33 pm   #42
ITimmins
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Default Re: Car radio aerial issue

Latest update:-
I’ve tested the theory by shorting L3 terminals and hey presto, the radio works normally with the cover screwed down fully and if I add a length of wire to the aerial socket reception improves dramatically. On MW I found several clear stations and even checked LW and managed to find a couple of stations albeit with quite a lot of background noise, but I am still testing indoors. Running for about 1 hour the reception remained loud and clear.

I’ve carefully checked for a wire break and probed at the terminals on the L3 coil but it’s dead.

My question now is whether my shorted L3 solution is usable longer term and what is the detrimental effect given it seems to be behaving normally to my mind?
Is it even possible to repair or replace this coil given the age of the radio??

Anyway, thank you to all contributors for helping to get to the source of the problem and, dare I say it, a working radio.
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Old 2nd Mar 2018, 11:52 pm   #43
'LIVEWIRE?'
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Default Re: Car radio aerial issue

If the radio is working reasonably well with L3 shorted out, I don't see how any harm can be caused by leaving it that way permanently. All it means is that there is one less tuned circuit in the RF stage, hence my comment about selectivity in an earlier post. IIRC, the later Radiomobile/Motorola radios (970/980, 727/828, 1070/1080, etc.) omitted one coil (L4), and coupled L3 to the base of Tr1 via a capacitor instead, and they were very good performers. What you are doing, out of necessity, is bypassing L3. Whether using a capacitor instead of just shorting out L3 would be better could only be found by trying it, and some experimenting to find the optimum value cap. would be required. Since shorting out L3 doesn't upset ant DC conditions I'd be inclined to leave it that way. Other than from a donor set, you are never going to be able to replace the coil anyway. The reason for poor performance on LW with L3 shorted out is that, due to the low resistance of the primary of L4, the preset LW aerial coil, L2 is virtually earthed when the set is switched to LW, thus reducing the strength of the signals received via the aerial. If LW reception is important, bypassing L3 with a capacitor would be better. Somewhere between 4n7 (4700pf)and 10n (10000pf) should suffice (if I've done my sums right!)

Last edited by 'LIVEWIRE?'; 2nd Mar 2018 at 11:59 pm.
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Old 3rd Mar 2018, 12:42 am   #44
ITimmins
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Default Re: Car radio aerial issue

Thank you again for the helpful feedback. My next step will be to install it in the car and see how well it works and maybe I’ll substitute my bi-pass wire for a capacitor for better LW reception.

I have found this a very helpful forum and hope to find some other challenges in the future.
One thought that comes to mind is wiring in an MP3 socket......but I should probably start a new thread!

Thanks all
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Old 3rd Mar 2018, 9:31 am   #45
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Default Re: Car radio aerial issue

Over the years I have fitted sockets to many models of Car Radio to allow for the connection of MP3 Players, I-pods, and other audio devices. There is a 'sticky' thread on how to do this at the head of this section entitled 'wiring 'aux-in' connectors ', and a read of that will explain what's involved.
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Old 3rd Mar 2018, 11:17 am   #46
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Default Re: Car radio aerial issue

Having been reminded of the correct units to use in Reactance/Impedance calculations by Frank (Nuvistor), I would advise that a coupling capacitor of between 1000 & 5000pf could be used to bypass the o/c coil L3. A 1000pf cap. would have a reactance of about 1060 ohms @150kHz.
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Old 3rd Mar 2018, 12:50 pm   #47
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Default Re: Car radio aerial issue

That’s perfect and very much appreciated��
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