Service manual

At start-up of the main supply, C2515 can be assumed as being a short-circuit. UAB will
be 15 V (R3513, D6510) and UGS of the FET will be +5.4 V (voltage division over
R3512 and R3518). The FET will be driven into saturation (same as closing switch 'S').
The drain-current will increase linear in time. With other words: resistors R3513 and
R3518 will start the oscillator.
The voltage across the co-coupled coil (4, 5) is also positive and will keep the FET into
conductivity.
The drain-current will also flow through R3514//R3515. The voltage on the base of
TS7502 will be +0.8 V due to the stabilization circuit (which is explained further). At
increasing current, the emitter-voltage of TS7502 will get more negative. When this
voltage reaches -0.7 V, TS7502 will be driven into conductivity and consequently
connect the gate of TS7504 to earth; the FET will block (same as opening switch 'S').
The maximum drain-current is 0.7 V/ (R3514//R3515)= 1.6 A.
The voltage polarities on L5506 will invert, keeping the gate of TS7504 negative via the
co-coupled coil (4, 5). The voltage on the secondary winding of L5506 (7, 8) will be
positive, generating the +28 V audio supply voltage via D6507 and C2512.
The sudden current interruption in the primary coil will induce a counter-e.m.f. that
wants to maintain the current via the 'freewheel' diode D6508. This current is linear
decreasing in time and as it is also flowing through R3414//R3415, TS7502 will be
blocked after a certain period. The gate of the FET will be again made positive, is driven
into conductivity and the cycle starts again.
Stabilization of VBAT:
The output voltage VBAT will be determined by: VBAT = VIN * TON/(TON + TOFF) =
VIN * duty-cycle.
To stabilize the output voltage, a feedback loop is implemented, which will reduce TON
when VBAT increases and vice versa.