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FM from left side only.

Sglaw461

Sglaw461

Member
I posted this in the Pioneer forum, this might be a better place.
Client brought in a Pioneer SX-650 and said it was only working in mono. After a test it seems it is working in stereo on all inputs except FM. The Stereo light never comes on. I am good with amps, pre-amps etc but not so much with FM. I washed the tuner board with ipa, and let it completely dry but the problem still exists. There are loads of tantalum capacitors so I will test those next but was hoping someone with expertise in FM could offer some knowledge and tips of where to look for the problem.
 
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Check the antenna, but it probably needs an alignment to get the stereo light to come on. If you don't have audio output from the right channel, then that's something completely different.
 
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It could be a few issues but I always start with DeOxit on the front switches, anything that clicks when you move it will be the worst offender with the am/fm/tape/aux being the main offender. I've fixed dozens of equipment this way, if it doesn't work, use headphones to see if there is stereo out. Your skill level may be higher but it's always smart to start with the easy stuff. With a lot of equipment, I might have had two or three over the years where the issue is on one of the boards.
 
Thanks, aside from the FM the unit is working perfectly. Since FM is only on one side and the stereo lamp never illuminates I can guarantee that it is something on the FM board but my limited knowledge of FM circuits leaves me scratching my head. With the unit switched to mono it is still only one side of the FM broadcast in both speakers.
 
Sglaw461 said:
Thanks, aside from the FM the unit is working perfectly. Since FM is only on one side and the stereo lamp never illuminates I can guarantee that it is something on the FM board but my limited knowledge of FM circuits leaves me scratching my head. With the unit switched to mono it is still only one side of the FM broadcast in both speakers.
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That's good info. I would signal trace all the audio cables from the mpx board to the input switching circuit.
Also most of the time with no stereo lamp lit I find the lamp itself is burned out.
 
I would verify that your stereo lamp is not burned out. They often are. Signal generator and Oscope. Tracing a sine wave you narrow it down in a few minutes. Losing a channel. Most common like stated before dirty switches. Next muting device (Q7,Q8), , low pass filter (LPF1). Pretty rare but but could be the MPX chip (HA1196). Clean he switches first.
 
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