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Old 15th September 2012, 02:18 PM   #1
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Default Boston GT 50

Hi all
This amp doest not power on.Safe light on the front is on . Amp has no rail voltage.
Ther is no voltage on the gates of the power supply FETs.I am getting 13.02v on only
One pin of the PWM ic (will comfirm which pin later).All fuse test ok.Any pointers for this
Amp?
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Old 15th September 2012, 02:22 PM   #2
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Location: Louisiana
If you apply both remote and B+ at the same time, the amp probably won't power up. Connect B+ and ground, then apply remote voltage.
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Old 16th September 2012, 04:30 AM   #3
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I tried that but no luck.I think pin 15 should have the same voltage with pin 13
But I still can not find out why voltage is not reaching that pin.I read on the dvd these
two pins are connected sometimes but on this amp they are not connected together



Sg3525
1: .00
2: .00
3: .00
4: .00
5: .00
6: .00
7: .02
8: .13
9: .21
10: .00
11: .00
12: .00
13: 15.71
14: .00
15: .11
16: .00


Lm339D
1: .27
2: .73
3: .86
4: .04
5: .00
6: 13.28
7: .86
8: .73
9:.27
10: 15.55
11: .24
12: .73
13: 6.27
14: .84

Last edited by ian33; 16th September 2012 at 04:33 AM.
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Old 16th September 2012, 08:58 AM   #4
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Post photos of the area around those two ICs.
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Old 18th September 2012, 06:17 PM   #5
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pics
Attached Images
File Type: jpg bost1.JPG (365.6 KB, 59 views)
File Type: jpg bost2.JPG (330.5 KB, 55 views)
File Type: jpg bost3.JPG (551.2 KB, 53 views)
File Type: jpg bost4.JPG (292.8 KB, 56 views)
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Old 18th September 2012, 09:49 PM   #6
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Location: Louisiana
There should be a transistor near the LM339 that has it's collector directly connected to pin 15 of the SG3525. Find it and post the voltage on all 3 pins.
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Old 19th September 2012, 02:07 AM   #7
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Q77 mark 4G

Pin:1 15.44
Pin:2 15.47
Pin:3 .04

Q76 also has one leg connected to pin 15
Pin:1 .04
Pin:2 .04
Pin:3 .24
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Old 23rd September 2012, 08:43 PM   #8
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Post the pins on the 339 that are connected to the bases of those transistors (possibly through resistors).
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Old 25th September 2012, 01:53 AM   #9
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The base of Q76 is connected to pin 10 through 1500 and 90 ohm resistor. The base of Q77 is not connected to
the 339.I have 339 ic in stock should I replace it?

Any one have the schematic for this amp?
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Old 25th September 2012, 03:47 AM   #10
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Location: Louisiana
Is there a 4.7 ohm SMD resistor with one end connected to the B+ terminals and the other end connected to pin 3 of the 339?

It's likely to be relatively large. Is it within tolerance?
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