Grundig 80U

[reset]

So which is the AM detector load resistor, still not quite with it...

Lawrence.

   

Lawrence,

The circuit is quite complex. I was a little concerned that Amie selected that radio to repair but we live and learn. The more complex the problem the more can be learned. The datasheet is in German which is of no help for me.

Anywho the reflex of signals is a bit confusing to me without tracing the switches. To see the circuit action I would have to print out the schematic and use colored pencils to trace the signal paths. I suggest you examine the ECC85 circuits. I think you will find your answer there.

73

[pwdrive]

It's a strange one....take look at the schematic in the link below, it's for an Ekco A244....Very similar so far as the AGC is concerned insofar that the spare diode is across the AGC line just like in the 80U, the circuit description implies that it's for the voltage delay but in other receiver schematics I've seen the anode of the that diode is connected to HT via a high value resistor (10 meg or higher):

http://www.radiotechniek.nl/index.php?p=manualbrowser&l=first&s=downloadmanual&merk=Ekco&typen=Ekco_A244.pdf

There's an example here (Ekco U195) showing the forward bias arrangement with a 10 meg to HT from the anode for the AGC delay voltage:

http://www.radiotechniek.nl/index.php?p=manualbrowser&l=first&s=downloadmanual&merk=Ekco&typen=Ekco_U195.pdf

The Bush AC31 which uses an EBF80 uses the same AGC delay voltage trick as the Ekco U195 above:

http://www.radiotechniek.nl/index.php?p=manualbrowser&l=first&s=downloadmanual&merk=Bush&typen=Bush_AC31.pdf

Maybe in the first example the AGC voltage is clamped to the contact potential of the diode, until the -ve DC component from the detected signal exceeds it and takes over, in other words it keeps the AGC line voltage (and hence grid bias) constant on weak signals.

All interesting stuff....

Lawrence.

[pwdrive]

So which is the AM detector load resistor, still not quite with it...

Lawrence.

Lawrence,

The circuit is quite complex. I was a little concerned that Amie selected that radio to repair but we live and learn. The more complex the problem the more can be learned. The datasheet is in German which is of no help for me.

Anywho the reflex of signals is a bit confusing to me without tracing the switches. To see the circuit action I would have to print out the schematic and use colored pencils to trace the signal paths. I suggest you examine the ECC85 circuits. I think you will find your answer there.

73

The reflex circuit is no problem for me, the IF and detector aren't reflexed only the FM RF amp is reflexed....take a look at this for a description:

https://www.radiomuseum.org/forumdata/us...nglish.pdf

The switchery is very easy to understand and follow, in the 80U schematic the switch is in the AM position.

Lawrence.

[Amie]

A thought crossed my mind.

(Mum would say "short and lonely journey").

The second diode with the 2M resistor on the anode is for DC restoration, its audio that's clamped then integrated for use as AGC, the side effect is that the 2M resistor is allowing the detector to have enough of a load to work, even if that isn't the intention.

Actually I didn't come up with any of that it was dads old pal Don, if I pester him sometimes he coughs up a pearl. He's clever but a bit of a GOM!

(If you're reading this Don, I love you man.   )

Really, I still have no clue!

Amie.

[Terry]

Curiouser and curiouser ...

So far we've traced the path back to the AGC diode anode in the EBF80 - but there's more.

Back to the secondary of the 1st IFT, winding III, which we'll come back to, then on via R1 and the switch (which is shown in the MW position) to the grid of the ECC85. Also via R22 to the ferrite rod aerial but the coils are isolated from ground by both C15 in one leg and the tuning cap C42 in the other, so still no connection to ground.
So, back to winding III on the first IFT, the other end of which goes through the switch to the grid of the EBF80 - and that's it, journey's end! The entire path from the detector back to the mixer has no connection to ground other than by conduction of the AGC diode.

As Amie has conclusively proved, the resistive path through the FM discriminator is a red herring and there is no other resistive path to ground but isolating the AGC diode kills the set stone dead, so it looks as if my reasoning was right in the first place!

The circuit is quite complex. I was a little concerned that Amie selected that radio to repair but we live and learn.

I must admit to having the same doubts but it was clear in that opening post that she is no novice and followed a logical sequence of initial checks. By the end of that post I had no doubts at all.

The datasheet is in German which is of no help for me.

I suggest you examine the ECC85 circuits. I think you will find your answer there.

The only German you need to know is Schalter in Stellung MW gezelschner which means Switch shown in MW position. (I guessed, then checked with Google translate!)

You seem to be suggesting that you have spotted something which we have missed. We'd be delighted to know what it is!

[pwdrive]

Another Grundig three bottle job that's reflexed on FM...The 85 Musikgerat, the DC path of the AM detectors load resistor is more clearly defined in this circuit (R2, 270k).....:

https://www.doctsf.com/documents/affiche..._doc=46065&num_fic=1

Lawrence.

[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.

[pwdrive]

The  the -ve from the ratio detector to g3 is in effect FM AGC, the more -ve g3 is with respect to cathode the less the valve will conduct, it helps to prevent overloading of the ratio detector so far as I know.

I still think that the 300k is the main diode load for the AM detector but hey maybe that's just me.

Yet another example....Musikgerat 87....yep another three bottle reflexed FM job from Grundig but guess what...it has an extra switch to select between ratio detector (FM)
and AM...diode load is now totally obvious...yep, 270k, no more evidence needed with that one (click on thumbnail schematic):

https://www.doctsf.com/87-musikgerat/d36203/ref=20299

Lawrence

[Amie]

I still think that the 300k is the main diode load for the AM detector but hey maybe that's just me.

Lawrence

I'm sure it is, but not being there doesn't stop the set working (if not correctly) on MW.

Amie.  

[pwdrive]

Another one....ECL86 output job this time...Musikgerat 88, that's the last one I can find for now schematic wise.

https://elektrotanya.com/grundig_88_musi...ad.html#dl

Might be something about some of the models in the Grundig Technische Information series if there's a complete archive of such publications (have briefly looked through the americanradiohistory collection of those but no luck, however that archive appears to be incomplete:

https://www.americanradiohistory.com/Gru...hnical.htm

Lawrence.