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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 17th Sep 2018, 3:19 pm   #1
Andrew64
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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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Old 25th Sep 2018, 8:37 pm   #2
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Default Re: Akai GX-77 motor & other problems.

Progress made, see photo, the catch was not moving when the tape loading solenoid actuated due to hardened grease on the pivot (arrowed). The wheel and associated parts was stripped, cleaned and re-lubricated, when all was reassembled tape is now loading fine.

A close inspection of the 6 heads shows a slight ridge low on the right hand (forward) erase head and high on the left hand (reverse) erase head, no ridges are detectable on the other 4 heads although the shields make visual inspection harder but I can still run the nail of my small finger over them. The tape will always be in contact with the erase heads on this deck but the tape is lifted away from the inactive record and play heads depending on the direction the tape is moving, this will of course reduce the wear rate to approximately half that of the erase heads.

My thinking that the heads are in a reasonable condition was confirmed when I carried out a tape/source comparison test, I used a Deutsche Grammophon CD of the Planets as source material. My 67 year old ears could not detect any HF drop off at either speed but the background hiss on quiet passages was noticeably lower at 7 ips, to be expected me thinks! I am not too precious about these things so good enough for me.

Fast wind/rewind is problematical on these decks, mine is fair I think, the driven spool slows quite a bit as it becomes full of tape but nowhere near stops, I see online, mods concerned with fast wind/rewind problems. One involves adding relays and circuitry to apply some volts to the supply spool motor to help matters, I will not go down that road, I think there is a better way if needed, more on that later.

I have had both capstan flywheels out for shaft/bearing cleaning and lubrication, also the entire belt path has been cleaned, the motor pulley requiring most attention. I have decided not to completely strip the mechanicals, wiping of accessible sliding parts and lubrication will be done. All rotating shafts/bearings will be stripped/cleaned/lubricated although of the ones done so far none were dry. I haven't done the motors yet, I foresee problems in that direction as they run warmer and will tend to dry out more. I will need to study carefully how they were assembled, anybody have experience of stripping the three GX-77 motors for re-lubrication?
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Old 10th Oct 2018, 4:44 pm   #3
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Default Re: Akai GX-77 motor & other problems.

I have done all the rotating shafts/bearings accessible from the front now. Before stripping I mark the position of adjustable parts and note any end float where applicable. I take lots of photos and only do one assembly at a time, re-install it and go on to the next. I changed the reel table drive belts and a drop of oil was applied to each of the reel table drive motor shafts. The slide for the tape loading roller was cleaned and lubricated when I had the loading roller out, the tape motion detector disc is a press fit on the roller shaft, tightish but easily removed with care, again I took careful note of the end float and restored it on reassembly. Pots and switches have been treated to a moderate amount of Servisol

I have re-tested playback performance in both directions and my ministrations have not caused any noticeable deterioration and the various switches and pots don't crackle or dropout any longer.

And now to that capstan motor! Wish me a clear head and steady hands guys.......
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Old 15th Oct 2018, 11:25 pm   #4
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Default Re: Akai GX-77 motor & other problems.

The Capstan Motor strip.

Before removing the motor I recorded a mains sine wave signal at 3 and 7 ips. I compared these using Lissajous figures on my Heathkit IO-18U scope. This revealed that at 7 the playback gives a steady but slowly changing phase relationship, at 3 there is some wow, the trace stays steady for about second then advances a few degrees and steadies again, this repeats over about a 2 second cycle, notwithstanding this I cannot detect wow when listening to music recorded at this speed so it is not that bad. The figures also revealed the speeds are not an exact multiple of each other, which one or maybe both is is out I didn't fathom, I have requested a suitable tape in the 'Wants' section and if some kind person can make me one I will use that at some stage to correct matters.

I have carefully stripped the motor now and no real surprises here, It's an electrically/magnetically and acoustically well shielded permanent magnet DC motor with 3 coils on the armature connected to a 3 segment commutator, it runs in a pair of self aligning plain bearings. At the front end of the motor is a toothed wheel that with a corresponding assembly attached to the magnet provides the motor speed dependant frequency feedback required by the control board. The pulley is a slip fit on the shaft held in place with a smear of Loctite or similar. It had a quantity of black brush/commutator dust as I suspected it would. After a blow out and inspection it is apparent the internals are mostly in fair condition, no apparent wear to shaft/bearings. The exceptions are the brushes/commutator....the non-technical description 'shagged' comes to mind! See the photos. The com OD is 5mm on the unworn section and 4.75mm across the humps, now I'm estimating it will be down to about 4.5 mm in the valleys, how much is gone from the brushes is anybody's guess.

There is some good news though, the lack of spark erosion on the segment edges indicate not a lot of sparking has been going on. In the relaxed state there is about 1.5 mm between the brushes so they won't 'lift off' the com anytime soon. I estimate there is about mm of brush left before the metal cups touch the com so there is some life left in the old dog yet!.

What to do? To refurbish a motor such as this you can skim the com down to get to rid of the grooves and fit a new set of brushes. Skimming this com is problematic – even if you could find somebody to do it, it would be easy to wreck it and as for finding brushes well they are probably unobtainium even on the planet Unobtainia. (Unless you know different, PM me if you do!). In view of this I have decided to not attempt any repairs to the com or present brushes, just carefully lube, reassemble and reinstall the motor.
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