grundig: Grundig radion model 1004W_FM detection?
It should be very interesting to know how does FM-detection work in this model? Is it some kinf of "slope detector"
The Grundig 1004W is a model from the beginning of the FM era. In these times the radio factories still experimented with different concepts for combined AM/FM receivers.
- The mixer is a heptode ECH42 for AM and FM.
- The first IF stage uses an EF41. However this stage is used for FM IF only, not for AM IF. Additionally it works as reflex amplifier for the AF signal.
- The second IF stage uses an EAF42, which has one diode only for detection of AM and FM signals. For FM detection indeed slope demodulation [in German: Flanken-Demodulation] is used.
Whereas slope demodulation for TV applications was familiar, and posed no problem because the FM IF is fixed due to the frequency difference between picture carrier and sound carrier.
For FM Radio reception there is a problem in so far as there are two IF filter slopes which are suited for FM demodulation. This gives an ambiguity when the station is tuned in.
- Behind the volume control the demodulated AF signal goes to the EF41 (first IF) and is amplified there. Behind the FM IF circuit the amplified AF signal goes to the grid of the EL41.
- The EF41 stage has a reduced AGC. For FM only no AGC would be better, because the EF41 works as an amplitude limiter. However for the AF signal this AGC is helpful in order to equalize the loudnes of the received stations, especially for AM stations.
The best schematic for analysing the receiver ist the 3rd one on the model page.
Thank you very much for your comprehensive and clear explanation of the 1004 operational principles. This is quite expectional construction and obviously also the cost economy has been in mind, when this model was made.
In Finland FM broadcasts started in 1953 as well as manufacturing of FM local receivers but this kind of concept I have never seen here.
Slope demodulation in TV receivers I do know due to my work experience.