22-08-2019, 03:45 PM
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!
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 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!
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!
(22-08-2019, 02:33 PM)reset Wrote: 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.
(22-08-2019, 02:33 PM)reset Wrote: 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!
