Service Notes; Circuit Description - Philips PM 3344 Manual

9
SERVICE NOTES
Circuit description
A. PREAMPLIFIER CHANNEL A (Fig. 18)
The incoming signal is fed to a ten step attenuator,
direct or via a capacitor, depending on the position of
switch AC — DC (SK2), which reduces the signal
amplitude by a factor 1, 2, 5, 10 etc. up to 1000, as
selected with switch V/cm (SK3).
Source follower TS101 provides an impedance trans-
formation to drive the following transistor amplifier.
A second source follower TS103 is symmetrically
connected to TS101 to minimise drift.
Furthermore, a provision is made to balance the
amplifier by varying the gate potential of TS103 with
DC.BAL(R3 coarse and R2 fine).
The second stage is formed by emitter-followers TS102,
TS106 and TS104, TS107. They offer a low impedance
signal source for the next stage and for the trigger-
amplifier w$icfi_receives its signals via R120 and R130.
As continuous attenuator CONT (R4) and gain adjust-
ment GAIN ADJ (R1) are connected between the
emitters of this stage, no shift will occur in case of
correct do balance, when continuous attenuator CONT
(R4) is operated (the emitters have the same do poten-
tial).
Moreover, the trigger voltage is balanced before it is
applied to the trigger amplifier.
The FET amplifier is protected by diode GR102.
The third stage and the first stage of the final amplifier
form the series and the shunt feedback transistor
circuit TS108 — TS501, in push-pull with an identical
circuit TS109 — TS503.
The amplification factor of this circuit is mainly
determined by the ratio of the values of R506 (or R517)
and the resistance at the emitter of TS108 (or TS109)
and resistor R508 (or R516).
By means of potentiometer SHIFT (RS) in the emitter
circuit of TS108 — TS109 the currents in these tran-
sistors can be varied.
This variation results in a vertical shift of the display.
Capacitors C111 and C112 and resistor R142 provide
a square-wave response correction.
Resistor R 134 — R 146 and R 124 R 129 provide a
0
constant dissipation of the relevant transistors so that
no sag occurs in case of square waves of low frequency.
Between the third stage and the final amplifier, switch
NORMAL-OFF-INVERT (SK4) has been inserted.
This is a 3-position rotary switch. In the first and the
third position the leads of the pre-amplifier and the
electronic switch are interchanged so that the signal is
inverted 180°. In centre position of the switch the
channel is disconnected from the output amplifier by
means of the electronic switch.
When three channels are switched off, the pulse gener-
ator is stopped and no blanking pulses are fed to the
cathode-ray tube.
The INV-OFF-NORMAL-switch (SK4) is followed by
the electronic switch consisting of diodes GR104,
GR106...GR109 and GR111 and the resistors R144,
R147 and R148. A pulse from the ring counter is fed to
junctions GR107 — GR109 and GR106 — GR108.
If the pulse from the ring counter is
negative
with respect
to the input level of the output stage, diodes
GR 106...GR 109 will be blocked, while GR 104 and
GR111 are conductive. This means that the relevant
pre-amplifier is connected to the output stage.
If the pulse of the ring counter is
positive
with respect
to the input level of the output stage, diodes GR106...
GR 109 will become conductive and GR 104 and GR 111
will be blocked. The output stage is now switched off
from the relevant pre-amplifier.
The pre-amplifier is then supplied with current from
the ring counter via conductive diodes GR106...GR109.
As a blocked diode always possesses a certain capaci-
tance there is some capacitive transmission of the signal.
To minimize this effect the circuit is doubled.
GR106 and GR108 short-circuit the signal, after which
any remaining signal is further attenuated by resistors
R147 and R148 and conductive diodes GR107 and
GR 109.
R144 serves to equalize the current through GR106 —
GR108 and GR107 GR109.