JBL LSR28P Technical & Service Manual

Also see for LSR28P: Owner's manual

37shorted SW1; To verify correct operation, interrupt power while monitoring collector ofQ17. 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 TestAmplifier 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 possibleshorts in the areas where these parts are. All the above parts form the “CurrentAmplifier” (providing voltage gain).If output voltage is centered (close to ground potential), look for shorts to ground onoutput leg (+LF test point TP4).If output voltage is centered (+LF test point TP4), and output leg is not shorted, therecould be a problem with the bias reference. Jumper across C1. Power unit. If excessivecurrent 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. Measureresistance of R11 and R21, the correct value is 150 ohms. If R11 or R21 values isincorrect, or if voltage across these parts is excessive proceed to “Voltage Amp Test” ifR11 and R21 testing does not uncover a problem, and shorting C1 does bring the powerconsumption to normal, proceed to “Bias Reference Test.” Remove jumper across C1If the output voltage is offset (+LF test point) from ground there could be a problem inthe voltage amp. Jumper across C1 and then bring this point to ground. If the outputvoltage centers itself, remove jumper and proceed to “Voltage Amplifier Test.” If theoutput voltage does not center, then the problem is in the current amplifier. Check forshorts on all elements (Base to emitter, base to collector, collector to emitter) of Q6, Q7,Q8 and Q9.