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| Troubleshooting Noise in Cassette Player Technics RS-M17 |
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| dankinzelman:
Hi there, this is my first post here. I am a beginner with a little bit of experience repairing and modifying tube guitar amplifiers. I am currently restoring a Technics RS-M17 cassette player. I have repaired the transport and rebuilt some switches, and now it makes music, but there is some really bad noise in one channel (actually in both, but its much worse on the left so I want to fix that problem first). I've attached a complete schematic as well as a zoom on the area where I think the noise is originating. I have very little to no understanding of solid-state amplification circuits so please be patient with me. The heads feed into a preamp IC called the QVITA7122BPB (the chip itself just says TA7122BP), apparently no longer available. I put a scope on the output with the motor muting switch shorted and found the RCA output noise is highly correlated to the ouput signal at pin 6 of the IC. I've attached a sample of the noise, with transport stopped and motor muting switch shorted (ie no signal arriving from the tape heads). As a first step I tried swapping the ICs between left and right channels and the noise stayed on the left channel, so I presume there is a noisy passive component somewhere in the network surrounding the chip. There is no perceptible noise on pin 2 of the chip, which I presume is the input pin (right?). Shorting pin 5 to ground seems to significantly reduce the noise, but I did it by accident and don't want to damage anything so I've only done it a few times briefly, and don't know if the chip still amplifies with that shorted to ground, just that the noise is significantly lower with pin 5 grounded. I tried swapping C11 with a new part and the noise did not go away. I'm not sure where to go from here. What should I look at next? I'm happy to simply follow orders, but If you feel like explaining your reasoning, it would be much appreciated. I'm also happy to read articles/watch videos if you have any relevant suggestions. I'm here to learn. Also, any suggestions for easy modifications I can make to improve the noise/distortion performance of the player, especially easy drop-pin replacements for outdated ICs or transistors would be appreciated. The transport is in good shape and seems well built, so I think it could be a nice project to try to get it sounding as good as possible. |
| coromonadalix:
have you check all the supply lines ?? the head wires grounding ?? had some who simply cutted out ver the years i redid a old high quality Sony deck "Esprit series", redid all the psu capacitors, and the noise floor drastically dropped if inside you have looong slide switch for record playback etc .... clean them with deoxit .... check pcb groundings screws / posts |
| dankinzelman:
--- Quote from: coromonadalix on November 29, 2024, 12:07:49 pm ---have you check all the supply lines ?? the head wires grounding ?? had some who simply cutted out ver the years i redid a old high quality Sony deck "Esprit series", redid all the psu capacitors, and the noise floor drastically dropped if inside you have looong slide switch for record playback etc .... clean them with deoxit .... check pcb groundings screws / posts --- End quote --- Thank you. I already disassembled the looooong slide switch and cleaned it. I also lost one of the contacts, but it's the one that turns the recording led on, it could have been much worse. I will check solder joints for the head grounds and pcb groundings. The chassis is floating (no earth ground). Should I install a 3-wire cord and give it an earth ground reference? The case is mostly plastic, metal parts like top cover, face plate and part of bottom cover are connected by aluminum grounding straps. What point should I use as my main ground reference for measuring/checking ground? Negative termanl of first filter cap? |
| coromonadalix:
take your line in ground as reference, but you should have a good ground line on the pcb, test points etc ... i did saw once ground shield of the heads wiring cut .... to my knowledge i never saw the 7122 going bad, mostly connections / solder / the famous rec play switch, and supply issues |
| floobydust:
A very common problem with cassette decks is a dirty rec/play switch. Best to turn the unit so the switch is vertical, put paper towel under it (switch) and spray a contact cleaner with PPE like DeoxIT. DO NOT GET ANY SPRAY ON THE BELTS, CLUTCHES, IDLER WHEELS ETC. ANYTHING RUBBER. Then, move the switch back and forth 20 times to scrape off the oxide. If you don't have contact cleaner, you can move the rec/play switch back and forth by hand a zillion times. Not with power on and a tape in, that will cause erase blips on the tape and wreck any recording. Back in the day, we would press the record tab sense button up, and move the transport play/stop many times as another way to move this switch. edit: added board pic, it's the long switch with the connecting arm. If someone is aggressive with a tape head demagnetizer, the head's output voltage can blow the head preamp IC. This is rare but does happen due to transformer action and the high level magnetic field. I would not fault the IC until the switch is confirmed clean. In play mode carefully measure the DCV on each pin to look for trouble. Other things that go wrong (rarely) is the head coax cable breaks due to flexing. But an open-circuit there will crackle of cause hum pickup. |
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