Servicing Technics RS-M85-MK2 Tape Deck
This deck was brought to me with the complaint that the tape does not work in play mode, but rewind and fast forward was working. I opened and cleaned the set as usual. This set had visited me during May, 2013 with the same complaint. The complaint was due to worn out rubber on the gear wheel that rotates the take-up wheel. What I did was removing it and pasting a rubber belt of same width by cutting it precisely to fit in the slot where the original rubber ring was, then grinding it to shape by fixing a small abrasive stone on a tape recorder motor! Because the take up wheel should rotate in synchronization with the release of tape by the pinch roller.
The wheel should neither pull nor slacken. If it pulls, the tape will get elongated and get spoiled in the long run. If it is slackened, it will get rolled back on the pinch roller. If the collection is not just right enough tight, the tape will sway and the cassette wheel will get jammed. This is such a complicated mechanism that the design specifies the torque, speed and rpm of each wheels right from motor to wheels! But because of my year long experience in handling these, I do it manually without using any of the measuring tools otherwise required for proper adjustment. The set worked very well all these years and was making a revisit to my workshop after a gap of 4 years!
I checked and found the complaint is true. This is a very complicated and complex deck with a lot of wires going up and down and when you look at it, we would not feel like taking up the set for repair. Why a deck is being made in such a complicated way, is a question which remains convincingly not answered for a very long time!
In order to remove the mechanism, we need to remove the control board right behind it, which is covered by a shield, disconnect all the connectors of this board, and then locate the screws (almost hidden from easy view!) mounting the mechanism, and then unscrew them, tilt the mechanism in a particular way, to remove it! I dismantled the mechanism after going through all these complicated procedures.
Removed the inner cover of the mechanism that shields the play and gear wheels (black colour, fixed by four tiny screws, which cannot be directly accessed by a screw driver. We need to hold a long tiny screw driver in a slanting position and unscrew! Otherwise, we have to dismantle the door assembly completely and then we can do it! But that is also not very easy!)
When I could finally see the gear wheel, I saw that it had collected dirt and had a polished look. Rubber wheels have a tendency to become hardened and or collect a lot dust, which will form a slippery coating on top of it, because of the pores on the surface. We need to occasionally clean it using cotton dampened with IPA. For that we need to run the play mechanism, hold the cotton close enough and if necessary use a Zero emery paper. But in the process, care should be taken to ensure that only the dirt gets removed and the rubber ring is not eaten away! I did this delicate cleaning, by applying power to the motor externally from my desktop PS and holding the mechanism in play mode by pressing the solenoid down using a metal clip. We can do this even by removing the gear wheel. But the cleaning may not be even as we may not be able to apply the pressure all around the ring evenly. So, this is the best method, though cumbersome. I could not do the cleaning by running the mechanism by its own power supply, as there is a hall effect sensor for auto switch off. A belt is connected to the counter wheel which has a magnet ring and a hall effect sensor.
Then I connected the board and mechanism outside the unit and tried. It worked well.
Left it run for more than two hours by changing the sides. It worked very well.
I forgot to mention that this mechanism uses two motors. One is a direct drive capstan motor (a brushless DC Motor, with auto adjusting speed regulator) and another small motor fit on the top, which works on, rewind, fast forward and play mode. Its duty is only to turn the take-up wheel or release wheel. Hope you can locate the arrow mark inserted in the following picture (look in the middle of the mechanism). This is a 5V DC motor.
Having observed it working very well, I refit the mechanism back in its place. Again tested it for proper functioning, using its own power supply and control mechanisms.
You can have a look at the inside:
In the second picture, you can see the mechanism inner cover, about which I was referring to earlier.
Now, one word about an omission that I made by an oversight, at the time of checking the set to see the complaint. When this set was brought to me for the first time, I was told that the set had undergone a modification in the power transformer. Instead of 120V AC input, the Tx was changed to 230V input.I could have immediately marked on the transformer and/or at the rear so that this is known to any technician who opens the set. But somehow, I overlooked it and paid heavily for it this time. Observing what is written on the rear cover, I used a 230V to 120V Converter transformer before connecting this set. But when I tried to play with it, it did not work at all. As I had totally forgotten about the modification, I was groping in dark as to what happened. I looked through the service manual that I had downloaded during its first visit and found all voltages almost half! So combed the power supply board for any short circuits or other complications, removed some of the semiconductors and checked. What to say, I did all sort of stupid things that ought not have been done, including suspecting the mains transformer and repeatedly checked whether it was getting hot due to a partial short! When finally, better sense started working, I called up the customer and asked him whether the set was working otherwise well in his home, or was it not at all playing. He told me it was working very well except for play. Then I asked him, what type of converter transformer he is using for working this set. At that time, he reminded me that it works on 230V AC in! How many hours that I have wasted because of one negligence! This was a lesson for me! So, I marked it this time promptly:
Played the set continuously without any problem. This set has an odd looking rear, but easy for the user. This design is not at all advisable for the rear panel, as dust will directly enter into the contacts. You can also see a remote control socket. But this set is bought by the user from second hand market, and it came to him without the remote.
Thus, this set, which has already a pain-in-the-neck look, gave me enough ‘labor’ pains, though the net result was satisfactory! Wiped off my sweat and fit it back to its original shape!
This article was prepared for you by Parasuraman Subramanian from India. He is 68 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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