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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 20th Oct 2007, 11:45 pm   #1
Steve_P
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Default Civilian Receiver - MW Only.

Now I suppose at some time in the future I will probably look really stupid, but if anyone can help....

Been worked on before, but being that way inclined, I switched on. Smoke - off quick.

Open it up, look inside, notice R13 was smoking. So, replace all 3 smoothing caps, R14, R13, valves out except rectifier.

Kept on smoking from R13.

Disconnect output transformer.

Kept on smoking from R13.

Remove Rectifier.

Kept on smoking from R13.

Disconnect R8.

Kept on smoking from R13.

Disconnect R13.

Smokes from R14. Put R13 back in - Smokes from R13. R13 has 74 Volts AC on it!!

Part of Circuit below. HELP!!!!!

Cheers,

Steve P.
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Old 21st Oct 2007, 12:10 am   #2
PJL
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Default Re: Civilian Receiver - MW Only.

Sounds like magic but how about transformer insulation to screen gone? ...Peter
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Old 21st Oct 2007, 1:36 am   #3
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Default Re: Civilian Receiver - MW Only.

I think Peter has something there. There isn't much left!!
Also check transformer wiring , wiring around rect socket, and rect socket it self.
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Old 21st Oct 2007, 7:44 am   #4
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Default Re: Civilian Receiver - MW Only.

Yep, mains tranny has gone S/C secondary to chassis
Try lifting the tranny off the chassis, and disconnecting any earth connections, run it up with the tranny sat on a bit of insulation....


Hang on a moment - isnt R13 there to provide part of the bias line - might be worth checking the bias components around the set, just incase someone has done something strange...

Cheers
Sean
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Old 21st Oct 2007, 8:37 am   #5
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Default Re: Civilian Receiver - MW Only.

Hold on a mo,

I don't think the tranny is kaput, Steve says that R13 smokes, indicating shorted HT winding, when he disconnects R13, R14 smokes,
Well there is no path back to ground for R14 as it is isolated by the newly fitted smothers,
also it isn't connected to the suspected shorted HT winding, it is connected to the rectifier heater winding!!

eh?

Help! anyone?

Lee
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Old 21st Oct 2007, 9:25 am   #6
Paul Stenning
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Default Re: Civilian Receiver - MW Only.

Having spent the last 10 minutes looking at the diagram I can see no logical cause of that problem if the set is wired to the diagram. I suspect that the previous bodger has made some errors. The fact that removing the rectifier valve doesn't help would make me look for shorts around the wiring of that socket. It looks like some boring wiring checking is needed.

It is possible that a short between windings or from a winding to chassis in the transformer could cause this, but I wouldn't suspect that until the wiring has been checked.

If they have used that fabric-type sleeving that goes sticky, remember that it also goes conductive. So a HT wire covered with that laying against the chassis could be sorting to the chassis, but only when significant voltage is applied.
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Old 21st Oct 2007, 6:10 pm   #7
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Default Re: Civilian Receiver - MW Only.

It is the transformer. AC Volts, so it has to be. The AGC line from the transformer is taken from the middle of the AC to the rectifier. Because it's AC, it was getting through the Smoothing Cap and to deck via R14 when R13 was removed.

Drat....

Thanks lads. Isn't it odd how a simple (but difficult to repair) fault can be hard to find sometimes. Especially with my illogical fault finding methods!!

Cheers,

Steve P.
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Old 21st Oct 2007, 6:28 pm   #8
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Default Re: Civilian Receiver - MW Only.

That's a shame. Can you get Mike Barker (or maybe Ed Dining) to rewire it, or can you use some of the windings plus a small filament transformer or whatever to make up the difference?
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Old 21st Oct 2007, 6:38 pm   #9
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Default Re: Civilian Receiver - MW Only.

Already on it Paul! When you can't do something one way, do it another....

Cheers,

Steve P.
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Old 21st Oct 2007, 7:22 pm   #10
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Default Re: Civilian Receiver - MW Only.

Hi Steve, the tranny looks a pretty bog standard type, and from memory there is a reasonable amount of space on the chassis so a replacement should not be a problem.
If you need originality it will have to be a re-wind.

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Old 23rd Oct 2007, 2:24 pm   #11
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Default Re: Civilian Receiver - MW Only.

Steve

Have you measured the resistance across the HT. Looks to me like it could just excess HT current...or do you mean that R13 smokes immediately (before the rectifier has a chance to heat?

TimR
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Old 23rd Oct 2007, 2:48 pm   #12
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Default Re: Civilian Receiver - MW Only.

Actually, R13 used to overheat with the valves out!

It was getting AC from the Transformer. The AC taps to the rectifier had a centre tap, which was also used to provide 0v for the bias circuit and also one of the three smoothing caps was connected to here and DC Output from the Rectifier and R14.

With R13 out of circuit, the AC voltage went to Deck via R14 and the Smoothing Cap. Being AC, it partially went through the capacitor.

With the lead to the transformer 0v disconnected, the AC was OK. Valves back in and it plays, just! So I'm going to try and modify it...

Cheers,

Steve P.
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