This is a complete step-by-step tutorial for how to adjust the Dolby levels, record levels and adjust the bias on your WM-D6 or WM-D6C. Applies to all generations and all PCB revisions. This tutorial starts on the assumption that the head is not worn or pitted and the azimuth is correctly adjusted. It also excludes electronic problems, such as bad EQ modules. Those could also cause tapes to sound muffled when played, but can also cause the unit to record tapes incorrectly. There are 2 EQ modules, one for playback, one for record. This is a common problem which should be fixed on any D6C. New EQ modules can be found here: https://www.ebay.com/itm/263537248339 Regarding the rec level and bias adjustment, some people like to do the adjustment to one specific tape and only use the unit to record on that type of tape. While this is the ideal scenario, I imagine that in reality most people will want to use the walkman with different types of tapes. I personally like to do that, because I also collect blank tapes. The good news is: if you align the machine using a good cobalt-doped ferric (TYPE II), it will work fine on most tapes (even on TYPE I and TYPE IV) without significant differences. The only big difference are true chrome tapes (like BASF Chromdioxid series). These have very different magnetic properties and are best recorded on a high-end will external calibration pots anyway. In the pictures below is the test rig I have built myself to load the output with 47K. It is very simple, using components that can easily be found.
DOLBY LEVEL ADJUSTMENT - You will need either a digital oscillscope (an analog one with digital measurements will also work) or TrueRMS multimeter. - You will need a Dolby level tape (330Hz@200nWb/m ANSI). I don't recommend using a P-4-L300 tape. - You will need to load the LINE OUT with 2 x 47K resistors. If you don't, the levels will be higher than they need to be. The level you should be getting is 340mV RMS with a Dolby tape 200nWb/m ANSI. The VU-meter will indicate -5dB when the level is adjusted correctly. You need to adjust 2 trimpots: RV-101 LEFT CHANNEL RV-201 RIGHT CHANNEL Stop the unit and start it again a few times and check the level remains the same.
RECORD LEVEL ADJUSTMENT - You will need a function generator, digital audio player or computer. If using a DAP or computer sound card, it is necessary to use an uncompressed wav file, as mp3 will not have a constant level. - You will also need the oscilloscope or multimeter used previously. - You will need a blank tape of your own choosing. But it needs to be one that has not been recorded before. - You will also need to load the LINE OUT with 2 47K resistors. Feed a 330Hz 0dB (775mV RMS) signal to LINE IN. Put the unit in record mode and measure the value on LINE OUT. Write it down on a piece of paper. Now rewind the tape and play it back. Check that the value in playback is the same as the value you wrote down. If not, adjust accordingly. You will need to repeat this procedure a couple of times to get it right. It doesn't need to be perfect, a few mV RMS difference is ok. The tripots that need to be adjusted are: RV-102 LEFT CHANNEL RV-202 RIGHT CHANNEL Note that the REC level trimpots should be rotated counterclockwise to increase the level. The idea is to match the level that you get when monitoring the recording the same as the one you get on playback.
BIAS ADJUSTMENT - You will need a function generator, digital audio player or computer. If using a DAP or computer sound card, it is necessary to use an uncompressed wav file, as mp3 will not have a constant level. - You will also need the oscilloscope or multimeter used previously. - You will need a blank tape of your own choosing. Needs to be the same as the one you used for setting the record level. Feed a 10Khz -10dB (250mV RMS) signal to LINE IN. Put the unit in record mode and measure the value on LINE OUT. Write it down o a piece of paper. Rewind the tape and play it back. Check that the value in playback is the same as the one you wrote down. If the value in playback is higher, you need to increase the capacitance. You can bridge more than one pad if necessary. If the value in playback is lower, you need to decrease the capacitance. You can unsolder more than one pad if necessary. Of course, repeat the process a couple of times until you get it close. Given there are only discrete values of capacitors, you won't be able to get a perfect result. In order to have a good dynamic range, it's better that the output at 10Khz to be a litttle lower than the input, not a little higher. That's because the tape's SOL is the biggest limitation.
