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Old 17th Sep 2018, 3:19 pm   #1
MeerKat
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Join Date: Jan 2016
Location: Ramsgate, Kent, UK.
Posts: 296
Default Akai GX-77 motor & other problems.

For various reasons I acquired this deck untested, to save length of narrative I will not detail them here.

When I first applied power nothing happened, no lights or movement, nothing in fact. Inspection of the PSU board showed F4 blown, this fuse provides 4 supplies via an IC regulator, 2 discrete transistor regulators plus the raw unregulated rectified capacitor smoothed supply. These 4 supplies feed a lot of the electronics and the capstan motor.
Searching the net revealed the motors are weak points plus the complicated mechanicals will fail to work properly once the grease starts to harden, as this deck is at least 33 years old and has seen a lot of service I started looking in these areas.

I disconnected the supply to the capstan circuit and this cured the fuse blowing problem. To check it was only the motor and not the control circuit I connected my low voltage current controlled supply direct to the capstan motor and sure enough it was virtually short circuit, as I cursed my bad fortune I left the supply on at just over an amp and low and behold the short suddenly cleared and the motor spun up. Now these are DC motors so must have brushes and after extended service a lot carbon brush dust will be generated and can short out the motor, I suspect the current had blown a path through this dust removing the short thus allowing the motor to run. I will need to strip the motor (if possible!) to confirm this, clean it and see what condition the brushes and commutator are in, I may be able to get some more service out of it, it's dated Dec 81 so is 37 years old!

I restored the supplies to normal and switched on, the capstan spun up no problem but when I pressed the Tape Loading button I heard a solenoid actuate but the tape did not load so I need to look into that, hardened grease I suspect.

Now thinking about the way forward, I plan to get the loading sorted and the mechanicals to a state where I can do a high quality recording and see the difference when switching between source and tape, if my old ears can't hear a significant difference the heads will be good enough for me. I don't have test tapes and associated test gear so this method will have to suffice.

If the heads are acceptable I will completely strip the mechanicals, clean and re-lube with silicone oil and then I'll have to tackle that capstan motor.

Any advice will be gladly received.
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Last edited by Station X; 24th Sep 2018 at 12:16 pm. Reason: White space added.
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