Electronics > Repair
Troubleshooting Noise in Cassette Player Technics RS-M17
floobydust:
OP, it seems weirder because the head preamp IC voltages are all reasonable and the same between channels.
Pin 2 is the input and that sick scope waveform there could be capacitor C1 10uF 16V or others maybe at pin3 that are bad, or the 100R resistor to pin 2 went high value, or the IC has some strange failure.
I thought you can use a multimeter on diode-test and check between pin 2-3 look for an E-B junction voltage drop - but we already see that when power is on.
Pin 1 is a common bias for both channels which can cause oddball problems if that cap below is open.
The IC is only three transistors inside: TA7122 schematic
dankinzelman:
I've done some more digging.
R7 was not actually 100k from the factory, but rather 100R (I mistakenly followed the schematic instead of reading the resistor I had removed). So I think that explains the waveform, once I replaced it with a new 100R resistor the waveform came out normal, but the noise persisted.
The power at pin 7 is noisy, and corresponds roughly to the noise on the outputs. I traced it back towards the power supply. Noise is worse upstream of R27. I can't find R402 on the board, and measure no resistance between C404 and the emitter of Q401, so it seems like R402 is not present in my player (can anyone verify this?).
I replaced C401, 403 and 404 and noise reduced significantly, but is still there, and presents more strongly on the left channel, especially some low frequency intermittent noise, with peaks at 100, 150, 200hz and 2khz. Should I replace Q401? What would be an appropriate modern equivalent?
This noise is present at pin 1 of IC1, but not on the other side of R13. It's also not present on the right channel (or much less so in any case). This made me suspicious of R13, but changing that didn't help either. I can't find R21 on the board, although I do measure ~10k of resistance between R13 and R17. I replaced C7 (connecting to ground between R13 and R21) and C13. Still no improvement.
floobydust:
If the 12VDC rail is noisy, I would first fix that. But it supplies both channels with common feeds, so why is one channel a problem? Hmm.
It might be a problem with the voltage regulator (zener D406 MA1150LF 11V? or Q401). I would look at TO-126 2SD882 or 2SC669 etc. but lately I prefer to use a TO-220 part instead (flipped), if Q401 is cooked or damaged.
Noise can also be caused by too much of a load on the voltage regulator. I would look at the voltage drop on R402 1V2? for 36mA?
The switched rail Q403 supplies muting transistors but I can't see it drawing much current if a muting transistor is bad.
R7 (tape head to GND) should be 100k, not 100R that will short out the head and give no signal. Unless it's R1 (in series with tape head) you changed.
dankinzelman:
Thanks for your response.
A few observations:
1. I can't find R402 on the board, and measuring between Q401's emitter and the positive leg of C404 shows no resistance, so I think I have no R402 in my unit. Voltages are a bit low all around (~10-15%).
2. Not sure if this adds any useful info, but the unit takes almost a second to unmute when I press play, sound is initially very distorted as it fades in. Could there be some problem in the muting circuit? It mutes immediately when I stop, but unmuting takes some time.
3. I will look again but I'm pretty sure the resistor marked R7 was a 100R resistor.
4. Can you please tell me a specific part I can get from Mouser to replace Q401? What about a replacement for D406? Sorry, I'm really a beginner and don't want to get this wrong.
floobydust:
I guess the schematic does not match. They might have added R402 later. Another way is to see if the rail is overloaded is to see if Q401 runs too hot.
For Q401, 2SC1846S regulator transistor, it has a built-in insulator TO-126 (SOT-32) package which is quite hard to find, I've never seen that and Panasonic has them as obsolete. "TO-126B package which requires no insulation plate for installation to the heat sink".
It's on a heatsink? Then I'd just use an insulating washer and consider BD139-16 (cheaper than lower voltage BD135 or 137) or 2SD882.
D406 is MA1150 Zener 15V 400mW. (so it's a +14.3V rail I guess, not 12V).
I would use https://www.mouser.com/c/?q=BZX55B15[BZX55B15 or BZX79-C15 for $0.10
Check voltage across filter capacitor C401 is more than say 16V I hope, enough extra for the voltage regulator input.
Muting transistors sometimes go bad and cause distortion or a very quiet channel. I'd leave it for now, until the power supply is working OK.
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