Its using the stock foil antenna inside the radio.

Would this be a good circuit to use for the source follower? I assume the "in" would come from the detector, and "out" would go to the MPX decoder input? What values would have to be changed if any?

Try connecting a pair of rabbit ears (indoor TV antenna) if you have one, or just two lengths of wire, maybe 4 - 6 feet long each, and I guarantee a large improvement.

If you are on cable TV, you can also connect it to the cable, using a Balun.

If you have an outside TV antenna, it would be by far your best skyhook for FM.

The gain derived from any of these methods will enable many more stereo broadcasts to come through, over what you are now using.

Your set, and it's antenna, was designed for monaural FM reception, and stereo requires a stronger signal; it's just the nature of the beast. So what was adequate for what your set was made for, is not for what you are now trying to accomplish.

You may not need to change any of the circuitry once you've improved the antenna, given that it does now get stereo.

I don't really think it is reception issue. It seems that connecting the decoder affects the FM detector's ability to perform properly. It is also possible that the previous stage driving the ratio detector transformer goes into oscillation when the decoder is connected, thus keeping the tuning eye closed. It's hard to tell because only a small portion of the circuit is readable here. Need to see more. But fifties advice is certainly valid because good stereo reception needs about 22 db greater signal over mono.

Dave

If the antenna was the issue, why don't the other stations come in mono? The IC has automatic mono/stereo switching. There are just a few strong stations coming in. Everything else is just static.




Okay, I tried that last night. There was no difference.

So far, I'm sold on the "source follower" theory, but in all honesty, I don't know how to make it. I will split the FM circuit up here into three parts. Hopefully it will be more readable.

The two transistor buffer circuit that you posted above would work, but is more complicated than what it necessary. You could get away with a single JFET something like this:

Thanks Bob. I will try to scrounge up some parts and make some modifications. I will report back when I test it again, probably a few days. I would also like to eventually run the JFET and stereo decoder off a voltage doubler from the 6V filaments, if possible. But before I do that, do you think they would draw too much current from the power transformer, possibly burning it out?

That sort of tells me that the schematic might be correct and that the optional Telefunken-made MPX adapter must have a complementary pre-emphasis circuit at it's input. We'll never really know without seeing a schematic of it.

Dave

Does the tuner have a tube stage after the detector? If so the simplest mod will be to remove the de-emphasis circuit components and take the audio from the audio out jack if mono operation is no longer desired.

The detector goes to an eye tube and then through the tape switch to the audio stage. There is no audio out jack, just a phono input and tape input jack. The Telefunken 5183W is a receiver, not a tuner. But seen as how you mention it, what is the purpose of the de-emphasis circuit components in the first place?

Oh ok I missed that part about it being a receiver.

Have you tried using an external antenna yet?

I doubt that you will ever get successful results without it.

To remove the pre-emphasis put in the transmitted FM or else it'll sound like a coaxial tin can and tweeter.

The mono audio from an FM station is transmitted with an industry standard treble boost known as "pre-emphasis". Every receiver is designed with a complementary treble cut known as "de-emphasis". The purpose of this system is for an improved signal-to-noise ratio at the higher audio frequencies. But the stereo information is transmitted at supersonic frequencies above 15 kHz which includes a 19 kHz pilot tone to syncronize the stereo demodulator with the main channel and the derived L-R audio which is transmitted as upper and lower AM sidebands of a surpressed 38 kHz carrier. What feeds the FM transmitter is known as a composite baseband signal which may also include other subcarriers that the station chooses to send. The de-emphasis circuit, in doing it's job to correct the tonal balance of the main (mono) signal, also removes the pilot tone and stereo information in the process. But, in order to perform it's job of re-constructing the original Left and Right channels in, for example, a stereo recording, an FM multiplex adapter needs to see the entire composite baseband signal as transmitted by the station. So it has to be connected to the output of the FM detector before any de-emphasis takes place in the receiver.

Dave

Thanks, Dave. There sure is a lot going on in there. I wonder if the de-emphasis circuit is added in again somewhere inside the LM1800 chip, after the stereo signals are separated. Wouldn't that make sense? Otherwise, wouldn't you get tinny stereo output? I would love to see the schematic of the original Telefunken decoder to see how they did it.

That's right....after the left and right channels are derived, de-emphasis is applied to each channel. That's what the caps and resistors are for connected to pins 3 and 6 of the IC. Wondering if Telefunken's circuit is tube or solid state. I'll bet Dennis Daly would know. He might even have a schematic.

Dave

I found this, a schematic for the Telefunken Opus 2430 stereo decoder:

The input has a trap to eliminate SCA subcarriers (not associated with the stereo information) but there is no pre-emphasis compensation. It does, however, go into a high impedance (grid) input.

Are you using a short (less than 1 foot) piece of shielded wire to feed your stereo decoder?

Dave

maybe the 390 pF capacitors on the outputs perform the de-emphasis.

Ignoring the 330ks they'd be close at 39uS (Euro de-mphasis being 50uS) but maybe there's additional capacitance in whatever the output goes to or maybe they're intentionally leaving it a little 'bright' for tone purposes.

I said ignoring the 330ks because I'm not sure what the effect is of coupling that into the screen.