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 Post subject: Radio Craftsmen RC 10 tuner
PostPosted: Jan Thu 10, 2019 1:55 am 
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Joined: Dec Sun 29, 2013 10:11 pm
Posts: 115
Location: Richmond, Va.
Greetings all - I’m working on a Radio Craftsmen Model RC-10 tuner, and its associated amplifier, Model RC-2. This equipment was bought by my father as a spare tuner in 1976 from a neighborhood yard sale, from the original owner. It has been kept in normal room temperature conditions ever since my father bought it, although he never used it.

Links to schematic: https://www.radiomuseum.org/r/radio10_a ... er_10.html
(Sams schematic, easier to follow)
http://www.nostalgiaair.org/pagesbymode ... 016045.pdf
(Radio Craftsmen schematic, harder to follow)

In restoring it for my use, I found R20 ( 1K, V3 12AT7 mixer plate resistor) looked a little burned, so I replaced it. I replaced all paper and electrolytic caps, and a couple of other resistors I found out of tolerance. It then played well on FM, or so I thought, but not at all on AM. AM gave only a very noticeable 60 hz hum, and extremely weak audio. I checked numerous things that could lead to hum (lead placement, tube shorts, etc.), but to no avail. Using the resistance charts led me to find that R8, ( 1K , V2 6CB6 2nd FM, AM, RF amplifier plate and screen resistor) was missing from the chassis. The connections looked like it had never been installed at the factory. Replacing that brought back the AM, and the FM was better than before. (I used the resistance charts because the voltages given were taken with a lower impedance meter than my VTVM.)

It played well for several hours, over several days. The next day I used it for a couple of hours, and turned it off. Later I turned it on (FM), and all I got was the FM background noise, although AM was still OK. After about 20 to 30 seconds, smoke began coming from under the chassis, and I turned it off immediately. I found that R20, which I had replaced earlier because it looked a little burned, was now burned. It still measured 1K, so I left it in (temporarily). The tuner continues to play well, after being turned on and off several times. Voltages on V3 measure OK, and nothing is now overheating. I have carefully checked for things touching that should not touch, and for stray wire clips and solder blobs, and found none. Resistances of coils L11 and L12 are normal, not open or shorted to chassis. NOTE: the resistance from L 12 primary center tap to chassis measures about 15 to 20 megohms (it varies); I expected it to be infinite. It has no loud static crashes typical with SMD.

It appears to me that L 12 may have an intermittent short to chassis. My questions for those more expert than I: 1. Has anyone found a missing component, such as R8 above, in this equipment? 2. Does anyone have any other suggestions for the cause of the burned resistor?

Many thanks for any advice from the experts.

BTW - I’m using it with an E-V Aristocrat enclosure, which I see mentioned here recently, with a 12TRXB speaker, and it sounds great!

CharlesD


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 Post subject: Re: Radio Craftsmen RC 10 tuner
PostPosted: Jan Thu 10, 2019 7:06 pm 
Member

Joined: May Sat 06, 2006 4:03 am
Posts: 3268
Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
This link will get SAMS 110-12 for those interested.

Bll J

https://app.box.com/s/vfbda5et69elp5yny4cxwuj3fdsuq931


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 Post subject: Re: Radio Craftsmen RC 10 tuner
PostPosted: Jan Thu 10, 2019 11:39 pm 
Member

Joined: Dec Sun 29, 2013 10:11 pm
Posts: 115
Location: Richmond, Va.
Bill J -- Thank you for the better link.

CharlesD


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