Soviet East German Radio

[pwdrive]

Maybe he means 5 volt heater rectifiers?

Lawrence.

[pwdrive]

For example notice the IF and mixer stages in American HAM sets. As I said before cathode signal injection is often found. The HAM schematics I find easier. I will comment on the detector stages later.

Although cathode injection was used, the most popular set up across the pond was the pentagrid mixer, sometime self oscillating, sometimes fed from a separate oscillator, on this side of the pond the triode hexode and the triode heptode became the norm.

In general Ham schematics as printed were quit well laid out and easy to follow but then you get schematics like the AR88 and the BC348 which if you're not used to them can be a bit of a headache:

http://bama.edebris.com/manuals/rca/ar88/

http://bama.edebris.com/manuals/military/bc348/

Lawrence.

[Nowhere-Man]

Maybe he means 5 volt heater rectifiers?

Lawrence.

I don't always know what I mean but you understood me correctly. If the 6.3 of the EZ80 (being in parallel) were on the regular 5 volt winding, there would be a problem. Or certainly not ideal. As it is the EZ80 is in parallel with all the other 6.3 heaters.
Pretty much all of my HAM schematics have two heater windings as normal with only the rectifier heaters on the 5 volts.

[ppppenguin]

It's true that many older rectifiers had 5V heaters. Such as the GZ34. That isn't the most important reason why the rectifier commonly had a separate heater winding. Even if all the valves in the set had the same heater voltage a GZ34 or other rectifiers of that period would have needed a sparate heater winding.

Hint: A lot of earlier rectifiers were directly heated. The GZ34 has the cathode internally joined to one side of the heater which amounts to much the same thing.

GZ34 information: http://www.r-type.org/exhib/aaa0013.htm

[Nowhere-Man]

Here's tonights new discovery: verhaltnisgleichrichter. This appears to be L23 - 25 in the EBF89 circuit. This cluster of windings between the EBF EABC80 is the hardest bit yet. I may sometimes use another schematic just to compare to try and get a basic summary.
By this stage I know a feed from C60 to R12 will drop 135 volts across R12 assuming the ECH81 draws about 4 milliamps. That would put 95 or so volts across the oscillator. Safest way to do that really is restoring 4, 5 and 6.

[Nowhere-Man]

The system runs strangely. Tonight, having gotten into the EABC80 and so-called verhaltnisgleichrichter, I decided now is the time to learn the basics of FM. So, the first cram has been phase modulation and frequency modulation. And explanations of the modulation index. So far, so good. Tonight I learned how amplitude in a modulating signal can deviate a carrier frequency. For the next few nights I'll be doing this and then finish the circuit diagram. Without this theory I won't really grasp the scope of the schematic.

[Crackle]

Your going a bit deep to repair a radio, but if it helps you to understand!

Mike

[Nowhere-Man]

Your going a bit deep to repair a radio, but if it helps you to understand!

Mike

It's the way I'm wired, I guess, but I admit it's a bit strange. I seem to be almost incapable of just zooming in on one fault but instead wind up needing a deeper overall understanding. Really the truth is I know almost zilch about FM or how it works. It did turn out so far to be simpler than amplitude modulation. The main thing so far for me to understand is the modulating signal will change the carrier frequency. Tonight I will carry on with the explanations. Hopefully by the end of it all I'll understand how my Saalburg has two IF's (455 KHz and 10 MHz).

[pwdrive]

You're nearly there with the two IF's, 455kHz is the AM IF and 10.7MHz is the FM IF, you will already see that in the IF amplifier, the primary tuned circuit of the AM transformer and the primary tuned circuit of the FM transformer are connected in series between the HT feed and the anode of the IF amplifier, in comparative terms the AM tuned circuit will be high Q and it's inductance value will be high, the FM tuned circuit will be a lower Q, and it's inductance value will be low, the higher the Q the narrower the bandwidth, the lower the Q the wider the bandwidth, this suits the type of transmissions being discussed just fine because the bandwidth of the AM transmission is only a few kHz while that of the FM transmission is much higher and is several 10's of kHz.

You should also be able to see how those two tuned primaries can work in series at the different IF frequencies, you've already done the understanding of the reactance bit for L and C so let's take the AM 455kHz tuned circuit first, at that frequency it will be resonant and it's impedance will be high, the tuning capacitor of the FM transformer will have a high reactance at that frequency because the capacitor has a relatively low value, but the value of the inductor is low and therefore it's reactance at 455kHz will be low so in effect the anode end of the 455kHz transformer is now connected to the IF amplifiers anode via a low reactance path, eg: the anode load at that frequency in effect just consists of the high impedance primary tuned circuit of the 455kHz transformer, this is what we want because the maximum voltage will be developed across it at resonance.

Now the FM 10.MHz tuned circuit, one end is connected directly to the anode of the IF amplifier and the other end to one end of the AM 455kHz transformer, all still in series, this time were dealing with an IF of 10.7MHz, the FM transformer is resonant and therefore has a high impedance at 10.7MHz but the HT end of the transformer needs a low impedance path to chassis in order to decouple it, the decoupling capacitors are C40 and C41 which are connected in series, the path to them has to be via the AM IF transformers tuned circuit, that tuned circuit has a high value of inductance and a high value of capacitance relative to the values used in the FM transformers tuned circuit, this means that the AM IF transformers tuned circuit inductor will have a high reactance value at the FM IF frequency but the capacitors reactance will be low, around 50 ohms in fact, so now in effect we have a high impedance anode load at 10.7MHz which will develop the maximum voltage across it at that frequency, this again is what we want.

Hope I've got all that right and that is of some help.

Lawrence.

[Nowhere-Man]

Yeah, very helpful. I'll use that for reference. Thanks.