I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.

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On 08 Apr 2006 21:50:01 GMT, (Was Immer) wrote:

I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.

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

--
Kind regards,
Gerard Bok

Was Immer wrote:
I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.

----------------------------------------------
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----------------------------------------------

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.

Was Immer wrote:
I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.

----------------------------------------------
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----------------------------------------------

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! g

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

http://www.somerset.net/arm/fm_only_stereos.html

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:
23F 4060
SR-1594 --- I believe this is actual chassis #, but both are
wrriten.

The manin board number with tuner on it:
1610471


I'd really love to get this unit back to %100, i have so many
memories with it.

The main board has NO ICs, only transistors and coils + pots.

Thanks.

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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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wrote in message
.. .
Was Immer wrote:
I have an old stereo system from the 70s that has an 8track, record
player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I tested the
voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not touch the
little ceramic capacitors. I tested the 2 diodes. The model is an Elgin
RM-4392 but I cant find any thing concerning a service manual.

What should I look for? All capacitors (aside from ceramic) have been
replaced.

----------------------------------------------
Posted with NewsLeecher v1.0 beta 28
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----------------------------------------------

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! g

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

http://www.somerset.net/arm/fm_only_stereos.html

Regards
Lee in Toronto

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

On 09 Apr 2006 07:20:09 GMT, (Was Immer) wrote:

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.

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.

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.

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.

--
Kind regards,
Gerard Bok

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.

"Was Immer" wrote in message
ng.com...
I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.


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

On Sat, 08 Apr 2006 22:38:31 -0400, wrote:

You guys are SO young! g

I built a number of these from kits way back when.

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
50's

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.

--

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On 08 Apr 2006 21:50:01 GMT, (Was Immer) wrote:

I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.

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.
--

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On 08 Apr 2006 21:50:01 GMT, (Was Immer) wrote:

I have an old stereo system from the 70s that has an 8track,
record player and tuner + aux

Anyhow, it has AM, FM mono and MPX

But on MPX it plays mono only... I am unsure what to look for.
I first thought it was the stereo lite that was broken, but I
tested the voltage going to the light and it is absent.

What should I look for?

I replaced all the capacitors in the entire board. I did not
touch the little ceramic capacitors. I tested the 2 diodes.

The model is an Elgin RM-4392 but I cant find any thing
concerning a service manual.

What should I look for? All capacitors (aside from ceramic)
have been replaced.

----------------------------------------------
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----------------------------------------------

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