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Old 1st February 2012, 12:52 AM   #1
ivkata is offline ivkata  United States
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Arrow boston acoustics gt-28 protection mode

got this amp no pebay in a lot of broken units .Its stuck in protection mode and as far as i see there is no repairs ever done to the unit .None of the power/outputs are bad and I'm not getting any rail voltage and get voltage on the psu. the PWM IC is sg3525ap and the only reagins I'm getting is leg 8:-0.25 and the power on leg 13: 13.5 volts all other legs read 0v
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Old 1st February 2012, 01:42 AM   #2
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Pin 15 must have 12v for the IC to operate.

Post the DC voltage on all pins of the LM339.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
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Old 1st February 2012, 02:19 AM   #3
ivkata is offline ivkata  United States
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Pin 1: 0.12
Pin 2: 0
Pin 3: 13.2
Pin 4: 0.2
Pin 5: 0.1
Pin 6: 5.4
Pin 7: 2.2
Pin 8: 0
Pin 9: 0.1
Pin 10: 0.01
Pin 11: 0
Pin 12: 0
Pin 13: 0.35
Pin 14: 2.1
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Old 1st February 2012, 11:46 AM   #4
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Email me
babin_perry@yahoo.com
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Old 1st February 2012, 02:21 PM   #5
ivkata is offline ivkata  United States
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Ok did trace the transistor and in fact it has 13.2 volts on its collector
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Old 1st February 2012, 02:38 PM   #6
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Check the voltage on pin 16 again. If there is no voltage, trace the circuit between that transistor's collector and pin 15 of the IC to see if you have an open trace, bad solder connection or if there is something in the circuit between them.
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Old 1st February 2012, 02:54 PM   #7
ivkata is offline ivkata  United States
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false testing on my side ,the transistor dosent have any voltage on its collector.trace to pin 15 if fine ,no voltage on pin 16 as well
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Old 1st February 2012, 03:00 PM   #8
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Pin 16 voltage is produced internal to the IC and requires that you have voltage on pin 15.

Post the DC voltage on all legs of Q43 as well as the one corresponding to Q45 on the schematic diagram.
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Old 1st February 2012, 03:38 PM   #9
ivkata is offline ivkata  United States
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i feel so dumb right now.problem solved.
newbie mistake .I was powering the amp with remote and B+ applied at the same time so this was triggering protection mode.Whit power and then remote applied amp power on and plays nice.Thank you Perry for responding
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Old 3rd February 2012, 07:18 PM   #10
speaker is offline speaker  United States
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The GT-28 is a beast of an amp. It runs fine with a 2-ohm mono load, on the bridged output pair.
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