Troubleshooting Noise in Cassette Player Technics RS-M17

[wraper]

I suggest replacing all tiny electrolytic capacitors, generally they don't last long compared to larger ones. As of noise, could be something sensitive powered through resistor + smoothing capacitor with that capacitor being dead.

[floobydust]

I see no R402 (on the output of Q401) on your board pic. You'd have to find somewhere else to measure the (guess around +13.7VDC) rail, at C404 or on R27/C13 by IC1.
Right now if the voltage is too low or unstable, Q401 could be damaged or overloaded. I've seen Sony let these parts run very hot for a short lifetime, intentionally would not heatsink and repair shops knew their games.
The regulator also powers the LED VU meters IC502, so they would add load if the LEDs are all on I think. So the rail should be stable even if the VM meter LED's are blinking.

One thing about this board is the use of carbon-print resistors, a little black rectangle on the PCB that makes a resistor. This can be extra confusing as not all resistors are through-hole parts.

[dankinzelman]

Wow yes, I had no idea about carbon print resistors. I've spent literally hours looking for those resistors, and they're just those little black squares on the board?

Anyway, I measured R27 to be 268R, and there are 12.4V before and 10.7V after it. Is that too much current draw? Upstream is the same as on schematic, but voltages at IC 1 and 2 are about 10-15 percent lower than on the schematic. I checked again for R33 and resistance between emitter and R27 is 0.5R, so I think R402 doesn't exist. After disconnecting the tape heads, I measured 91.7k for R7 and 95.5k for R8. Is it worth scratching through them and replacing them with new, through-hole parts?

Also, I noticed that the rail gets really noisy when I activate the transport, and the noise appears on the scope together with the noise on the headphones. At idle, the upstream side is fairly smooth (with muting circuit activated), I've attached a video where you can see what happens when I press play.

Here's the video link https://youtu.be/q1yeR-zryws

Top trace is before R27 and bottom trace is after R27, at maximum scope gain, 1mV/div. I tried disconnecting the motor to see if that helped and it becomes very noisy even at idle with the motor disconnected.

Could the headphone amp be drawing too much current or something? Should I try disconnecting the headphone amp by lifting a leg of R99 and R100? Also, the player takes almost two seconds to unmute, is this a useful clue? And what can I do to improve it, is it likely a dying electrolytic cap?

[floobydust]

I'm confused now because the "noise" you are showing isn't a problem I think - the scope is on 1mV/div (very sensitive) so you are seeing normal AC wandering of the voltage regulator. It's fine to see even 100mV of noise there.
What matters is the (multimeter) DCV it outputs, to see if the regulator is low or what. Can you check what is across C404? And the head preamp IC is still getting low voltage for power?

[dankinzelman]

I read 12.33V across C404.

Pin 7 of each IC is at 10.56V.

I understand what you're saying, I think the noise is appearing at the output, though. It seems to be getting amplified. I will try to look tomorrow with the scope and verify this.

Also, I really appreciate you taking the time to help with this.

[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).