AKAI GXC-760D: Servicing the Reel Motors
While using this tape deck I have noticed that the tape transport stops completely from time to time. As I have changed the only two belts in this machine there is nothing else suspect but the reel motors. In my case the take-up reel motor. So it is time to dismantle it and see whether I can fix it.
In this deck, the reel motors are of type GSM-300 and have three functions: motor, generator, and magnetic brake. These motors are now no longer available (NLA). So fixing them is imperative in order to use the GXC-760D machine. Otherwise another solution would be to find a similar deck for spare parts and recover the motors hoping they won't exhibit the same issues.
Let's go with the problematic reel motor first. I won't detail the removal procedure since it is straightforward if you study the mechanical block.
This is the motor outer shell. There are two screws that secure the bottom cap in place.
This is the motor inner shell. There are three screws that secure the bottom cap in place.
This is the stator with the beautiful copper wire. You can clearly spot the carbon and oxidation build-up on the collector. I am going to remove this altogether and clean everything.
The bottom cap contains the carbon brushes. In this motor the brushes are quite worn. But there is nothing I can do to straighten this up. What I can do however is to replace the dampening sponge material between each brush arm and the tensioning spring. Well it is not quite a spring but it works similarly. This material is worn already and it easily disintegrates when touching it. I have cut two pieces from a similar material that I had around and then glued them in place of the old dampening material.
This is the restored motor stator.
The motor is reassembled.
I have reapplied a drop of quality grease to the bronze bushings. The motor was installed back from where it came from.
Since I was at this point, I decided to work on the supply reel motor as well. The steps are identical so I will just put some pictures.
The motor servicing operation is lengthy and requires patience. It took me about one hour per motor from disassembly to reassembly. You need an additional hour to disassemble the mechanical block and about the same time to reassemble it as it was.
While working on these motors I also preventively changed two electrolytic capacitors on the motor power supply distribution printed circuit board that is bolted to the mechanical block. These are rated 0.47 uF / 50 V. I have replaced them with Kemet modern parts rated 470 nF / 50 V. Normally I would have changed these on a full capacitor replacement job. But given this PCB is hard to reach and I already had the mechanical block disassembled, the operation seemed straightforward.
This is the completed board.
Next I was eager to test the serviced motors. I have reassembled the mechanical block and loaded a cassette. The two motors accelerated very quickly and snapped my tape in a split-second. I observed that both the supply and the take-up reels were spinning in reverse. I cannot explain why this was happening since I noted carefully the way the wires were originally soldered. Upon re-soldering I have carefully respected these notes. Total disaster. After inverting the connections to both motors it appears that everything works as it should. Play mode delivers constant torque. Fast forward and fast rewind is smooth again.
Now I will throw-in my 2 cents opinion. One should never ever run these motors on full speed without a tape loaded. The motors will accelerate very fast and the brushes will quickly deteriorate. The collector will become clogged in carbon dust and spark debris. In the end the motors will either work intermittently or loose torque. Or both at the same time. I would go even further to say that it wouldn't be advised to fast forward or fast rewind tapes on this old deck. Once the brushes are completely deteriorated it will be very hard to replace them. Assuming that this operation can be done.
That's about it. Off I go to listen some tapes now.