JBL LSR28P, Technical & Service Manual

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shorted SW1; To verify correct operation, interrupt power while monitoring collector of
Q17. The voltage should quickly change to + 15 and, eventually, zero volts. If it does not,
check for open C29 and, or Q17;
With negative voltage applied to pin 6 of U1B, pin 7 of U1B should be at +15V. If not,
check for short or open trace around pins 5 and 6 or replace the defective IC U1.
Low Frequency Power Amplifier Test
Amplifier pulls excessive power when board is connected.
Verify correct values of Q6, Q7 (2SC3201), Q8, Q9 (2SD1763A), and Q5 (2SB1186A).
Check that D11 and D12 are not shorted and for proper polarity. Look for possible
shorts in the areas where these parts are. All the above parts form the “Current
Amplifier” (providing voltage gain).
If output voltage is centered (close to ground potential), look for shorts to ground on
output leg (+LF test point TP4).
If output voltage is centered (+LF test point TP4), and output leg is not shorted, there
could be a problem with the bias reference. Jumper across C1. Power unit. If excessive
current is not pulled, bring AC supply to full (120VAC). Measure across R11 and R21.
The voltage across these parts should not be any greater than 2.5 VDC. Measure
resistance of R11 and R21, the correct value is 150 ohms. If R11 or R21 values is
incorrect, or if voltage across these parts is excessive proceed to “Voltage Amp Test” if
R11 and R21 testing does not uncover a problem, and shorting C1 does bring the power
consumption to normal, proceed to “Bias Reference Test.” Remove jumper across C1
If the output voltage is offset (+LF test point) from ground there could be a problem in
the voltage amp. Jumper across C1 and then bring this point to ground. If the output
voltage centers itself, remove jumper and proceed to “Voltage Amplifier Test.” If the
output voltage does not center, then the problem is in the current amplifier. Check for
shorts on all elements (Base to emitter, base to collector, collector to emitter) of Q6, Q7,
Q8 and Q9.