Case history 5-7-06
? Make: Crown MA2400 serial 221281, with main board D6978-7
Defect: amplifier right channel remaining in ODEP protection status; no output. Right channel relay does not click on.
Electronic internal examination:
? power supply does not measure + 55v like it should; it was wildly mismatched (something like +85 and –25);
? in-circuit ohmmeter measurement of several output transistors revealed collector-emitter dead shorts in two.
? in-circuit ohmmeter measurement of several 0.20? emitter resistors revealed that three had been blown open
? visual examination of the main board revealed a burned resistor R236 (the input to the ODEP circuit)
it was decided at this point to replace all 12 output transistors, the 4 drivers, the 3 emitter resistors and the bad ODEP resistor R236.
After the rebuild, the amp was connected to a oscilloscope and a signal generator, and turned on slow with a variac and AC ammeter. When driven with a 1Khz signal, we found that the rebuilt channel put out a half wave only – the negative half of the signal was clipped off. The rebuilt right channel was still defective.
We examined the two pre-driver transistors Q215 and Q216 to see if they were defective, and they were not. We checked the other components on the output board (the flyback diodes D405, D406, D407, D408; the bias string diodes D409; 410; 411; 412) for defects; they were not. We disconnected the four ODEP sensing resistors R224, R227; R236; R238 to see if the ODEP circuit had been damaged in the initial amplifier blowout and was affecting amplification. This did nothing; the signal remained missing it’s negative half.
We decided to measure the voltages of the right channel circuit and the left channel and compare them to each other. If we saw a circuit point on the bad channel that looked different from the good channel, it was hoped that point would lead us to the defective portion of the amp.
We measured the voltages of the 10 pins of ribbon cable J500 and J600 (the good channel) and J700 and J800 (the bad channel) and wrote them down next to each other on a chart, and on the schematic. We highlighted the voltages that were off from the good channel with a yellow marker.
? J700 pin 6 to be –0.17 volts when it should have been 0.9V
? J800 pin 5 at +1.86V instead of –1.1V
These circuit points are part of the low side bias string. The service manual says the amplifier bias voltage is supposed to be 300 to 320mv as measured across R421. We measured this voltage to be zero.
We decided to check the pre-driver transistors again (to see if we had made a mistake before) and all the components immediately around the pre-driver and connected to J700 and J800. We checked resistor R247 (200?) in circuit with the ohmmeter and found it to be open. It was not burned and did not look unusual.
Replacement of this resistor brought the output signal back to normal.
Total time: about six hours.
We would very much like to have done this faster. If anyone has faster ways to isolate and troubleshoot bad sections of a mac, please let us know.