Electronics > Repair
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.
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