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1971 Panasonic IC FET FM-AM Multiplex Stereo

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Right Speaker Channel Dead - Almost Silent

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I just recently picked up an old Panasonic RE-7430 stereo thing, but with the tuning dial and stereo indicator completely dead. I thought the rest of the system didn't work, but it actually did. However, only the left channel works and no sound comes out of the right. I replaced the lightbulb behind the tuning dial, and the stereo indicator, so now both work. However, I can hear a small click through the right speaker whenever the stereo indicator light turns on, and some quiet and tinny version of whatever I'm listening to if I use the balance to turn off the left channel and turn the volume all the way up, but nothing else. This means that the balance slider basically just works like another volume slider for the left channel! When I plug in headphones, both the left and right channels work, but the stereo is a little dodgy from old balance and volume sliders. I'm not sure what's causing the right speaker channel to not work, may I please get some help?

Thanks so much!

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Hi @wozmon

Check if the headphone jack is OK.

I couldn't find a free download for the service manual online but found this link where it can be purchased.

It's a bit awkward to use but if you select the image of the wiring diagram that is posted in the link and then zoom in, you can follow the entire circuit by moving the cursor around.

Here's an image showing the wiring diagram (not zoomed in) and an enlarged section of the diagram, showing the speakers/headphones jack connection.

As can be seen the audio output is the same for both options, speakers and headphones, except that the headphone output is fed through 2 x 330 Ohm resistors (one each for L + R audio) and the speakers are not. They're connected when the headphone jack is removed from the socket, bypassing the resistors.

It may be that the R audio output headphone socket contacts are corroded, high resistance or have even lost tension so that they don't make properly when they should

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Are you saying that the right speaker channel is reliant on the headphone jack?

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@wozmon

Yes. Both speakers are directly connected through the headphone jack.

When there are no headphones inserted, the audio for both speakers i.e. L+R, pass through the headphone jack contacts which are closed and connects to each speaker.

When the headphones are inserted, the headphone plug opens the jack contacts so that the audio to the speakers is cut off, and it goes to the headphones instead, via 330 Ohm resistors to lower the sound as you don't want to blow the speakers in the headphones or distort the sound due to the different impedance that most headphone speakers have as compared to stereo speakers

You can see it drawn in the right side image of the schematics I posted above.

Trace back from each "speaker" shown above the "headphone" jack in the circuit (right hand side) and you can see the contacts in the jack.

Imagine that when the headphone plug is inserted in the jack it pushes the contact springs away from the connections going back into the board i.e. headphone contacts 6-7 and 2-3 are opened.

When the headphones are removed from the jack they close again and audio is connected to the speakers again. It is also still there for the headphones but nothing's plugged in to hear it ;-)

Given that you said that both the L+R audio was OK when listening using headphones but the R audio wasn't heard on speakers the that is where I think the problem is. Either that or in the wiring (or connections) between the headphone jack and the R speaker terminal where the speakers connect to in the radio.

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Thank you, but I think the contacts in the headphone jack might be okay. I tested for continuity between the speaker connections on the board going to the headphone jack and the actual RCA connectors, and everything shows up fine. I also tested the 6-7 and 2-3 contacts, and both of them are closed. I did notice, however, that there is a 1.3 kohm resistance reading between the positive connections of the RCA jacks, but I don't know if this is normal or not.

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I also thought that I'd mention that when I plug in headphones, the left channel actually sounds a little quieter than the right sometimes, but this might also be because of old and dodgy sliders. I already cleaned the sliders with contact cleaner, and sliding or wiggling them still affects the left channel a little. However, this is only with the headphones, and I don't hear any crackling through the left speaker channel when I adjust the slider.

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I also tried shorting the contacts closed, but still nothing except a click when the stereo indicator lights up, and a crackle from twisting plugging in the connector.

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