Crown D-150 - Checkout Procedure - J KM2 Input; with Load

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SYMPTOM
Output unstable when
adjusting level pot
PROBABLE CAUSE AND REMEDY
positive check the + side. If
offset is negative check the
+ side.
Defective bias diodes (02-D5).
Replace. :
NOTE;
As a general procedure, use a
voltmeter to compare voltage
readings with those shown on
the unit schematic. Begin at the
input, move stage by stage
through: the circuitry, and note
any Jarge.deviations. from the
quiescent vollage levels shown.
Determine whether specific
components are defective or
simply responding to a problem
elsewhere.
Check .componerits-on Main. PC
Board by alternately heating
and cooling (use small sokier
ivon and circuit cooler spray)
while observing the oscillation
on the oscilloscope, Concen-
trate on transistors and diodes.
Significant changes in the oscilla-
tion may pin-point a defective
component.
Check diodes and resistors by
paralleling witha capacitor.
This may help isolate the source
of an unwanted oscillation.
CAUTION! In some cases
this technique may cause
oscillation so use with care.
Check capacitors by paralleling
ther with a known good capac-
itor of the same value. 1 oscilla-
tion is eliminated, an open
capacitor may be indicated.
Dirty internal contacts on level
pot. Temporary correction of
this problem may sometimes
be had by turning the pot back
and forth repeatedly to allow
the contacts to clean them-
selves. If this is unsuccess-
ful, replace the pot.
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25
5.7.5 Checkout Procedure — 1 KHz Input,
With Load
Most of the common service problems should have manifested
themselves previously and the remaining test procedures shoukt
be fairly routine. if troubles do occur, a likely source is the output
Stages, since this is the first test where these stages are subjected
toa load.
1. Connect an 8-ohm load to the output. (Refer to paragraph
§.6.2).
2. Monitor the output with an oscilloscope and an accurate AC ~
voltmeter.
3. Connect a sine wave oscillator set at-1 KHz (+10dB out) to the
amplifier input.
4. Turn up the level control of the channel under test: The output
must clip at over 98 watts (28 volts). Typically, the clip point is
105 watts or above, with the ac line voltage at 120 volts. The
waveform should be clean, and should clip on and ar
matrically.with ne. ringing. ;
5. Change load to 4 onms. Clip point should occur at approx:
imately 170 watts (26 volts): :
5.7.6 Checkout Procedure — Protection |
Circuit
Tests
,
The
protection
circuit
test
is
a
clipping
test
designed
to
verity
the
operation
of
the
limiting
circuits.
1. Set the amplifier output at approximately 28V across 8 ohms.
2. Switch the foad to 2 ohms. At this load impedance the output
will cause enough power supply sag to clip the output but may
not cause the sharp'clip indicative of limiter circuit action.
Slowing
the
oscilloscope
trace
to
look
for
power
supply
ripple
at
the clip level will indicate whether the power supply or limiter
circuits are causing the clip.
3. in order to insure that the limiter circuits are operating, it may
be necessary to driva a 1-ohmn load. If the oscilloscope trace
still shows only power supply clipping, the associated limiters)
is not turning on and the limiter circuitry is defective.
PROBABLE CAUSE AND REMEDY
SYMPTOM
+ signal does not clip Check components in + limiter
on
2-ohm
load
(Q6
circuit).
4
- signal does not clip Check components in - limiter.
on 2-ohm load (Q7) circuit.
Either + or - signal Check limiter circuits. Refer to
clips at wrong level Figure. 5-2 for clipping levels.
‘on 2-ohm load.
neater nininnnenine
PATA EREB EN BAIRIRAL.