Soviet East German Radio

[Nowhere-Man]

The ECH81 heptode will be the first FM IF amplifier. It's supplied at 10.7 megacycles by L7 L8. L19 and L20 listed as the 10 Meg bandfilters then pass to the amplification stage. It helps me to maybe try and split the circuit. You know, one odd thing about this radio was I had to repair the ganged moving vane tuner as it had been disabled. Makes me wonder if someone purposefully changed something. I mean you get people trying to turn them into amps.

[pwdrive]

I referred back to Lawrence's summary of the Saalburg circuit. The main areas of focus for me at the moment are the two IF's and the EBF89 plus the EABC80. I am literally racking my brains to trace the diodes and, of course, there is the discriminator circuit. I think Lawrence indicated the centre tap between L23 and L24. Anyway maybe it will be understood why I take time out to address textbook theory. Discriminator circuits won't be understood without some grappling with ARRL tutorials.
I haven't fully grasped Lawrence's explanation yet but I will get there. Tomorrow I will print it.
Notice one diode on EBF89 is grounded out and the grid used instead. Likewise the diode on EABC80.
The Foster Seeley discriminator circuit may help me if I locate it.
When I step back and say all the circuit makes sense, then I will go to work more fully.

It might help you understand better if you print out a copy of the schematic and highlight the AM and FM signal paths with two different coloured marker pens.

The EBF89 contains two diodes but only one of them is used, the one that is used is used for AM detection and it also provides the AGC voltage, the EABC80 contains three diodes but only two of them are used, those two that are used are used for FM detection, the FM detector in the Saalburg is of the ratio detector type, it's not a Foster Seeley type, the Foster Seeley type needs a limiter stage before it.

It might also help by analyzing schematics in block/stage form by recognizing chunks of circuits and understanding what their function is, ie: what goes in and what comes out.

Lawrence.

[Nowhere-Man]

There is this here:
http://www.radio-electronics.com/info/rf...inator.php
The subject comes up a lot on various forums.

[pwdrive]

There is this here:
http://www.radio-electronics.com/info/rf...inator.php
The subject comes up a lot on various forums.

That's a classic layout for the ratio detector, the two diodes shown in opposition (facing the opposite way) the Foster Seeley has the two diodes facing the same way as it were, also the Foster Seeley needs a limiter stage before it, that's in the form of an extra IF amplifier stage, the valve in that stage limits the amplitude of the signal fed into the discriminator by clipping the signal and thus removing any unwanted AM component from it such as interference.

Lawrence.

[Nowhere-Man]

Still been working trying to locate electronics companies in the Russian Federation. Not all of them export. The stuff they sell is often a combination of Chinese manufactured or domestic produced stock. I'm even considering trying to get my foot in the door as a sales agent, although probably that is unlikely.

[Nowhere-Man]

There is this here:
http://www.radio-electronics.com/info/rf...inator.php
The subject comes up a lot on various forums.

That's a classic layout for the ratio detector, the two diodes shown in opposition (facing the opposite way) the Foster Seeley has the two diodes facing the same way as it were, also the Foster Seeley needs a limiter stage before it, that's in the form of an extra IF amplifier stage, the valve in that stage limits the amplitude of the signal fed into the discriminator by clipping the signal and thus removing any unwanted AM component from it such as interference.

Lawrence.

Do you think it will help me? Last night I felt like my brain was going through a grinder. It does get clearer as you ponder it but, as I said, there have been times I needed to just go back and look at the very basic theory. I even had to go back to AM a little because often in the FM tutorials they compare bandwidth to AM. One thing I did see with fresh insight was how at a resonant frequency (say in am), the current will drop off at resonance according to the value of the series resistance. That is, either broad or sharp. ARRL does have some information on FM and PM but it comes pretty late in the book. Sometimes ARRL doesn't offer the best explanations but other times it  can be useful.

[pwdrive]

Everything helps....

Lawrence.

[Nowhere-Man]

The explanation in ARRL is better than that diagram above. ARRL shows a primary at the IF frequency. The secondary is centre tapped and voltage introduced from primary to the tap by a 100 picofarad capacitor. It then says the phases at the resonant carrier frequency lag and lead by the same phase angle. So, I assume that's the ratio. The out of phase voltages each side of the tap pass to a double diode. They're rectified as two DC voltages of opposite polarity. Then it says changes of frequency in the circuit will cause the amplitudes of the two DC voltages to increase and decrease relatively. By that I deduce deviation in the carrier through the original modulating signal will be converted to amplitudes. That is about my best effort towards understanding. If correct it's better than two weeks ago when I was clueless.

[Nowhere-Man]

Page 329 of ARRL shows that diagram. It helps valves are shown and not semiconductors.

[Nowhere-Man]

I think it might be starting to make sense, Lawrence. Looking at L23 and L24 they are at the IF of 10.7 megacycles. L24 is the tapped secondary. On my ARRL schematic the out of phase voltages are rectified by anodes. Here (Saalburg)it seems a cathode is used? As in the other diagram (where semiconductor diodes are shown). In other words I get the impression the rectifier circuits are a little different in various cases. The main thing is I think it's starting to make some sense. At least I hope so as I spent hours racking my brains.