原创 What a lazy motor!

In the early half of the 1970s I purchased (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 analogue 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 authorised 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 synchronisation—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 magnetisation 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.

He submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).