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Old 18th Jan 2019, 9:10 pm   #1
llewellyn1012
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Join Date: Oct 2018
Location: Basingstoke, Hampshire, UK.
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Default Cossor 480K radio sensitivity

Hi

I have been working on a Cossor 480K for several months now in my spare time.
I am getting a signal through to the speakers but the sensitivity is very low, the strongest is Radio 4 and there are several short wave stations still very low output.

I have changed the coupling capacitor C22 and the output stage valve V4, tuned the If stages, RF and Oscillator stages SW/MW/LW using a signal generator input and scoped the output, also the IF Filter.

But still get poor sensitivity.

I am beginning to suspect the output transformer T1. Has anyone got the DC resistances of T1, or is there another way of checking T1.

I am using a 50ft copper wire stretched horizontally in the garden.

Any advice would be gratefully received

Regards
Llewellyn
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Old 18th Jan 2019, 9:43 pm   #2
Nuvistor
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Default Re: Cossor 480K radio sensitivity

The manual states approx 400 ohms for T1 primary.
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Old 18th Jan 2019, 9:50 pm   #3
G8UWM-MildMartin
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Location: Stockport, Greater Manchester, UK.
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Default Re: Cossor 480K radio sensitivity

T1 is an unlikely suspect.
What happens if an a.f. signal is applied to the PU sockets?
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Old 18th Jan 2019, 11:07 pm   #4
cathoderay57
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Default Re: Cossor 480K radio sensitivity

Hi, what are you using for an aerial? Also it would be worth measuring the valve voltages (Anodes and screens (grid 2)) and comparing them with the ones quoted in the service sheet. Often that can give a clue as to which stages are under-performing. V4 being a 6v6 I think is almost indestructible and they seem to last forever. As a complete guess I would check the anode voltage of the OM4 (= EBC33) and its anode resistor - sometimes these go very high resistance. Cheers, Jerry
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