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MM phono input problem, AU-D7

Uncle Bambi

Uncle Bambi

Funky Tut
I finished overhauling and re-capping the AU-D7 chronicled in a different thread, and have discovered a perplexing problem. I did not test this functionality before I worked on the unit, so I do not know if it was per-existing or if I introduced it.

With a TT attached to the MM input, the right channel is missing. If I select the MC input, both channels are present but muted and fuzzy sounding. If I leave it on MM, I can get the right channel to play if I turn the "mode" knob from "stereo" to "L+R". I've cleaned both the input jacks as well as the controls. So, the signal is coming into the unit, and is getting lost somewhere along the MM path.

Any ideas?
 
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Most likely dirty switch, or broken connection/dry joint, damaged ribbon cable perhaps?

I think you will need some kind of signal tracer (an oscilloscope +sig gen would be best) to find where the signal is being lost.

Alternatively
You could turn the volume down low and touch parts of the circuit with a small screwdriver with all but the very tip insulated (to avoid unwanted shorts). Work backwards from the power amp until you lose signal (hum) when you touch signal path components with the screwdriver, refer to the schematic/board diagrams for where. Reminder keep the volume low and be very careful - watch out for unwanted shorts

(NB: It would be best to use a set of 'beater' speakers for this test).

You might find it that way.
 
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Oh, Lord.

No O scope in this locality, and the alternative method you describe sounds fraught with danger.

Ugh.

Thanks for the help, though!
 
Uncle Bambi said:
Oh, Lord.

No O scope in this locality, and the alternative method you describe sounds fraught with danger.

Ugh.

Thanks for the help, though!
Click to expand...

You are absolutely right, it is fraught with danger, I am glad I got that across. Just trying to illustrate that there is no substitute for proper test equipment when you run into difficulties - a multimeter isn't enough.

It's kind of a 'catch 22' - if you are savvy enough to use this so called 'method' without blowing the thing up, then the likelihood is you have the right test gear to do it properly in the first place.
 
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Hyperion said:
You are absolutely right, it is fraught with danger, I am glad I got that across.
Click to expand...

Yes - yes you did. :yes:

It would be something I'd be very interested in learning, but I'd want someone with the skill to show me how.

Guess I'm at a dead-end with this one then, and it's useless to me if I can't spin records with it. I'll tinker with the switch some and see if that is fixable. but I won't hold my breath....... :sigh:
 
just plug in some rca jump cords and touch the tip of them with your finger and see if the symptoms are the same. Does not sound like your problem sounds right....

edit the mode switch is a red herring, it should act like that. I suspect dirty selector switch. It switches the MC MM transformer in and out.
 
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DCinDC said:
just plug in some rca jump cords and touch the tip of them with your finger and see if the symptoms are the same. Does not sound like your problem sounds right....

edit the mode switch is a red herring, it should act like that. I suspect dirty selector switch. It switches the MC MM transformer in and out.
Click to expand...

OK - will do in the morning.

The selector switch - that's the boxy thing that the ribbon from the MM/MC buttons goes into, yes? I've squirted that with DeoxIT, and it didn't change the symptoms. I'll try again. Is there any way to open that switch up without destroying it?
 
You guys are frickin' awesome. :ntwrthy:

I took the covers off again to see what I could see. Using the "buzz" trick with a set of input patch cables, I was able to verify that there was something amiss on the right channel and commenced to troubleshooting.

I took the cover off the slide switch, and manually moved it back and forth with no change. I hosed it with DeoxIT - no dice. And then...... I saw it. R12.

I had pulled several components on this board to clean up the Sansui glue that was starting it's corrosive nastiness. I removed, cleaned, and reinstalled a handful of resistors, and went on with my re-capping. Turns out I missed soldering one leg on R12, which appears to connect the right channel MM switch position with the drivers for the phono circuit. My de-soldering tool does such a thorough job cleaning out the hole in the PCB board that there was absolutely no connection at all, not even an intermittent one.

D'oh.

This was a self-inflicted wound. And even thought I tested basic amp function after every repair step, I did not test the phono inputs..... I used my iTouch on the AUX input to play and test.

Details, details, details.......... just when I think I'm getting good at this, something blows up in my face and then circles around to bite me in the ass.
 
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