Troubleshooting a MacDonald Am/FM/Mpx Stereo

[type6]

I have a old stereo that I am repairing. Its a old MacDonald Am/FM/ Mpx. The problem is, it operating on Fm mono. Even on Mpx. I''ve checked ever thing I know of. But the Stereo light will not come on. I've read every book on it. But I can't same to find the problem.

 

[Danger]

This is probably a stupid question, but have you checked to make sure that the light isn't burnt out?

 

[sophiecentaur]

I have a old stereo that I am repairing. Its a old MacDonald Am/FM/ Mpx. The problem is, it operating on Fm mono. Even on Mpx. I''ve checked ever thing I know of. But the Stereo light will not come on. I've read every book on it. But I can't same to find the problem.

Do you actually hear stereo when the light is not on and the signal is strong enough for other receivers to work correctly? Most receivers inhibit the stereo decoder if the signal is too weak because the audible noise level is so much worse when you add the stereo difference signal.
The problem could either be with the decoder or with the RF / IF stages if it doesn't work with a good signal.

 

[type6]

This is probably a stupid question, but have you checked to make sure that the light isn't burnt out?

yes I have! I've checked all the capacitors. Resisters, But the sound come in . Mono, you can hear it! Compared to my other stereo I have. In fact when I tune into a station that is broad casted in stereo! I get no volts to the stereo light. Its dead! Oh well I can sell it for parts.

 

[jim hardy]

Without some knowledge of its internals this will be hard to fix.

When a station broadcasts in stereo there is a 19khz tone accompanying the sound signal , its job is to turn on the stereo decoding circuitry and the light.

read up on it here.
http://en.wikipedia.org/wiki/FM_broadcasting

Are you equipped to find that part of your radio's circuitry?

good luck

 

[sophiecentaur]

yes I have! I've checked all the capacitors. Resisters, But the sound come in . Mono, you can hear it! Compared to my other stereo I have. In fact when I tune into a station that is broad casted in stereo! I get no volts to the stereo light. Its dead! Oh well I can sell it for parts.

You still haven't made any comment about the level of signal that you are getting. This can make a big difference. An old receiver on the fringe of a service area is unlikely to work in stereo. What sort of antenna are you using? Using a simple dipole, at the top of the house can improve signal strength. No fancy engineering needed. Just separate the end of a length of coax so the inner and braid are spread out. Make it about a half wavelength, end to end. This will be a lot more effective than the whip on top of the set. Perhaps you are referring to a portable without an ext. antenna input, though - in which case get as high as you can with it - or drive to somewhere near the transmitter!

 

[type6]

Without some knowledge of its internals this will be hard to fix.

When a station broadcasts in stereo there is a 19khz tone accompanying the sound signal , its job is to turn on the stereo decoding circuitry and the light.

read up on it here.
http://en.wikipedia.org/wiki/FM_broadcasting

Are you equipped to find that part of your radio's circuitry?

good luck[/QUOTE

I know all about that! What gets me. Is I have a out side atennna

hooked up to it. It come in mono. Even on MPX . With all the channels. I tune in. Then I hooked up my Sansui. Stereo light come on. It works find. My friend is going nuts trying to find the problem. I said throw it out.

 

[sophiecentaur]

You seem to have established that it is not a limitation due to signal strength so is could be anything from a dodgy solder joint on the input connector, to the RF section gain, to the local oscillator / mixer, to the if amplifier(s) or filters, to the decoder itself. If you look inside, there is always a chance that you could find a loose component or lead - or something that looks like it has been very hot.
It could be a long job and would need a fair amount of diagnostic equipment plus a manual for the receiver (probably obtainable for free). You could buy a new receiver for not many quid - but that would be no fun.
Not worth getting into the problem really unless you plan a lifetime of equipment repair, in which case the experience could be good for you.

 

[type6]

You still haven't made any comment about the level of signal that you are getting. This can make a big difference. An old receiver on the fringe of a service area is unlikely to work in stereo. What sort of antenna are you using? Using a simple dipole, at the top of the house can improve signal strength. No fancy engineering needed. Just separate the end of a length of coax so the inner and braid are spread out. Make it about a half wavelength, end to end. This will be a lot more effective than the whip on top of the set. Perhaps you are referring to a portable without an ext. antenna input, though - in which case get as high as you can with it - or drive to somewhere near the transmitter!

Its a roof antenna for radio's. I have a CB unit. It works great. For my Sansui. It just won't work for this old radio. I'll tell my friend to just junk it. The newer radio will pull in signal with just a little table top antenna.

 

[type6]

Do you actually hear stereo when the light is not on and the signal is strong enough for other receivers to work correctly? Most receivers inhibit the stereo decoder if the signal is too weak because the audible noise level is so much worse when you add the stereo difference signal.
The problem could either be with the decoder or with the RF / IF stages if it doesn't work with a good signal.

Nope! No stereo at all! Just mono. Its a big differents when you hear stereo! Its a old radio!

 

[jim hardy]

There's a site "AudioKarma" where hobbyists swap information about their old stereo gear.
I found a thread on my Fisher RS1022, you might try there on your Macdonald.