Service manual

The HOP (located on the SSB) generates the line-drive pulses (LINEDRIVE1), which
have a frequency of 31468 Hz (T = 31.77s).
When the LINEDRIVE1 signal is high, TS7409 and TS7408 will conduct. A constant DC
voltage will be applied across L5410, causing a linear increasing current through this
coil. The secondary voltage of L5410 has a negative polarity so that TS7421 will block.
When switching on the set, the current through L5410 is supplied by the 5V2 Standby
supply (via D6407), and taken over by the +11D voltage (via D6408) of the main supply.
When the LINEDRIVE1 signal becomes low, TS7409 and TS7408 will block. The
voltage polarity across the primary winding of L5410 will invert. The positive voltage on
the secondary winding will now drive TS7421 into conductivity. Because of the storage
time of the line transistor (TS7421), L5410 cannot transfer its energy immediately to the
secondary side. This may result in high voltage peaks on the collector of TS7409 and
TS7408. To prevent that these peaks will damage the transistors, a 'snubber' circuit
(C2414, C2412, and R3411) will suppress them.
When the LINEDRIVE1 signal is high again, the above-described sequence starts
again. Circuit L5411 and R3409 will increase the switch-off time of the line transistor.
The line stage will be started via the 'slow start' principle. During start-up, the HOP
generates line drive pulses with a small TON and a high frequency (50 kHz); TOFF will
be constant and TON will be gradually increased until the duty-cycle is 50 % (normal
condition). The time interval from start to normal condition takes about 150mSec.
When switching off, the same procedure is followed, but now in reverse order.