Readjustment Of Power Block - Nakamichi 620 Service Manual

4. READJUSTMENT OF POWER BLOCK
The 620 uses no semi-fixed parts to enhance reliability. As
long as all parts meet the specification, the published
characteristics can be obtained without readjustment.
Generally, no readjustment is required if only defective
parts are replaced at repair.
Observe the following precautions when repairing defec-
tive parts:
4.1.
4.2.
4.3.
Relocating a wiring can cause larger distortion,
Do not relocate the wiring,
Fully tighten or tetighten the screws on the chassis
to decrease the resistance between GND terminals.
If a new semiconductor is installed in the power
block, a perfect balance should be held between it
and the existing semiconductors in the block. An
imperfect balance can cause larger distortion or un-
wanted oscillation.
To maintain a good baiance, connect an 812 100W
load resistance to the output terminal, measure
distortion and check that it meets the following
requirements: (In this case, the residual distortion
factor of the instrument shouid be Iower than the
specified value.)
(1) Output 1 watt, 1 and 10 KHz input Signals;
less than 0.005%
(2) Output 100 watts, 1 and 10 KHz input sig-
nais;
less than 0.007%
Described here are the possible causes for defects
and the recommended remedial Steps:
(1) The characteristic of one transistor does not
match that of the other transistor when they
are used as a pair.
(2) Usually, the idling current of power tran-
sistors Q019 and Q020 is approx. 20mA -
30mA. If it is Iower, the distortion at 1 watt
will increase. In such a case, solder additional
33K12 parallel to R018 and R019 (33Kft),
respectively, from the dip side of the printed
circuit board.
(3) Improper locations of power supply wiring
will increase the distortion at 100 watts.
(4) If oscillation occurs, the distortion will be-
come large at both 1 watt and 100 watts.
Sometimes, oscillation cannot be observed
even on a synchroscope if it invoives fre-
quencies as high as several MHz. Osciilation
may have occurred if the distortion is large
though the external operating voltage remains
normal. Try the following Steps to eliminate
oscillation:
(a) Add ceramic capacitors of the same
capacity parallel to C010 and C011
(0.047^, 100V).
(b) Increase C013 (100pF) (by adding a
47pF in parallel) or decrease the capaci-
tance (by repiacing it with a 47p or
removing it. If C013 is removed, make a
short circuit by jumper wire.)
(c) Short R017 (2712) or increase it to
4712.
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