In the first half of the 1970s I bought (second hand, of course…) a monster cassette tape deck. Monster for many reasons -- I'll just name a few.
While back then the energy to engage and disengage the tape heads and mechanisms usually came from the user's finger, that deck employs (big) solenoids and a control logic made of relays and SCR. This permits functions like auto repeat and (wired) remote control. Pure science-fiction at the time.
It also featured a Dolby-B that could be calibrated, all made by discreet components. No ICs! No LEDs! It only had micro-miniature lamps, of course. The capstan was directly the axle of a brushless motor (direct drive!) working also as a flywheel.
After years of faithful operation I started hearing some flutter, then suddenly the motor stopped. I dismantled the deck to investigate. The brushless motor control part was all analog with germanium power transistors. I made some static measurements and checked some wires and soldering: Nothing was wrong. I turned the deck on and the motor ran. How could I fix something that was working?
After a week or so the motor stopped again. Dismount again and repeat the checks -- nothing. I turned the deck on and the motor runs.
Things repeated like that, with up-time intervals randomly going from a day or less to a couple of months. Then one day my hand slipped, and I made a short on the motor connector while the circuit was energized. I saw a spark.
Okay, I knew it, now either the motor or the controller was gone -- R.I.P. Wrong: the motor was running beautifully and kept working for a year and half! Then it stopped again. I repeated the checks and the situation was a replica of the pre-short era.
My patience reached the limit and I bundled the deck off to an authorized repair shop. It took more than six months and a lot of money to get it back. The motor worked barely a week. Replacing the motor and controller was impossible, as it was too difficult to find a replacement and too costly. Time to buy a new deck!
Years later I tried once again to investigate: After measuring the continuity of the rotor-position-sensing coils of the motor with an ohmeter, I discovered that the motor worked for a while. Got it!
Here is what I diagnosed: The position sensing coil's permanent magnets slowly lost their magnetism. The controller had an insufficient, weak position feedback and was missing synchronization -- therefore the motor couldn't run. The DC current of the ohmmeter (mind the polarity!) restores a bit of magnetism and the motor runs for some days. Obviously, when I made the short, the current was more intense and the magnetization lasted much longer.
The fix was then a no-brainer. That deck is still operating in my backup Hi-Fi stack.
Spagni Maurizio has been an electronics enthusiast for a long time, He transformed his hobby into his job. He's an electronics designer but often does some repairing for himself and his friends for every kind of electronic device.
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