Hi all, I have a strange issue I haven't seen covered: Playing tapes back in my WM-D6C works great, no problems. During recording everything sounds fine too (through the headphone out monitor). Sadly when I go back to listen to what I just recorded, the left channel sounds very loud and scratchy. In other words the WM-D6C is recording a bunch of noisy, fuzzy, scratchiness to the left track on the tape. So far I've cleaned both heads, I've cleaned the record volume pot, I've tried recording through the mic in (same issue), checked for loose or disconnected wires, and the problem persists. I've thought about removing the muting module but playback works fine in both channels so I don't think that's the issue. I have a parts machine so I was thinking of replacing the erase head? Any thoughts?
In the first instance I would suggest checking and cleaning the S301 record/playback slide switch on the main PCB. It is the one which is located in from the record button and is actuated by a fitting on the record lever. If that is OK, then your problem could be either a lack of bias on the left channel or a problem with the signal path to the LH head. You would need to trace the signal, using an oscilloscope, from the Dolby IC (IC101) through the record amp (IC302), through the bias trap and to the head. I think it's unlikely that the Dolby IC or the record amp are faulty but one, or more, of the capacitors or other components surrounding them may have failed. It is also quite possible that you have a dry solder joint or a cracked PCB track. If the D6C is like the D6 then you can sometimes get cracks in PCB tracks which are invisible, even with a strong magnification lens. A lack of bias (65 kHz sinewave superimposed over the audio signal) can cause a recording to sound broken and scratchy, although I would need to hear the recording to know if this is the problem. We know that the bias / erase oscillator is working because the RH channel is OK. You could try checking the capacitor array C149 and C150 and the condition of the PCB tracks connecting them to the bias trap and the head. Also, check if any of the solder bridges are in place and is the pattern the same as for the RH channel. Although the fault tracing may sound daunting, you have an identical, and working, circuit which you can compare against the faulty one. This makes life a lot easier.
Thanks for the quick and detailed response! I will definitely try to look into all these things despite not having an oscilloscope (although I do own a hacked CRT TV). Before jumping in I can say that the solder bridge pattern on the L side is different from the R. My understanding is that these patterns are often different? I own another WM-D6C which works well and it also has different solder bridge patterns on the L and R side. I also forgot to mention that the machine was recording great for a while then just started scratching out of nowhere. It was not gradual. It was sudden.
I have come across machines with different solder bridge patterns on the bias setting but I did wonder if it might indicate that somebody has tried to sort out the bias in the past. If the scratchness happened suddenly then it looks like a component or switch failure, dry joint or broken track. Is the fault intermittent or does it happen all the time now?
In the 3 generations of D6C I own (1st, 3rd, 4th) all have different factory bias settings, presumably to match Sony blanks of the time. Out of these units, the 1st generation one has different settings for both channels. The rest have the same setting for both channels. I usually set them to early 90s XLII, a good mid point for Type II tapes.
Once it started it's been all the time (not intermittent). I'm getting some deoxit in the mail tomorrow so I'll try some on the record switch and take it from there.
My experience with the play/rec switch is that Deoxit does not help for long, it has to be desoldered, disassembled and the contact surfaces of the switch components lightly "lapped" with very fine sandpaper (2000 grit) or even deoxit soaked paper.
Back with another update – So unfortunately now the unit has gone mute for the most part. There is no music out of the headphone jack, however with my stereo turned rediculously high i can hear faint music out of the line out. I carefully removed the mute module to eliminate that possibility and now I hear a faint thump in both the headphone out and line out. I also checked the play/rec switch and confirmed that it is connecting all the points that it should be (i used a working unit as side-by-side comparison). I also checked capacitors C318, C305 as well as all the capacitors along the red line in the service manual diagram (it has a red line which follows the audio signal from tape head to headphone out). Everything seems to be in working order. I painstakingly checked all the wires in the "jungle" for continuity and connection. Questions: Are there other capacitors I should check? Doesn't it seem that if both the L and R playback audio went out simultaneously that it might have to do with something outside the audio paths in the circuit?
