Elec T Ri Cala Djus T M En Ts; Ad Ju St M E Nt S ,A Llmo De L S - TANDBERG 3400 X Service Manual

1 1 . ELE C TR I C AL AD J UST M ENT S
1 1 . 1 OU T P UT A M F LI F IE RS ( 3 4 OO X } .
Quiescent Current and Symmetrical
Clipping (3400X)
The amplifier symmetry (balance) is determined by
fixed components and by d.c. feedbak to stabilize
the circuits against temperature variations. There-
fore in general it will not be necessary to alter any
component values.
Deterrnine the quiescent current by measuring the
voltage across emitter resistors R813 and R814
( R 913 and R 914) us i ng a d .c . v ol tm ete r.
Adjust R80B (R908) to a meter reading of approx.
17 m V givi ng a quie s ce nt c urr en t of 3 0 m A .
Check the symmetry (balerrce) with an oscilloscope
connected in parallel with a 4 ohm resistor to the
LOUDSPEAKER
outputs position 2, external
speaker only. Apply a400 Hz signal to the amplifier
input and increase the input voltage until clipping
is observed on the oscilloscope.
The clipping should be symmetrical eurd should
occut at an output voltage of 7.7 V (corresponds
to 15 W in 4 ohm s ) .
Another method for checking the balance is to
measure at the junction between the emitter
r esis tor s R B 13 and R 81 4 (R 91 3 a nd R9 14 ). Wh en
the symmetry is correct, the voltage is half the
supply voltage. The balance can be adjusted by
altering the value of the resistors RB00 (R900) or
R801 (R 90 1). If th es e re si s to rs h av e to be a l te re d
drastically, it is an indication of some other kind of
fault in the amplifier.
Checking the Distortion (3400X)
The maximum permissible distortion from the
output amplifier at full output power is 1%. To
check the distortion, connect a distortion meter
and an a.c. voltmeter in parallel across the LOUD-
SPEAKER output, position 2, for the channel in
question. The output must be loaded with a 4 ohm
resistor.
Feed a 400 Hz sig'nal to the RADIO input when
the recorder is set in the amplifier mode.
Increase the lcvel of the input signal so that the
o u t p u t a . c . v o l t m e t er i n d i c a t e s 7 . 7 V ( 1 5 W i n
4 oh m s ) . T he di s t or t i o n s h o u l d no t e x c e e d 1 %.
Tone Controls (3400X)
Connect an a.c. voltmeter to each of the LOUD
SPEAKER outputs, position 2 (external spe:rker
only) and set the recorder in the stereo amplifier
mode (none of the function selector button depressed).
d e p r e s s ed ) .
Set the tone controls to their centre positions and
feed a 1000 Hz signal to the RADIOinputs.
Set
the INPUT LEVEL ard OUTPUT LEVEL controls
to obtain :rn output level of approximately
200 mV. Use the reading as 0 dB reference and
alter the frequency to check that the bass control
has t 15 dB range at 100 Hz. Check the treble
r e s p o ns e t o b e + 1 5 d B a t 1 0, 00 0 H z .
I-ig. 11.1 ;Tcljustments points on thc output bctartl