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My Pioneer SX-780 just caught Mono

Type IV Tape

Type IV Tape

New Member
THE INCIDENT

I was listening to XM Radio over the AUX input when I heard the audio fluctuate. I thought maybe there had been a power spike (despite being plugged into a surge protector). Suddenly the right channel became very weak and garbled. Occasionally it would break back through to full strength, only to return to its garbled state. However, I noticed no change on the right channel VU meter.

TROUBLESHOOTING

I output my left and right audio channels into my subwoofer over A, and from the sub out to my speakers. I completely broke signal path protocol in my panic and checked my sub first. I cycled the power and checked connections to no avail.

I then tested my left and right balance on my receiver, but nothing changed as right channel was still garbled on output. I switched it from stereo to mono and the audio came back on the right speaker. However, switching it back to stereo resulted in the same garbled audio on right. Shortly after audio returned to the right speaker, but the stereo indicator lamp had gone dark.

I checked all of my audio input sources. Starting with broadcasting Moving In Stereo by The Cars through my stereo FM transmitter connected to my laptop. The stereo indicator did not light nor did it while scanning through the dial. Checking my phono input I played Moving In Stereo on my turntable to no avail. Despite the stereo source the indicator did not change. Aux input also remained mono. Headphones were mono as well.

CURRENTLY

Switching from stereo to mono mode results in no changes to the audio. Stereo separation is completely gone.

I had noticed the stereo indicator had gone dark two days ago without the audio experiencing any issues. However, I assumed I was off frequency on the FM dial or the station wasn't strong enough for the FM carrier.

If anyone has ever experienced the same issue, I would love to know how they fixed it.
 
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XM Radio Stereo comes INTO the receiver on the L and R AUX interconnect cables.

The ONLY way stereo is actually decoded (and the indicator turns on) is if it comes in on the FM tuner from an FM Stereo broadcast..

The only thing you can test with the XM radio is to swap the L & R AUX input plugs to see if the missing or bad audio stays put or switches channels.
IF it stays put, it's the SX-780's fault, if it MOVES, then it's either the cable or the XM Radio.

The stereo / mono switch ONLY combines the L + R channels, just AFTER the function selector and monitor (1 & 2) switches.
Generally we say to deoxit all the switches, including the low filter.
 
Welcome Aboard!

To paraphrase MTF, the "stereo" indicator should only function when the FM section is receiving and decoding a stereo broadcast. You can have any input source (tape, aux, whatever) in actual stereo but the little red LED will not light unless the FM tuner is receiving in stereo. Switching to mono mode simply proves the output stages are intact, which is a good thing.

After cleaning the controls as advised, your symptom may or may not be resolved. If not, there are other possibilities, but cleaning the controls will be critical to eliminating them as possibilities since they are very frequently a problem. It might also be helpful to check the cable connections and the input jacks for contamination. The shells of the RAC jacks can be wiped externally with a cleaner, and the inner sockets can be wiped with a pipe cleaner dipped in cleaning solution. All cleaning needs to be performed with power off.

If you suspect the switching and/or AUX input jacks, the XM receiver could be connected to the TAPE inputs since they operate at the same signal level as the AUX inputs.

Other than cleaning and exercising the FM de-emphasis switch on the rear panel of the unit, do not clean anything on the tuner section unless following the explicit directions here: https://audiokarma.org/forums/index.php?threads/how-to-clean-a-tuner-analog.144163/
 
FIRST

Thank you MarkTheFixer and Watthour for all your help.

RE-TROUBLESHOOTING

OK, I did a small recheck. Phono input is in fact in stereo as checked by The Cars.
AUX input is in fact in stereo. Once again checked by The Cars.

There is no stereo over FM even when broadcasting from my transmitter.
I switched the FM de-emphasis switch down to 75μs and back up to 25μs.
(That is how it was set when I received it)

QUESTIONS

The manual states that 25μs is for Stations broadcasting with Dolby FM Noise Reduction.
A separate adapter is required for that? All I can perceive is that output is quieter at 75μs.
And the manual states that setting the low filter to 15Hrz could damage my speakers?

CONCLUSION

I don't know what happened last night to cause right audio get all funky over AUX.
Now it seems that the gremlins have only affected the stereo on FM.
The only station that is strong enough in my area for stereo sucks.
And my favorite classic rock station was killed off by CBS years ago anyway.
So, as long as that is all that is wrong, I'm going to be alright.

Thanks again for all your help!
 
Last edited:
Swap your XM RCA's from the XM receiver. Make the red left and the white right. I always use the red RCA for the right as Red has a "R" in it. See if the issue follows to the left channel.
We'er thinking something is wrong with the right output of the XM receiver. The 780 seems to be working correctly.
Copy?

Another note: I found a XM receiver at a yard sale a few years back. Plugged it in an found out it had a lifetime subscription. I really enjoy the service.
 
Type IV Tape said:
RE-TROUBLESHOOTING
There is no stereo over FM even when broadcasting from my transmitter.
I switched the FM de-emphasis switch down to 75μs and back up to 25μs.
(That is how it was set when I received it)

QUESTIONS ...The manual states that 25μs is for Stations broadcasting with Dolby FM Noise Reduction.

CONCLUSION...The only station that is strong enough in my area for stereo sucks.
Click to expand...

First, in North America (and Korea) FM broadcasts are timed for 75µS de-emphasis. It is usually best to use that setting.

Second, it appears that at least one station (sucky or not) will produce a stereo signal. If that's the case, the FM front end may not be capturing adequate signal for the decoder to divide that into separate channels. That could indicate a tuner alignment problem. However, back in Post #3 there is a link to instructions for cleaning the tuner section. That could be done (and should be before any alignment) to potentially restore operation. Follow the instructions carefully.
 
I missed the issue.
He's using a transmitter style one.
Sorry for the stupid attack.
Back on board.

:lurk:
 
avionic said:
Is he using some kind of an external antenna.
Click to expand...

I built a custom dipole antenna specifically for receiving FM broadcasts.
Where I live, it is very hilly and mountainous so its near impossible to get a good signal without a well tuned external antenna.

Also, I'm glad to be here. I've lurked in the shadows here in the past to read about other vintage audio equipment.
And after seeing how helpful and enthusiastic the members were here, I knew I could find help with my predicament.

zebulon1 said:
See if the issue follows to the left channel.
We'er thinking something is wrong with the right output of the XM receiver. The 780 seems to be working correctly.
Copy?
Click to expand...

Whatever the gremlins did last night only appears to have affected FM stereo. Everything else works.
 
zebulon1 said:
Swap your XM RCA's from the XM receiver. Make the red left and the white right. I always use the red RCA for the right as Red has a "R" in it. See if the issue follows to the left channel.
We'er thinking something is wrong with the right output of the XM receiver. The 780 seems to be working correctly.
Copy?

Another note: I found a XM receiver at a yard sale a few years back. Plugged it in an found out it had a lifetime subscription. I really enjoy the service.
Click to expand...
 
I had a Sirius receiver stolen about 15 years ago that I had purchased a lifetime subscription for. I didn’t replace it at the time, so I just let the thief relish in the fact that he had stolen a unit with a lifetime subscription. Fast forward 10 years and I bought a new truck with built in Sirius. That’s when I switched it over. I suppose that I should have bought one right away just to screw him or her over, but I figured the audio gods would smile on me for giving them a gift. I did hope however, that they would be in the middle of listening to it when it finally cut out.
 
UPDATE

Yesterday I ran my receiver all day with no new issues. (Still no stereo on FM)

Today the gremlins returned to mess with the right channel again.
All sources were low and garbled on the right.
The balance knob became tight and scratchy when turning it to right.
After a little exercising the tight scratchiness began to move to the left as well.

Yet once again, switching the mode down to mono and back up to stereo fixed it.
It looks like it may be time for me to use some deoxit as has been suggested.
 
I know this is an old thread but did OP ever figure out the problem? My SX-780 is having the exact same issue. My right channel is quiet/garbled/crackly in stereo but is fine in mono. Doesn't matter if I'm using aux or phono. Also, same settings it was working fine until yesterday.
 
Apparently not or he did and didn't finish the thread. Please start a new thread and list exactly what it's doing. Left, Right, Both channels? Which sources, AM/FM/Aux/Phono/Tape monitor (plug a CD player in and test). We'll go from there. Hi-Jacking an old thread isn't considered good form.
 
larryderouin said:
. Hi-Jacking an old thread isn't considered good form.
Click to expand...

oof, "hi-jacking" seems like a strong word. I searched the threads for my issue, found my issue and posted in the thread. Thanks for checking in though. After searching some other threads, and not posting in them, I've got some contact cleaner and will be following these directions: https://audiokarma.org/forums/index.php?threads/the-idiots-guide-to-using-deoxit-revisited.207005/
If I'm still having trouble after I clean everything, I'll be sure to start a new thread.
 
AdamTM said:
oof, "hi-jacking" seems like a strong word. I searched the threads for my issue, found my issue and posted in the thread. Thanks for checking in though. After searching some other threads, and not posting in them, I've got some contact cleaner and will be following these directions: https://audiokarma.org/forums/index.php?threads/the-idiots-guide-to-using-deoxit-revisited.207005/
If I'm still having trouble after I clean everything, I'll be sure to start a new thread.
Click to expand...

Ever resolved your issue? Having the same exact issue with my SX-780. I don’t think your hijacking though. Hope to hear from you soon. Thanks.
 
nodols said:
Ever resolved your issue? Having the same exact issue with my SX-780. I don’t think your hijacking though. Hope to hear from you soon. Thanks.
Click to expand...
Hey I'm not sure if you're still around almost a year later. I never got an email about your reply. Using deoxit on my input selector did the trick and I no longer have this issue.
 
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