JVC cassette deck won't stop at the end of a tape

[LeeDerbyshire]

My 80's JVC cassette deck no longer switches off at the end of a tape. I found a small metal plate next to a solenoid that disengages the mechanism when pressed, so my guess is that something is failing to activate the solenoid. The solenoid activates and stops the deck when the power is switched off, so I guess that it's okay. There is also a hall-effect sensor on the tape counter that does seem to produce a voltage pulse as the tape moves (I only have a digital meter, so it's very hard to measure, but it looks like there is a pulse of about 10V every second or so, although I'd have thought that was a high output for such a device). So my guess is that there is something between the sensor and the solenoid that is failing to switch it all off. Does anyone have any ideas?

 

[Technomaniac]

Look for a transistor in between, and any electrolytic in that circuitry. And high value resistors. Probably the hall-effect pulses have to keep a capacitor charged up to a certain level, and when the pulses cease the charge falls away and triggers the transistor which pulls in the solenoid. Just watch if you're replacing an electro, there might be a non-polarised one in there.

 

[LeeDerbyshire]

Okay, I did a little digging. I don't know much about hall-effect configuration, but one contact is grounded. I assume another provides power, although a pulse can also be measured there - maybe a side-effect of its operation? The third contact does go to the negative side of an electrolytic (I can't see the value, but it is only about 12mm long). On the negative side of the electrolytic, there is the (very approximately) 10V hall pulse when the tape moves, and then it drops to about 0.3V when it stops. On the other side of the electrolytic, there seems always to be about 20V, no matter what is happening. It never drops. This 20V goes via a resistor to the base of a transistor (again, there is too much in the way to see any markings). Does this all sound normal? Has that capacitor perhaps gone open circuit?

 

[Technomaniac]

The cap COULD be faulty. The end which varies between 10 and 0.3 is likely to be the end which connects (and I'm assuming a bit here) through a resistor to the base of the transistor which should operate the solenoid. The electrolytic connects to the positive rail and so the transistor's emitter is likely connected to that rail also. See if you can find the resistor going to the base, make sure it hasn't gone high, and replace the capacitor. When you have a transistor pulling in a relay or solenoid, you must have a diode connected across the relay or solenoid coil so that the large reverse-voltage pulse produced by quickly switching the coil is suppressed and so doesn't destroy the transistor. If it has that diode, the transistor is probably OK, but if you can get to the transistor leads, with the unit OFF, you can check with your meter that you can read a diode between base and emitter, and another diode between base and collector. Obviously there is no short between collector and emitter or the solenoid would be pulled in permanently.

 

[Technomaniac]

I have just re-read your first posting, and I'm wondering. When you described the sensor as HALL EFFECT I was assuming that you were wrong about this. But now I'm thinking - If it IS really a Hall effect sensor, the only way it will detect the end of the tape is if you have a metallic patch between tape and leader. Do your cassettes have such a patch?

 

[Bluejets]

I have just re-read your first posting, and I'm wondering. When you described the sensor as HALL EFFECT I was assuming that you were wrong about this. But now I'm thinking - If it IS really a Hall effect sensor, the only way it will detect the end of the tape is if you have a metallic patch between tape and leader. Do your cassettes have such a patch?

Might be a pulse from the reel and when the tape runs out , so do the pulses.

 

[LeeDerbyshire]

I have just re-read your first posting, and I'm wondering. When you described the sensor as HALL EFFECT I was assuming that you were wrong about this. But now I'm thinking - If it IS really a Hall effect sensor, the only way it will detect the end of the tape is if you have a metallic patch between tape and leader. Do your cassettes have such a patch?

What I'm assuming is a hall-effect sensor is placed close to a circular magnet on the end of the belt drive pulley that moves the tape rotation counter. As long as the counter is moving, there is a pulse.

Anyway, I removed the capacitor from the board, and (sadly) my capacitance meter says it's good, at 12uF (marked 10uF).

 

[Technomaniac]

Then I guess its down to the transistor or the solenoid or the diode across it. If you ARE getting a voltage change at the solenoid, an electro across the rail feeding the solenoid, or shorted turns in the coil. Or a magnetised core. I HAVE had shorted diodes across solenoids in photocopiers. Sometimes they are hidden under the top layer of insulation. Of course that means you can't reverse the coil connections or the diode would be backwards.

 

[LeeDerbyshire]

The solenoid does work. If the power is switched off when a tape is playing, the solenoid is activated and everything is disengaged. I'm not sure how it manages to do this after the power has been removed, but I guess something is discharging through it.

There are a couple of large black (i.e. not glass) diodes (with thick leads) in the vicinity, and I'll check those, and I'll pull out the transistor and see if I can find a way of testing it.

 

[Technomaniac]

If you have TWO diodes, maybe they're using a voltage doubler (can't imagine WHY) which would give you two more electros to check.