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+/- 15V Supply Test
Slowly ramp up the Variac, never allowing the supplies to exceed +/-16V. Measure + 15V
and -15V supplies at Q2 and Q4. Both should be the correct value +/-15V, respectively.
If a supply is very close to the ground potential (Less than +/- 100mV) check for shorts on
the +/- 15V supplies lines. IC's and reference diodes, D1 or D4; could be open or
incorrect reference bias sources R18 and R26; open series pass devices Q2 and Q4; If the
initial +/- 36V supply is absent, proceed to "+/- 36V Supply Test".
If a supply is out of specification (deviation of more than .5V but not close to ground
potential) check for:
Incorrect value of D1, D4, R18 or R26;
Proper polarity on C4 and C8;
Excessive current being pulled (Do U1-U3 run hot? etc.);
Verify that Q2 or Q4 is not shorted from base to emitter.
If supply voltage is abnormally is high verify that:
D1 and D4 are of correct type and are not open
R18 and R26 are the correct value
Q2 is a 2SD1763A and that Q4 is a 2SB1186A
Q2 and Q4 are not shorted Base to collector, or collector emitter
(+/- 36V Supply Test)
Look for traces between main bridge, D13, and the main filter capacitors, C16, C17. Look of
open traces between D13 and power input connector, J1. Verify that D13 is not open. Return
to "Main Test Loop" after fault found.
(+/- 70V Supply Test)
The low frequency power amp high voltage stages might be pulling excessive current. Check
for correct value of R28 and R30 (3.24K) and verify 1.4 VDC across them when board is
powered. Check for correct value of R22, R29, (464 ohms), R21, and R11 (150 ohms) and
verify 870mVDC (+/- 100mV) across them when board is powered. Check for correct
values of R6, R23 and R25 (0.1 ohms, 3W) and verify <5 mV (or similar to bias setting)
across them when board is powered. If any of the above is not true proceed to "Low
Frequency Power Amp Test."
Power up boards and measure voltage at junction of D15, R29 and C27. The voltage should
be around 34 VDC. If voltage is negative check polarity of D18;
If okay, power up board while monitoring the collector of Q17. The voltage should ramp
from zero to around -8V in around 2 seconds (at which time LED's, D5 and D6 should
illuminate) and eventually toggle to -15V. If collector remains close to zero or at a somewhat
positive voltage check for collector to emitter short on Q17; for incorrect value R34; and for
MAIN VOLTAGE TESTS
Amplifier Muting Test.
36
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