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I'm currently trying to repair an old Sony TCD-D7 DAT-Walkman.

It suffers from unbalanced audio output. The right channel is much quieter than the left one during playback.

The headphone amplifier circuit of it looks like this:

enter image description here

Full schematics can be seen on pages 18 and 19, part names on pages 27 of the service manual.

Unfortunately, I have not been able to identify a faulty component. Maybe there's something broken within one of the ICs. Now I wonder whether there's a way of modifying the circuit itself to increase the output volume of the right channel, as a somewhat "hacked" solution.

As far as I can tell, there are two inverting opamp circuits in this circuit, one for each channel. Changing the resistor values of R227 and R127 respectively should hypothetically change the amplification factor of the whole circuit, no? When I tried to do so, nothing happend apart from strange noises coming through the output.

What have I done wrong? Is there something I have missed? Any other ideas on how to modify the output volume?

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  • \$\begingroup\$ 'Scope the NJM3416V's pins 1 and 7 (to pin 4) and you should see about 1.8V at idle and equal AC signal riding on this. If not, do the same with pins 6 and 2 to 4 and if that looks fine, then the chip is bad. Otherwise it's in the mute section or input signals. \$\endgroup\$
    – rdtsc
    Jun 27 at 12:00
  • \$\begingroup\$ That device is probably 25 years old and has a lot of electrolytic capacitors, which at that age are a common point of failure. \$\endgroup\$
    – GodJihyo
    Jun 27 at 13:26
  • \$\begingroup\$ @GodJihyo I thought that, too, but I measured all capacitors from the DAC to the actual output and they seem to be still ok. \$\endgroup\$
    – DLCom
    Jun 27 at 13:44
  • \$\begingroup\$ Connect the two collectors of Q314 to force equal input signals to both OpAmp circuits. Then you know if the problem is in this output stage or far before. \$\endgroup\$
    – Jens
    Jun 27 at 15:59

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This is where an oscilloscope comes in really handy.

You have a good channel and a bad channel. Use the oscilloscope to trace back from the headphone jack back to the ADC output. At each stage you see if the left and right channel are approximately equal. If not, go a step closer to the ADC. When you find them equal, the failure is one step closer to the output than the probed point.

As an alternative, you could use an amplifier and a wire to probe various points in the left and right channels. Wire to line in on an amplifier, tie the grounds together, then (carefully) touch the bare end of the wire to op-amp outputs. Trace the circuit back as described above, but using your ears instead of looking at the scope.

Increasing the amplification may not help. It is probably already (by design) close to the usable limits. If you jack up the amplification before the failed part (or connection) then the amplifier will distort. If you jack up the amplification after the failed spot, you'll amplify the noise as well as the signal. Either way will probably sound crummy.

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