WM-D6C stereo imbalance when monitoring recording from Line Out

Hello all!

I have an almost pristine WM-D6C that works really well, for the most part. Playback is flawless, but one of the issues is that recording is not balanced, and the right channel is noticeably louder than the left.

"It's a simple case of adjusting the record level," or so I thought. The first thing I noticed while following Valentin's tutorial in this very forum is that once I put the walkman in record, and monitor the line out from my oscilloscope, I see that the two channels are already out of balance. But if I plug my signal source straight into the oscilloscope, they levels are balanced.

See screenshots from oscilloscope below:

Direct feed from source to oscilloscope:

Monitoring Line in from Line Out, Rec Level 6

Monitoring Line in from Line Out, Rec Level 9


Is this expected, or something that I should investigate? Thanks!

Ps: This was all done using batteries as the power supply.

Edit: Removed incorrect measurement

 

Hi, @brunophilipe !

Most recorders that have a stereo record level potentiometer have some imbalance between the channels because the resistance of the 2 carbon traces is not identical (you can see that yourself by measuring with a multimeter).
I've always seen some imbalance on the D6C's I've repaired (as I have said, some is normal), however your measurements indicate an imbalance a little on the high side, especially at higher rec levels.

However, there is another thing that might cause an imbalance, however this would also create additional symptoms:
- a failing muting module might attenuate just one of the channels a little bit (https://stereo2go.com/forums/threads/wm-d6c-muting-issue.3282/);
- failing EQ modules (there is a REC and a PLAY one) These would cause the recordings made by the walkman sound muffled (when the REC one is gone), or the playback to sound muffled (when the PB one is damaged);

At what frequency are you testing ? This could point out if EQ modules are to be considered as a potential culprit or not.

Depending on the generation of your D6C, you might want to replace these as a precautionary measure. New modules are available on ebay:
https://www.ebay.com/itm/263537248339
https://www.ebay.com/itm/263851487685

Also clean the rec level potentiometer with contact cleaner and do the measurements again.

 

Thank you for the explanation @Valentin !

My unit has serial number 139011, so it has the original board. The measurement was done at 330Hz.

Also, is a conventional temperature-controlled soldering iron suitable to do these replacements or would I need something more specialized like a desoldering vacuum gun?

Edit: Another detail: The sound is very crisp, both recorded and playing back, but when I adjusted the playback level POTs I had to crank it quite a bit louder than it was originally to get crisp sound out when Dolby is enabled.

 

Ok, so given the 330Hz test frequency the EQ modules are to be excluded. Your serial number indicates one of the older D6C (I have seen failed EQ/muting modules on units in that range), however that does not mean this is your problem.

I recommend measuring the signal at the points indicated on the schematic, in order to see where exactly the signal gets attenuated. Maybe it's just a potentiometer with higher tolerance than the average.



As for the potted modules, if you want to replace them as a precautionary measure, using a normal temp controlled soldering station along with a good solder wick and flux is enough to desolder those modules (this is how I do it, of course there are other methods).
For reference (it's not your case), the newer green double sided PCB with SMD components do not even have those potted modules anymore, the circuits being made with discrete components directly on the PCB.

EDIT: The Dolby levels need to be adjusted accurately using a 200nWb/m calibration tape (you need to get 340mV RMS on the LINE OUT loaded with 47K). You can obviously adjust it by ear, but it won't be perfect.