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Sansui 7070 only plays in MONO in both speakers - HELP!

daveyh

daveyh

Super Member
I have tried to clean all the controls many times and it still only plays in MONO in both speakers. The amplifier section meters work on both sides and produce sound when in MONO mode. When the stereo button is pushed the stereo light comes on and it only plays on the left side and the right meter does not move nor does it produce sound for that channel. This also is the case in CD mode. While in MONO mode the balance works great for both channels. It does not however work in stereo mode. I can hear a slight signal coming out of the right side with in stereo mode when the stereo button is pushed and panned to the right.

The DC offset is set to 0v and the bias is set to 30mA as described in the SM.

What sound I look for? Any ideas?
 
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The right side audio signal is dropping out at a point before the mono switch. The good news is everything after the mono switch is working normally. Work all of the controls and see if anything changes.

- Pete
 
Look at board F-2621 and switches before. Is the board just before the mone switch so should be something there.
Screenshot_20220513-185255_Moon+ Reader.jpg
 
LBPete said:
The right side audio signal is dropping out at a point before the mono switch. The good news is everything after the mono switch is working normally. Work all of the controls and see if anything changes.

- Pete
Click to expand...

Thanks Pete! I worked and have cleaned the switches many times to no avail. I can hear the right channel ever so sightly and only after panning the signal to the right side and turning it almost all the way up. Mono works fine.
 
Checked all connections and cleaned everything. This also is the case in aux mode. While in MONO mode the balance works great for both channels. It does not however work in stereo mode. Anymore ideas?
 
I would check out the contacts on the Mode Switch (on F-2620), but all switches on a unit of this age could do with being looked at for oxidisation - like S702 & S703 for example, near F-2621 on the schematic. (Also recommend you check the switches on F-2621).

The above advice seems to be a repeating theme in this thread...

How are you cleaning the switches? - it doesn't sound like you are taking them apart for cleaning, so are you spraying something into them? and if so what?
 
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yep. they develop either a goo or some type of dry black grease, carbon...have had to take them apart. then static develops that deoxit won't cure. have had to scrub contacts with tooth brush, scrape xacto.. after using wd40, carb cleaner, alcohol, deoxit. queen's hair dryer... a pain.
 
Hyperion said:
I would check out the contacts on the Mode Switch (on F-2620), but all switches on a unit of this age could do with being looked at for oxidisation - like S702 & S703 for example, near F-2621 on the schematic. (Also recommend you check the switches on F-2621).

The above advice seems to be a repeating theme in this thread...

How are you cleaning the switches? - it doesn't sound like you are taking them apart for cleaning, so are you spraying something into them? and if so what?
Click to expand...
OK. I walked away from this for awhile. I am going to take the switches apart like suggested. I was cleaning by spraying it with 5. Thanks for the advice. Going to try this tomorrow.
 
Took the switch out and all apart and cleaned it again, Still did not work, When placed in stereo mode the stereo light comes on but it only plays on right speaker, Both speakers produce sound but only with the switch pushed in,

No distortion in mono on either speaker or the one the works in stereo mode.
 
not sure if your unit has a variable resistor/ pot, associated with the tuner, most usually do. they can be turned gently in either direction until the stereo effect engages. check the schematic.
 
It does it in all modes not just FM, Hook up a turntable or CD player and it only comes out on one speaker with the stereo button out and the stereo light still comes on. Would the pot effect this ?
 
I would look at all components associated with the pot on the schematic on tuner board...wiring, soldering, components, wiring plugs...
IMG_tune.jpg
 
Did you ever find out what was causing this? My 7070 also started doing the exact same thing.
Edit I moved the tape selector around and it fixed it self. Probably not a permanent fix but something to keep in mind for next time.
 
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FactorVIII said:
Did you ever find out what was causing this? My 7070 also started doing the exact same thing.
Edit I moved the tape selector around and it fixed it self. Probably not a permanent fix but something to keep in mind for next time.
Click to expand...
Yours likely just needs the controls cleaned from the sound of it. I bought a Pioneer that supposedly had a dead channel. It came back just by cleaning the controls and a Deoxit treatment. Same thing, dirty tape monitor button
 
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