Pinch Roller; End Of Tape Switch - Revox G36 Service Manual

2.1.4 Pinch roller

The pressure of the pinch roller can be measured by a gauge fixed to the pinch roller
arm as close as possible to the spindle of the pinch roller (fig. 4). A constant tone tape
should be played and the pinch roller withdrawn by the gauge until the note is heard
to change. A reading in the region of 1.7 kg should be obtained. Adjustment to this
value can usually be obtained by turning nut no 13 in fig. 5. Should this not provide
the necessary amount of correction it is advisable to check the position of the solenoid
no 11 in fig. 5 which will be correct when there is approx. 0.5 mm clearance at point
A (fig. 5) with the solenoid energized. It is essential that the solenoid's-slug fully
bottoms in the energized position and when properly adjusted, there should be no
movement of the slug when lifting the pinch roller off the capstan. Correct setting can
be obtained by loosening the nuts (12 in fig. 6) and moving the solenoid in the
required direction.

2.1.5 End of tape switch

A. Photoelectric end-of-tape-switch
Recorders with serial numbers from 58 000 onwards are equipped with a light
sensitive auto shut-off device. It consists of a light source (Osram 3644), the
photoresistor ORP 62 and a printed circuit section containing two transistors (see fig.
1).
The photoconductive cell is housed in the tape guide pin on the right hand side of the
recorder. It has a dark resistance of greater than 100 kΩ and this value drops to below
3300 Ω under illumination. The associated current amplifier energizes the push
button release solenoid when light reaches the photoconductor. For normal operation
of the recorder the sensing element is at its high dark resistance and both transistors
are biased into cut-off. Illumination of the photo-resistor changes the bias condition
on T1 thereby raising its collector current. A voltage drop develops across the
solenoid winding and T2 begins to conduct. Since T2 opens a current path parallel to
the photoconductor, positive feed back action sets in which causes the collector
current of T1 to rise quickly into saturation.