Braun SK6/RC4 Am circuitry

 

Recently I was looking at the circuit diagram of Braun model SK6 (RC4) radio. They have used a bit unusual circuitry on AM oscillator- mixer solution. Instead of “normal” ECH81 (or similar) it has replaced with EF89, acting as mixer + if amplifier and the 1st section of FM tube ECC85 operates as AM oscillator, when AM band (MW or LW) is selected.

AM oscillator signal is injected to the g3 and antenna signal to g1 of the EF89. First AM IF (455kHz) filter’s “cold end” is connected to g2 of the same EF89. I don’t have in my mind any other case where this kind of circuitry should be used on normal BC –receiver.

Therefore I should curious to learn what could be the reason for the solution like this. Does it offer some advantages either on FM (10,7MHz) or AM ?

Regards Ake

 

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Hello Mr. Nyholm

One from any example:

Blaupunkt WIEN ID: 828 use the same Circuit for AM-Mixing

Mixer = EF85,  Osc. FM- EC92

Knoll

 

Edit:

 Grundig 955 ID: 2077 and 1055 ID: 2078
and Graetz ID: 1758 also have a mixer with EF89
Text in German can be found here.
And additionell an Text in German, from GRUNDIG

These circuits inject the oscillator in the cathode of the pentode.

And   there  you can find, why the cold end of IF-Coil is connectet to screnn of the Mixer.

It is the Neutralisation for FM  and AM on 460kc on lower end on Mediumwave near

by the IF- Frequency

Next:

You ask why or advantage of this circuit

It is not the technology but the license requirements for principle Multiplicative mixing.

knoll

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Dear Mr. Knoll

Thank you very much of your comprehensive explanation to my question. I appreciate your support bring out alternative examples and related technical and commercial background to this issues. I do have some knowledge of licensing so also this reason is understandable.

Well done Mr. Knoll

Regard Ake

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