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Old 13th Sep 2017, 10:26 am   #1
kevinaston1
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Default Philco A-547B

I have been handed a Philco A547B for repair. I cannot do much yet as C27-C28 is open circuit, awaiting a replacement.

Do any members have experience of these sets?


Kevin
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Old 13th Sep 2017, 12:30 pm   #2
Tim
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Default Re: Philco A-547B

I did one a few years ago. Hope this helps.

Those little brown resistors are liable to high in value/open circuit

https://www.vintage-radio.com/recent...lco-a547b.html
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Old 13th Sep 2017, 1:03 pm   #3
ukcol
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Default Re: Philco A-547B

This is a fairly conventional post war 4 valve + rectifier superhet with the exception of the valve biasing arrangements. It does not use cathode resistors for biasing as was common practice by 1947 but instead uses an earlier technique.

Two resistors R20 and R21 (Trader Sheet 890) in series with the HT return line provide the negative bias for the values. C29 (25uF) provides the smoothing here and is likely to need replacement.

I always replace all the paper capacitors in valve sets as they are nearly always very leaking and replacements are relatively cheap. I would definitely replace C24 the coupling capacitor between the amplifier triode anode and the control grid of the output valve even if checks suggest it is OK. This capacitor (often referred to here as "that capacitor") can cause serious damage to the output valve and transformer.

Tim's excelent article has everything else covered I think.
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Old 20th Sep 2017, 12:33 pm   #4
kevinaston1
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Default Re: Philco A-547B

Many thanks for the help.

New problem though. Whilst removing the knobs, one of the "D" shaft springs fractured; more rust than metal, see the attached photograph.

Does anyone have any ideas as to sourcing replacements?

Regards.

Kevin
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Old 20th Sep 2017, 1:14 pm   #5
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Default Re: Philco A-547B

In cases where I have been unable to find an exact clip, it does work if you make one out of about 1.3 to 1.5mm brass strip and form it to the exact size. Gluing it in with epoxy resin near its free ends also helps. It won't give quite as tight a fit as the spring steel, but in most cases it will work. The best hope for an original is that someone has one in their box of knobs.
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