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IF Transistor explanation

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TheMac
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TheMac posted on Tue, Jan 12 2021 6:42 PM

Hello

I am reparing an Beomaster 700 with not so good fm reception. From what I can read on former posts on the beomaster 700 and 900, replacing the two IF transistors should do the the trick. The only problem is that I cant figure out what that is. I search on different electronic component suppliers (in Denmark), but cant find anything called this.

Do they go under another name? 

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Dillen
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Answered (Verified) Dillen replied on Wed, Jan 13 2021 7:40 AM
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RF = Radio Frequency = "what comes in from the antenna"
IF = Intermediate Frequency = "what lies between" (mixing down)
AF = Audio Frequency = "what you can hear"

The IF transistors are the transistors sitting between the IF metal cans.
B&O used AF116 or AF117.
Both types are known to short circuit inside from the metal housing to one of the pins, often the collector (the largest structure inside) due
to developing "tin whiskers". You can google that, it's quite amazing, 
Occasionally, an AF116/117 will burst to life if tapped with a pencil or similar (breaking the shorting whisker), but it will only be a short time before another whisker reaches in and shorts - still good for diagnosing, though if shorted "heavily" it may not always burst into life.
Cutting the screening pin will also some times bring back functionality, but only in the cases where no other shorts have developed.

A typical symptom of bad IF transistor(s) is lack of sensitivity. In severe cases complete silence.
It may affect AM or FM or both.

Martin 

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hostler
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Hey - can you provide more of the description / post that talks about these components.

I've not personally heard of IF transistors, but perhaps with some additional material it would help you find the answer you need.

Either cut & paste, or send a link.

Best

 

Tom

trackbeo
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Maybe IF="Intermediate Frequency"?  An example of "additional material" (is it even the same tuning method?) showing "IF transistors" as part of an R/C filter circuit would be at:

http://philsvalveradiosite.co.uk/philsvalveradiosite/pulsecountingfmreceiver_1.htm

sonavor
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I would think the writer was referring to the transistors used in the tuner's IF section. The service manual should show that section and what type of transistors that are there.

-sonavor

Dillen
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Answered (Verified) Dillen replied on Wed, Jan 13 2021 7:40 AM
Verified by TheMac

RF = Radio Frequency = "what comes in from the antenna"
IF = Intermediate Frequency = "what lies between" (mixing down)
AF = Audio Frequency = "what you can hear"

The IF transistors are the transistors sitting between the IF metal cans.
B&O used AF116 or AF117.
Both types are known to short circuit inside from the metal housing to one of the pins, often the collector (the largest structure inside) due
to developing "tin whiskers". You can google that, it's quite amazing, 
Occasionally, an AF116/117 will burst to life if tapped with a pencil or similar (breaking the shorting whisker), but it will only be a short time before another whisker reaches in and shorts - still good for diagnosing, though if shorted "heavily" it may not always burst into life.
Cutting the screening pin will also some times bring back functionality, but only in the cases where no other shorts have developed.

A typical symptom of bad IF transistor(s) is lack of sensitivity. In severe cases complete silence.
It may affect AM or FM or both.

Martin 

hostler
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Although not my query, but absolutely fascinating Dillen - thanks for the share.

It looks like AF116's and AF117's are available on the secondhand market for a direct replacement ... but there's also AF126's and AF127's that supposedly don't exhibit the Tin Whisker phenomena and are a comparable replacement.

Good luck TheMac!

 

 

Dillen
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Dillen replied on Wed, Jan 13 2021 9:27 AM

I would neither buy nor fit AF116 or AF117 again.
They will develop tin whiskers regardless of being used or not.

AF126 and AF127 will often work fine as replacements though I often use AF109r for 1.st IF and AF179 for 2nd IF.

Martin

TheMac
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TheMac replied on Thu, Jan 14 2021 6:16 PM

Okay.

Thank you very much for all the answers.

This has given me a better understanding of the terminology.

 

 

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