Electronics > Repair

Troubleshooting Noise in Cassette Player Technics RS-M17

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floobydust:
It's not easy to give troubleshooting help over text. I think we went off course, wrong rabbit hole.

Pin 7 is power for both TA7122 head preamps, coming off R27 270Ω to the +13V rail. You get +10.56V which is fine, I don't see a problem with that or the current draw there.
The rail seems a bit low at +12.33V I thought it should be over 14V but not yet worth going after. The noise you measured I think is not an issue at only a few mV on power.
+10.6V is enough for the head preamp IC's and if you think about it, same reading on both- yet one channel is dead... so that isn't it.

I would backup a bit. Forget about the noise measurements, thinking a few mV is trouble or even 0.2Ω Ohms reading means trouble - na it's a tiny amount. I think the ballpark of what a reading should be is part of a service technique to learn here.

During playback, touching a probe to anywhere at the head coil or downstream at R7, C1, IC pin 2 should peg the VU meters and be a very strong signal.
It's usually a dirty REC/PLAY switch or a broken (shielded) wire or solder joint connecting to the head that causes no sound and the stray noise pickup because of a floating connection to the head preamp input.

Again, I would go back and take ohmmeter readings from R7/C1 to GND and confirm the tape head resistance is showing up there, to the preamp IC- proving that the connection and REC/PLAY switch are OK. Compare that reading between the two channels.

dankinzelman:
In case it was not clear, both channels are functioning and playing back music, the left channel is simply annoyingly noisy whenever the muting circuit is not active (and this noise is superimposed on the audio output if I'm playing a tape). The noise tends to be very low frequency and intermittent, with a HF whining component to it. Right channel is nice and quiet, with no whine or rumble, just a light hiss, undetectable except when putting my ear to the speaker.

Probing pin 2 and C1/C2 pegs the meters as you suggested. C1-ground showed 225R, C2 showed 224R. I reflowed solder at the tape heads and swapped C1 with a new cap.

Noise is still there, and noise component at the output (both headphone and line out) does correspond very closely with the rail noise before R27, although with reversed polarity. After R27 the rail is quiet, but the noise seems to make it back into the signal chain somewhere along the line (output of the IC at pin 6 is noisy).

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