Grundig 80U

[Amie]

Hi.

We are told at school in combined science that we should apply the scientific principle of investigation.

So, before I put the set back in its case I tried to find out why the FM detector is permanently in circuit.

Using the oscilloscope to test, in FM, I found the EBF80 g3 went negative on strong signals. That must be deliberate as its connected to the FM detector.

In MW, there's little change to g3 voltage.

Injection of modulated MW IF signal to g1 produced a nice clean sine wave at the volume control, removing the 300k resistor to the FM demodulator made the wave form sawtooth... But didn't seem to change the tone in the speaker much.

I've looked at text book examples of valve diode detectors, and they often show the volume control as a load. Others show a very different det/AGC setup.

I think the 50K/300K resistors that join the top of C1 to the volume are just there to normalize the level/impedance from each detector, it doesn't need switching, so isn't.

The fact that the FM demodulator provided load to the MW detector is a component economy.

I conclude that I don't know what the connection between the FM demodulator and valve g3 is doing, but whatever it is, is a design feature.

The AGC clamp/delay diode section of the EBF80 anode is holding the R6 just below ground on negative transition of the signal and providing some of the detector load we are looking for

Finaly, the guy who designed this set deserved his days pay

How far off target am I ?

Thanks,
Amie.