grundig: Automatik function

Hello dear colleagues,

This radio has a push button under the volumecontrol, named "Automatik". On the schematic it is named "Abstimmautomatik" using a switch to turn it on/off in combination with a luminosity control on the magic eye.

On the schematic one can see the source of the function lies at the AF output (center tab) on the secondary winding of the FM ratiodetector passing by the "Automatik" switch but the line terminates in the AM oscillator section, not the FM oscillator. I'm familiar with the principle in a philips radio where that signal controls a varicap diode in the FM-tuner but in this radio there is an OA70 diode, not a varicap. 

Can someone explain the principle of frequency correction used in this radio, if it is an AFC at all?

Thanks in advance for the answer,

Dirk Bladt

Olens Radiomuseum VZW

 

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Hello Dirk, colleagues,

the switched automatic function is indeed an AFC for the F.M. range.

In general, the AFC function is based on the effect that the capacitance of the p-n junction of a semiconductor diode decreases with increasing of the reverse voltage and vice versa. Special diodes have been developed for this purpose, which show a relatively large change in capacitance at small voltage changes. However, these so-called capacitance variation diodes or varicap (first types were e.g. BA101 and BA102) were not yet available in 1959 when the Grundig 6098 was designed. But the used OA70 Germanium diode has only a small capacitance variation by altering it's reverse voltage.

In order to achieve the desired frequency correction with an OA70, a control voltage amplifier is required. For this purpose the triode of the ECH81 is used, which otherwise has no function at F.M.

The shift voltage of the symmetrical ratio detector is fed via R21, 2 inductors and R16/R15 to the control grid of the triode. At the triode anode the amplified control voltage appears with reversed phase at the working resistors R19, R20. This voltage is then fed via R7 to the cathode of the OA70. The anode is biased in reverse direction at the fixed level of the G2/G4 voltage of the heptode.

A negative voltage (related to ground) from the ratio detector will result in a more positive voltage at the Anode of the ECH81-triode, a positive voltage will decrease the anode voltage.

The successor of the Grundig 6098, the receiver 6199, uses the same circuitry for AFC.

Bernhard Nagel

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Thanks Bernard for this clarification.

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