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Old 16th Jan 2017, 2:01 pm   #1
longneck90
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Default Cossor 479 AC/E

Hello to everyone, I had a tread in this Forum a few weeks ago, looking for information about this radio. Presently, I am using Cossor 474 circuit diagram, which is different. My problem now, is this; When I power up the radio, I noticed that there is a voltage of 420 ADC, between the transformer chassis, and the main chassis. (I mean the actual steel chassis). When I disconnect the 'connector' between transformer chassis and main chassis, the neon electric tester shows a bright light, when touched to transformer chassis. As the chassis seems to be 'live' I can't ascertain if any voltage is coming out of transformer. The transformer is very sturdy, and shows no signs of stress. I have tested the transformer winding's for short circuit and open, but all seems to be well. I was hoping that some of the Forum members might be able to give me some advice, before I dismantle the transformer. Thanks in advance. All the best. John.
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Old 16th Jan 2017, 2:38 pm   #2
ms660
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Default Re: Cossor479 AC/E

Not sure what's meant by ADC?

With the power off I would first check for continuity between the PSU chassis and the receivers main chassis then if that's ok and the transformer has a primary winding that's isolated from the secondary winding eg: it's not an auto transformer then I would rewire the mains lead for a three core job and earth the chassis and make sure there's a 3 amp fuse (maximum) fitted to the mains plug, that will make sure the chassis doesn't indicate live or can become live when the receiver is powered up, remove the rectifier valve and power up

If the fuse in the plug blows or the RCD trips then that would indicate that the transformer is shot primary to core, if the fuse doesn't blow and the RCD doesn't trip then short the iron core of the mains transformer to chassis, with a screwdriver blade, there should be no sparks, if there is then the transformers shot, if no sparks then unplug from the mains and make sure the transformers core is connected to chassis somehow.

Plug rectifier valve back in and plan your next move.

A bit caveman I know but if the transformers shot it doesn't really matter how you go about testing it.

Lawrence.

Last edited by ms660; 16th Jan 2017 at 2:43 pm. Reason: Addition
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Old 16th Jan 2017, 3:44 pm   #3
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Default Re: Cossor479 AC/E

Thanks Lawrence, (ms660) By ADC, I meant the rectified voltage. (current) Thanks for all your information, which I will work with now, and then report back. All the best. John.
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Old 16th Jan 2017, 6:34 pm   #4
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Default Re: Cossor 479 AC/E

Hello to all. Lawrence, I am sorry about ADC. When you questioned it, it still did not hit me. It was only, when I was walking the 2 miles down to the shops, that revelation suddenly came. ADC makes no sense. I meant VDC. All the best. John.
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Old 19th Jan 2017, 7:55 pm   #5
longneck90
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Default Re: Cossor 479 AC/E

Hello to all. To Lawrence, I took your advice, introduced 3 core cable, earthed both chassis, and all seemed OK. When I connected Rectifier valve, with mains input at 140 Volts, the dial lights worked and the valve (rectifier) seemed OK, with heater very bright. Connected transformer chassis to main chassis. Valve heaters not lit. A difference of 40 volts between the 2 chassis. Momentarily, I tried 240 volts main input, with instantaneous burning and sparking underneath chassis or base of rectifier valve. I couldn't find the source of burning. I re-attached to 140 V mains input, and checked the voltage on the 3 electrolytic capacitors (reservoir etc). With negative lead on chassis, they each had a voltage of around 420 VDC. Coupled with the fact that I haven't got the correct circuit manual, I concluded that I was now entering dangerous territory, and the safest thing to do was abandon project until hopefully, in the future, I would come across the proper data manual for this particular receiver. Thanks for all the help and information. All the best. John.
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Old 19th Jan 2017, 9:40 pm   #6
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Default Re: Cossor 479 AC/E

Some of those measurements don't tally up, eg: 40 volts between two chassis's when all chassis's are earthed.

That said the transformer could be breaking down.

Lawrence.
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Old 20th Jan 2017, 12:54 am   #7
longneck90
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Default Re: Cossor 479 AC/E

Hello to all. Yes Lawrence. It is strange, as the chassis are all properly earthed. As well as that, there are connections to the rectifier valve pins, that are strange, and do not tally with circuit diagram that I have. Anyway, I have now put the radio aside, until I get the right manual. Thanks again. All the best. John.
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Old 20th Jan 2017, 8:16 am   #8
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Default Re: Cossor 479 AC/E

Hi John
Can I just confirm, when you say " I re-attached to 140 V mains input, " you do mean that you have connected the radio to a supply of 140v AC, from something like a variac transformer, with the voltage selection on the radio transformer still set at the normal 240v.
You are not setting the voltage tap to the 140v setting and applying 240v are you?

Mike
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Old 20th Jan 2017, 8:18 pm   #9
longneck90
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Default Re: Cossor 479 AC/E

Hello to all. To Mike (crackle). When I say 140 V, I mean the radio/chassis is powered through Variac, set and double checked, at 140 VAC. Thanks Mike. all the best. John.
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