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Drive from
Pream pi ifier
5 Amps
.
250 V
Fast Bl ow Fuses
(GBB 5)
27,-0
/S
Fig.3.7. Schematic diagram of the Power Output stage
The fuses used are very fast acting and designed specially for use with
semi-conductors. Any replacements used must be of a similar specification
and such that for a current of 4 times their rated value of 5 A (i.e. for a
current of 20 A) they will blow within 40 msec. and for ax 10 overload
within 5 msec.
The power transistors should also be checked for damage, the most
common mode of failure being a short circuit collector-to-emitter. A simple
resistance check can be made by placing one lead of the ohm-meter on the
power transistor case (collector) and the other on the relevant fuse-holder
(emitter), with the fuse removed (Fig.3.8). Resistance readings depend on
the polarisation of the ohm-meter and should be 200D. or 1 kD. for the NPN
transistors and 200D.-1 O·kD. for the PNP transistors. Junction break-down
will be indicated as a short-circuit.
The transistors are mounted in plug-in sockets and can be simply
remov-
ed. The four power transistors mounted on the bottom of the heat-sink can
be reached by removing the bottom plate of the amplifier case. Always
ensure that the size of the base and emitter pins of the replacement transist-
ors used is not greater than 1 mm
±
0.05 or the transistor holder will be
damaged.
20
•
'
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