Classe DR3 pwr sm

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VCLASSE AUDIO
DR-Z(3)

PURE CLASS A

POWER AMPLIFIER

DR-Z (3)
SERVICE MANUAL

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INIRDDUCIION

The design, construction and quality control of the DR-2 provide
very high reliability, MI'BF, and serviceability. Beyond our
crmtrol, however, lie external factors, or the hidden ill-fate
of a particular wnponent, which may cause unit failure. When
failure does Occur, it is limited to a. few parts which are

easy to find, reach and replace quickly. Sane or all of the
following parts may be involved (for each dxannel)

* OUTPUT TRANSISIORS - there are four ; two NPN devices on the
front heat sink and two PNP devices on the rear heat sink.

M are .
* R 5 and 3 - on the PCB, just below the connector. They

are both 75 ohms, 5 watt, 1% metal film. They feed the plus
and minus supplies to the PCB circuitry. When output transis-
tors fail, one or both may burn.

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* 16 - the bias trirrpot, W The value is
500 ohrrs, 20 turn. Occasionally it may be burnt and inoperat-
ive, but this is usually NOT the case. DO mm ASSUME: THE
TRIMPOIHASFAILEDIFTHEPARFISMERELYCHARREDONTHE ..
OUTSIDE ONLY, AS A RESULT OF R 31 BURNING. The trinpot can be
checked by measuring resistance across PIN 2 and 3 of the PCB

»wmector J 1. Turn the trimpot to see that its value
dianges normally. If it is OPEN or seized on one value,
replace it.

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* C 2, C 6 and C 11 - these are the three electrolytic capacitors. The value is lOOOuF/35 volts (680uF (1 So/yo)
in earlier units). One to three (rare) may blow. It if has OK 3
\N .

blown severely, it is obvious because it literally blows

apart. Note, however, that the capacitor "vent" faces down,

so look around the base of the part for a yellow residue, to
see if it has blown "quietly". We have not encountered any
other parts failing. The PCB transistors have never failed!
The severity of damage will also depend on how long the unit
has been left on after failure. A typical failure, for example,
will usually include one or two output transistors and R 31.
As already stated, failure of the trimpot (R l6) and the
capacitors (C 2, C 6, C ll) is less common.