Service manual

Transistor TS7100 will be driven and a negative voltage will be transposed to the
emitter of TS7101. When TS7101 conducts, the gate of the FET is at earth potential,
forcing the oscillator to stop. Due to the load, the secondary voltage UOUT will
decrease. At a certain voltage, optocoupler TS7104 will block and the oscillator will start
again.
Since there are no capacitors and there is a high amplification-factor in the feedback
circuit, the feedback is ultra-fast. This is why the ripple on UOUT is minimal. The
negative supply voltage (-20 V) used in the feedback circuit originates from the co-
coupling coil and is rectified through D6103.
Stabilization is not effected through duty-cycle control but through burst-mode of
TS7100.
Burst-mode is load dependent. If the power supply is less loaded, the secondary voltage
will have the tendency to increase more rapidly. If the load on the power supply
increases, then the oscillator stops less often, right up to the moment that the oscillator
is operating continuously: maximum load. If the power supply is now loaded even more,
the output voltage will decay. The maximum load is determined by the maximum
primary current set by R3108//3118//3119.
Protection:
If the optocoupler would fail, the secondary voltage will increase. This would have
disastrous consequences since many ICs (e.g. Painter, flash-RAM, DRAM) are fed with
this 5.2 V. In other words; very expensive repairs would be required.
We already know that the negative supply is directly dependent upon the secondary 5.2
V because of which the negative supply will increase proportionally as the secondary
voltage increases.
If the negative supply in the mean time reaches -30 V, D6106 will start to zener and
therefore TS7101 will start conducting. D6106 will take over the stabilization task of the
optocoupler, however, with a considerable spread: from -20V to -30V is a 50% increase,
thus UOUT will increase from 5.2V to max. 7.5V.