M90 : Cassette head won't stay up

[ampitupco]

I've got an M90 that works pretty well with the exception of playing a cassette. When I press play, the solenoid thunks, the head and pinch roller rise up, a tiny bit of sound can be heard, and then the head and pinch roller retract and the deck stops. This happens very quickly.

Fast Forward and Rewind work well.
Belts have been replaced (including the tape counter)
I've checked (with a multimeter!) and replaced the fusible resistors.
I've verified the reflective gear is in place, and a red light is visible on it that turns off when the deck stops.
No gears are cracked or broken that I can see.

I've read many of the posts here, but none seem to be related to this issue I am seeing. I feel like most everything that I can inspect is working and/or set to spec. I appreciate any response.

 

[Transistorized]

I am not a professional but I will tell you my thoughts on this issue as a guess and what I would be trying to help narrow the possibilities. Keep in mind, these things are relatively too complex to diagnose over the computer but here's what I would be thinking.

Since FF and Rewind work, this would seem to indicate that the magnetic hall effect sensor on the tape counter wheel is working. I am assuming that it does auto-stop on FF/Rew when the tape reaches the end correct? If so, I do not think that is what is triggering the logic circuit to shut down on play. I am thinking it may be more to do with the electronic eye / geared wheel issue or a possible mechanical deck issue. I am more mechanical minded than diagnosing logic circuits so I will focus on the deck to help you find out if its the deck or the logic circuit.

As far as play is concerned, you can manually engage play with the radio powered down and unplugged. Manually move the solenoid plunger pin inward towards the coil and hold it down while you rotate the flywheel in the correct direction. You will need to hold it down until the heads start moving upwards at least 50% of the way before you release the solenoid. Once the heads are fully raised up, the decks gearing should lock the mechanism into play. You should be able to sit there all day and turn the flywheel with no further mechanical reaction from the deck. If so, then your issue is not mechanical and is likely the logic circuit. If when done manually, it does cycle back around to stop, then you have a mechanical issue with the deck that needs sorted. If it does stay in play, to put the deck back into stop position, manually move the solenoid momentarily and release while turning the flywheel and the decks gearing should release the head assembly and return to stop.

This test, while not giving you an exact "there's your problem" scenario, will tell you if your issue is mechanical (deck) or electrical (logic circuit).

 

[Superduper]

You will need to hold it down until the heads start moving upwards at least 50% of the way before you release the solenoid.

For play, solenoid should be actuated for 3/4 of 1 rotation of the capstan.

As for gear, make sure the reflective sticker is present, I believe you said it is. The photocoupler has 2 devices on it, an LED and a photodetector. The photodetector, if factory, has maximum sensitivity at 800nm. That is infrared range, and not visible. You said the light is red. Red led that is visible will be less than 800nm. That doesn't mean that the detector can't detect it, and it's been ages since I've serviced an M90 deck and I can't say I recall ever seeing the photocoupler assembly emitting LED light, I might be wrong. Infrared LEDs might have a slight red glow as those emissions are at the edge of the spectrum range emitted, but the primary emission should be in the infrared range. If that LED was replaced with a regular red LED, it may not be emitting the necessary light that the photodetector requires. Conversely, if the infrared emitter is working but the photodetector is not, then the lack of a signal tells the system that the capstan is not turning and triggers a turn-off signal. You may be able to tell by observing whether the solenoid is triggered twice. Once to initiate play, and a second time to trigger shut off. However as you said this happens very quickly, you may not be able to see it, and an oscilloscope might be required to see these dual triggers to the solenoid.

 

[Tinman]

I haven't worked on one in a while myself but I do remember the light being visible if you cover over it or have the lights off.
It's VERY dim but visible.
If it was changed with a regular LED, it should be easy to see.

 

[BoomboxLover48]

I haven't seen any light in the deck. Never seen any lights from the interior of the deck assembly. Next time when I get a chance, I will pay more attention to this.
See some information below from my business presentations on UV Coatings.
It gives some idea on the visible spectrum.

I was a Coatings expert once.