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Old 16th Jul 2007, 7:36 pm   #1
plaka78
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Default Do I put it back to original state?

Hello to All


The person that I bought this Derwent Radio from has had it from new and told me that they purchased it in about 1948. They had it repaired in the early 50s. The
CY31 rectifier has been replace with a selenium rectifier as can be seen in the photo. Now the question is should I refit and rewire a CY31 or leave it as it is. Why was this modification carried out as the valve must have been readily available. I would think that the dropper must have been change as the heater voltage is correct for the remaining valves. Any suggestions ?

Tony
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Old 16th Jul 2007, 8:06 pm   #2
paulsherwin
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Default Re: Do I put it back to original state?

I agree that it's a very strange modification. I certainly wouldn't leave the selenium rectifier there - quite apart from looking wrong, it will be on the way out now and may fail disastrously. In the short run I'd fit a 1N4007 or similar 800V diode and a 200 ohm surge limiting resistor out of sight under the chassis. You can then consider refitting a valve rectifier at your leisure.

Paul
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Old 16th Jul 2007, 8:35 pm   #3
Ed_Dinning
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Default Re: Do I put it back to original state?

Hi Tony, there should be no problem obtaining a CY31, neither should it be expensive. Try some of the links to valve suppliers.

Ed
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Old 16th Jul 2007, 8:36 pm   #4
Neil Purling
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Default Re: Do I put it back to original state?

It looks like the chassis was made in a AC only and AC/DC version as the square hole could accomodate a HT & heater transformer.
However.......... Is the ceramic dropper unit adjustable? If so calculate a capacitive dropper, add the capacitor underneath and re-set the dropper to a value to serve as a surge limiter when you have the valve rectifier re-instated.
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Old 16th Jul 2007, 9:49 pm   #5
plaka78
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Default Re: Do I put it back to original state?

I do have a Raymond radio with exact same chassis which is an ac set. But the transformer is far to big to fit in the hole. The rivets holding the dropper plate are original like the other rivets used on the chassis. Which make me think it originally started of as an AC/DC set.

Tony
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Old 21st Jul 2007, 6:43 pm   #6
RoundyMooney
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Default Re: Do I put it back to original state?

I second the advice re: the selenium rectifier. It shouldn't be too hard to source a CY31 or similiar to replace, if you don't want to go back in there again...
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Old 29th Jul 2007, 7:04 pm   #7
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Default Re: Do I put it back to original state?

Hi Folks


I have decided to leave the modification as it is. I have the radio going for about 6hrs. It’s been working fine and the selenium rectifier appears quite happy and no sign of getting hot or smelling. By the way the Radio started life as a AC/DC set as stated on the manufacturers plate on the chassis

Tony
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Old 29th Jul 2007, 9:29 pm   #8
Gordon
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Default Re: Do I put it back to original state?

Just one query because I'm doing a restoration on an Ecko M23 which has a similar metal rectifier. Are these type of rectifiers copper oxide or selenium? I ask because my metal rectifier definitly doesn't work and was rusty. I took it to pieces to cure the rust and spray it with zinc oxide paint. This was to make it look original when hiding a silicon rectifier under the chassis. Was I in danger, when cleaning the rectifier, from selenium dust?
Gordon.
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Old 29th Jul 2007, 10:07 pm   #9
plaka78
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Default Re: Do I put it back to original state?

Hi Gordon

I will make a note of the number on the rectifier and what I can find out about it.
This rectifier was fitted round about 1952. It is in very good condition and give a good HT. It can be seen that it is a full wave rectifier with the two halfs wired in parallel. Watch this space and I will post the number and any data that I can find.
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