Hi everyone, I have essentially completed a full service on a nice GX51. It has all new electrolytics (which was interesting as the originals are unique), clutch gear, belt, etc. It works great but I am getting a very mild motor buzz coming through the headphones just in the forward direction, both channels. It gets louder with the volume pot. I have been checking the SMT caps for shorts but they seem ok. Any thoughts on where to look next? In the reverse direction, I put some resistance on the mechanism and it doesn’t buzz so I don’t think it is stress on the motor. Thank you!
Is that hum also present with the head disconnected ? Caps are unlikely to be the issue, since the ripple filter of both preamp and headphone amp is common for both channels and both FWD/REV.
Thanks Valentin, I appreciate your help. I will desolder the ribbon cable to the head and see if it is still there. Stay tuned.
Hi Valentin, when I disconnect the head, I get this loud fluttering in the audio Channel, both directions. I am holding the headphones up to my phone to record. It is too loud to hear if there is any motor noise. Thoughts? video here:
Ok, so connect the + side of the head coils input to the COM. But make sure you only use the unit in PLAY this way, not record, as in record you'll short the output of the bias oscillator. See if doing this makes that oscillation disappear or not.
OK, I have had a chance to start on this again. Thank you Valentin, shorting the pads as you noted did completely remove the oscillation. The issue still remains so I am assuming that the head is not contributing to the problem. It is very very faint but still remains a curiosity that it is in only one direction. Again, if I touch the capstan, the noise will change pitch. However, in the other direction, pressure on the motor does not create any motor noise so you could conclude the mechanism is not contributing. Any further thoughts Valentin or others before I close this one up? FYI, I thought is was worth sharing my solution for the 220uf and 330uf caps. I am not certain these were bad but I usually just replace all caps on the green button walkmans as they all seem to be bad on all models. You can get 5mm polymer caps at these values to replace the 4mm 'long' caps that are unobtanium. The actual position is key to avoid a ridge in the plastic cover and the slope for the battery. They seem to squeeze right in there! There is also a tight spot on the other side for a 220 that needs a polymer. Thank you again!
It's hard to point out what the exact problem could be. Something as simple as corrosion inside the FWD/REV switch creating a small parasitic capacitance can be enough to cause weird problems. What I can tell for sure is problem isn't the preamp/REC amp IC, since there is only one amp which is switched externally. And since the head was also excluded, it has to be something in the adjacent circuits or the PCB.
Thank you Valentin, I am coming to the same conclusion at this point. I’ll poke around a bit more but I really appreciate your help on this one. Enjoy the weekend. Jehr.
