Grundig 80U

[Amie]

adjust C36 for maximum

Tried that. It's a little better, but not much, I think the aerial rod has been repaired with glue at some time, that may be something to do with it?

Thanks,
Amie.

Part of the FM ratio detector circuit forms part of the AM detectors diode load circuit.

 On MW the set would work OK with the diodes missing, what forms the load? G3 of the EBF80 goes to ground through the 50K (R10) on the other side of the diodes from the winding on the IFT. AM audio, to volume control through R8, C1. Only DC load R6, 2M through other valve diode section? Any insights

Every day a school day

Thanks again,
Amie.

[pwdrive]

The only load path for the AM detector so far as I can make out is via R9, R10 etc, the question is, is that just to save another set of switch contacts or is there another reason for that arrangement....who knows?

Lawrence.

[Terry]

Not sure I can agree with Lawrence here. Ignore the FM bit and think in terms of a simple crystal set.

Normally, one side of the tuned circuit and one headphone lead go to earth and the other side of the tuned circuit goes through the diode to the other side of the headphones. It must still work if you transpose the diode and the tuned circuit and that is what is happening here, it is just that it uses a thermionic diode in place of a germanium diode (or crystal and cat's whisker).

There will be no AM IF signals when the set is switched to FM, so the diode will not be conducting so, therefore, the AM circuitry will not load the FM detector and the 300k resistor is sufficiently high enough to ensure there is no undue loading from the ratio detector when the set is on AM.

[pwdrive]

Not sure I can agree with Lawrence here. 

In what way?

Lawrence.

[Terry]

As Amie has already proved: when the diodes were missing there was no possible path to ground through the FM detector circuitry but the set still worked perfectly on AM.

[pwdrive]

I still don't understand how it can work properly with no load?

Can you describe as per the schematic so I can hopefully understand?

Lawrence.

[Terry]

One end of the tuned circuit formed by inductor I in the IF transformer in parallel with C12 is connected to ground via one of the diodes in the EBF80. C16 is the RF filter.

The signal then passes via R8 and C1 to the combination of volume control R23 and feedback injection resistor R11 which forms the load.

Does that help? If not, perhaps someone else would like to try.

[pwdrive]

I thought there had to be a DC path between anode and cathode, maybe I am mistaken, in other receivers I've worked on there's always been a DC path via a resistor or two and it's always been described as the detectors load resistor/resistance.

Lawrence.

[Terry]

Perhaps Grundig had a special batch of leaky capacitors made to use in the C1 position?

[pwdrive]

R15:

https://www.doctsf.com/documents/schemat.../VHF71.PDF

Lawrence.