70s Stereo Receiver only plays FM in MONO

How is it built ? Just some transistors and coils ? Integrated parts ? If so, which types ?

Maybe the VCO adjustment, if you can find it. Pretty common problem on older units. It will be a potentiometer near the stereo decoder IC. Tweak it a bit each way - if nothing happens put it back - you may have been adjusting the separation control. If the stereo light comes on reliably, you're done.

Mark Z.

Many of these old sets had a MPX (Multiplex) setting, but you needed a Multiplex adapter to decode the second channel.

You guys are SO young!

I built a number of these from kits way back when. Have a look at this link...

Regards Lee in Toronto

In regards to how it was built... No ICs, just coils and transistors.

I have a different unit that has the mpx decorder IC so I can tell the difference. This one has only coils and ICs.

Worked fine up until last week. Then all the sudden I lost the stereo and now plays only as mono.

The stereo light will not come on, even when I tune the station well, and attach an enternal FM antenna (didnt need to do that before, but I tried none the less).

As mentioned I replaced all the capcitors aside from the ceramic ones. There are several dozen ceramic ones so I dont know which I would have to.

Not sure where to look since this has no mpx decoder, which general area. I also tried to adjust the pots but that did nothing, I returned them all to their original positions.

In regards to some other person saying the radio needs an additional decoder IC, stereo worked fine up until about a week ago. Something failed, not sure what.

Any tips or ideas where to look? Which I could find a service manual for this model. I tried to flip around a few transistors... What I mean by this is I traced back from the stereo light and replaced the transistors with same ones from other parts of the board. The reason being I dont have any exact replacements and did not want to wait for an order to come. This did nothing. But I only flipped around a total of 4 transistors.

Thanks for any help.

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One additional questions regarding this unit that wont play FM Stereo:

Would anyone be able to cross reference it with another unit. I am trying to find a SAMS service manual but cannot find it for this unit.

The unit is a AM/FM 8track with BSR record player. Manufactured by ELGIN

Model: RM-4392

Chassis:

One more question regarding my stereo system that will play only FM mono.

As I mentioned I replaced all capacitors except the ceramic capacitors. Someone has told me that ceramic capacitors rarely fail and to not bother replacing them unless I see them are physically cracked or broken. Is this true?

Any other means I can test the ceramic caps?

If I dont need to bother with the ceramic caps. All I need to focus on are the resistors (which are easy to test) and the dozen+ transistors & zenor diodes (do these fail? how are zenor diodes tested, must I also unsolder one end to test?)

THANKS again

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But the guy said that it worked in stereo, with no addiditional parts, then just failed overnight ...

If you have tried adjusting a couple of pots that you found in the right area, these may well have been VCO and separation. The other very common thing to give stereo dropout on these older MPX decoders, is the setting of the demodulator coil. It only needs to be off by a fraction - certainly, not by enough to be able to actually hear the characteristic demodulation distortion that you get, when it is a long way off - for the 19kHz pilot tone output to drop below the threshold at which the decoder can use it.

You should be able to find the demodulator coil easily enough. It will be the one closest to where you've identified the decoder circuitry to be, and will probably have a plain black core ( others in the IF strip and RF often had colour painted cores - pink, white - in these older sets ). It will likely have a couple of diodes nearby. Only adjust using a suitable trimmer tool, NOT a screwdriver, as this will split the core and lock it solid. Make small adjustments, and take the tool away each time, and wait a couple of seconds. Some of these older decoders were slow to lock when pilot tone was detected.

Zener diodes can be checked by measuring the voltage across them, to see that this corresponds to their declared voltage rating. Ceramic caps are, in general, reliable, but it's not unknown for older ones, particularly disc types, to go leaky, or even short circuit. Unless you have a schematic to give you a clue, the only real way to check them is to lift one end, and measure with an ohm meter. Good luck with it.

Arfa

Don't touch those ceramics! Failures are rare.

But there is a real risk: some of them are part of tuned circuits. If you replace them, the new capacitor won't have exactly the same value, so you would need to retune that circuit. That's hard. And near impossible without a diagram, the right tools and good level of experiance.

That's also not such a smart move imho. Replacing transistors often requires retuning the circuits they are part of!

You should be looking for a 38 kHz signal. (Or possibly a 76 kHz one). That is most likely generated locally and gets synchonized by the 19 kHz stereo pilot signal as transmitted by the station. Once the 38 kHz signal is detected, your stereo indicator should light.

I had a mid 70's Sony receiver that did that. The cause was the lamp itself was open and obviously an integral part of the stereo decoding circuit. I just wired a resistor parallel to the lamp and the stereo was operational without the lamp. I think I tried a 1k resistor and it worked, so I just left it. As I remember, there was no voltage across the open lamp, but voltage was appeared across the parallel resistor.

Have you ruled out the AM/FM?MPX switch?

It's usually the simple things, and switch failure on 70s gear is rather common as the contacts oxidise.

Dave

I lived in Yonkers and could see Armstrong's tower on the NJ side of the Hudson - but never listened to his 50 MC experimental station - too young. Armstrong was from Yonkers.

I was building single toob FM regens that used slope detection in the

My first real tuner was a Paco kit built in the 60's with a magic eye tuning toob. Back when FM played nothing but classical and way into the night with no damn commercials whatsoever. The good ole days.

I later added a stereo decoder that appeared in the RCA toob manual. Which turned out to be a futile exercise as there was only two stereo stations at the time.

Second was a Heathkit receiver with a discrete decoder inside the cabinet - must have had 10 coils in it.

If you're not too concerned with keeping it original and can't fix it, I suggest you try an LM1800 PLL stereo decoder IC. One part with only a single adjustment pot. Drives an incandescent stereo indicator lamp directly.

I had a Heathkit that used lots of big ferrite coils and about 70 square inches of pcb. It got replaced with a LM1800 that uses 9 square inches of perf board. Works great.

If one of the capacitors you replaced was a polystyrene (tubular shape clear plastic), it has to be replaced with a polystyrene or silver mica capacitor or you'll never get the strereo section to work. Chuck