JVC M71 tape play issue

[BoomboxLover48]

Capstan shaft clearance is normally 1.00 mm to 1.50 mm. I've worked on M70 deck many times and totally swapped a deck mechanism without electrical components from our caution. The deck on my M70 collections had a problem with the record button not going down and stay there. Later I fixed that issue also.
Never had a problem with play mode stopping in a few seconds in any case or REW. If there is any drag in the play rpm the auto stop will activate and that is how the auto stop mechanism is designed to activate. Something is interrupting the smooth running of the take up reel axle which is in turn connected with a belt to the auto stop mechanism.

I would check the auto stop mechanism part where there is a sensor circuit with a reed switch and a ring magnet.

Check the clearance between the reed switch and the ring magnet. If the clearance is more than usual, it will activate auto stop mechanism.
In REW the clearance might me affecting more than in play mode. Reed switch is not detecting the changing magnetic field effectively.

1. Ring Magnet and Reed Switch: The auto-stop mechanism typically involves a ring magnet and a reed switch. The ring magnet is attached to one of the rotating parts of the cassette deck, such as the reel or capstan.
2. Magnetic Field Detection: As the tape plays, the ring magnet rotates, creating a changing magnetic field. The reed switch, positioned nearby, detects this magnetic field.
3. End of Tape Detection: When the tape reaches the end, the rotation stops, and the magnetic field no longer changes. The reed switch detects this lack of movement.
4. Triggering Auto-Stop: The reed switch then sends a signal to the control circuit, which triggers the auto-stop mechanism. This disengages the tape transport by stopping the motor and preventing any damage to tape or deck.

 

[cappedup]

I think I saw that reed mechanism while I was in there. (It’s all buttoned back up now). I remember thinking nothing looked amiss, but then had no idea what I was looking at either.

Right now I have the machine playing tapes through, which is amazing. REW is still non op, but I can live with that.

I really like your suggestions of what to look at there. I will keep that in mind next time it’s apart.

I left it with a slightly wobbling flywheel, I’d tightened up the vast majority of the wobble.

When you say the clearance is 1.0-1.5mm, where is that measuring from/to? Is that the in/out measurement, assuming a straight and true shaft?
My wobble is more like an earth orbit, the travel is ‘not true to the ecpliptic’. The wheel is off by about 1 degree, creating a very slight scraping noise on, I think, the mms device.

 

[BoomboxLover48]

The 1.00-1.50 mm clearance is the "up and down" movement clearance of the capstan (the axle of the flywheel). The capstan goes through two bush brass/alloy/bronze bearings (sleeve bearings- self-lubricating bronze bushings) that are in and out of entrance and exit hole on the chassis. Technically they are in place where it shouldn't wobble sideways. The top to bottom clearance is adjusted by a screw of the top plate. Basically, if you pull and push the capstan, it should not move more than 1.00 to 1.50 mm in or out. Thats the clearance I mentioned.