Pioneer tuning fork Service Manual page 24

a.
Disconnect R l and connect the emitters of Q3 and Q4 with
the output terminal via two resistors as illustrated in Fig. 2.
Their resistance should be about a half of R l . Low
resistance will allow excessive current in Q3 and Q4 and
will damage them. High resistance will decrease the out­
put. Their rated power is about 1/4 W .
Driver stage
Q3,
Cut
R1 1/2R1
Cut 1/2R1
Cut
Q4
Fig. 2 Circuit (c) — Grounding Emitters
The function of R l
In the circuit (c) R l inserted between the emitters of Q3
and Q4 is for maintaining idle current in Q3 and Q4 to
avoid non-linear range of transistor amplification charac­
teristic.
The current has been made about 5—10mA. In Fig. 4,
when the potential difference between T P 1 and T P 2 is set
to 60mV, Q F s collector current becomes 136mA.
Q P s Hfe is 110. Then the base current becomes 1.23mA.
To flow 5mA current in Q 3 , the current from (A) to ( § )
should be 3.8mA. To make a current of 3.8mA in R l , the
potential difference between (A) and ( § ) should be 1.26V
(0.6 + 0.06 + 0.6). So R l should be 330 O .
22
Power stage
+B
Q1
Cut
Output
Cut
Q2
- B
b .
Insert diodes in place of power transistors as shown in
Fig. 3. Leave R l connected.
Use powerful rectifying diodes used in mains current rec­
tifier or open the output load before checking the circuit
because the current in the diodes becomes larger than one
ampere when an input signal is applied although the cur­
rent is negligibly small when no signal is applied.
Diodes' polarities should also be verified to avoid damag­
ing them.
Driver stage
Q3
Cut
R1
Cut
Q4
Fig. 3 Circuit (c) — Inseting Diodes
Driver stage
Q3
5.03 mA
1.23 mA
R1
3.8 mA
330
Q4
Fig. 4 Circuit (c) — Emitters Floating
Power stage
+B
Q1
Cut
Output
Cut
Q2
- B
Power stage
+B
Q1
136 mA
TP1
0.22^
0.06V
Output
0.22
TP2
Q2
- B