My short-term Betamax emergency has been taken care of. I am going back through the machines that would not function. An interesting pair is the SL-HFR50 and companion HIFI add on unit. When I was testing the machine this unit play clean HiFi for about 10 sec and then quit.
I have pulled the machine apart, found a loose piece of plastic, not related to the issue, and have discover that the problem is somewhat related to the tape take-up reel drive.
The problem is not simple yet. The machine will FF and rewind fine but if you play a tape it stops after a coupe of seconds. In a belt driven machine this may indicate the capstan is not moving. Not the case here.
Upon closer examination I can see the capstan is moving but the tape is not being pulled in and the machine detects this and stops (spaghetti prevention).
I have open up the tape hub drive section, which is a real Rube Goldberg affair. There are many little plastic levers controlling drag, brakes etc..
The drive itself is beltless and uses a motor with circuit board mounted coils, and drive circuitry to move the tape hubs.
The odd thing is the motor does work in the FF and RW but fails to pull the tape in during PLAY. The mechanical levels appear to be in the right positions.
The other odd symptom is that when the FF reaches the end the FF does not shut off.
This leads me believe perhaps there is a fault in the motor drive and it is not properly detecting the motion, or is not driving all phases properly. We are dealing wit intelligent motor control here I think.
My curiosity is leading me down the path. We just finished making a cut-way on a beta tape so we can see what the hub are doing and when the PLAY is hit the take-up reel makes the faintest attempt to turn sometimes and then sits.
However, if I use my finger and apply a microscopic amount of torque to the take-up hub on the top the reel machine continues to play. So the problem is not a mechanical binding but failure of the motor to engage or the electronics to keep the tape hub motor turning.
We know the motor works at full speed/ torque so this is an interesting problem. Perhaps a position sensor on the motor board has failed and is not used at full but only during play.