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Old 27th April 2012, 12:51 AM   #1
diyAudio Member
Join Date: Sep 2006
Default Directed 750D

Amp is stuck in protection mode ... All outputs test fine..

Heres what i have on the 494

Pin 1:0
Pin 2:4.85
Pin 3:0.05
Pin 4:4.72
Pin 5:1.40
Pin 6:3.42
Pin 7:0
Pin 8:13.80
Pin 9:0
Pin 10:0
Pin 11:13.80
Pin 12:13.80
Pin 13:4.85
Pin 14:4.85
Pin 15:4.85
Pin 16:0

Any ideas where to check?
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Old 27th April 2012, 01:03 AM   #2
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
Pin 4 is high. If there is a PNP transistor that's connected to pins 14 and 4, pull it to see if the amp will power up. This will disable the protection circuit so clamp all transistors, small fuse or limiter....
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Old 27th April 2012, 01:10 AM   #3
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Join Date: Sep 2006
Removed the transistor amp powers up and idles fine also produces clean audio..

Guess i will replaced it and see if thats all it was..
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Old 27th April 2012, 01:12 AM   #4
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
That's the last component in the protection circuit. Similar amplifiers have thermistors, comparators and other transistors driving that transistor so if that's not the problem, you'll have to follow the circuit back and see what's driving the base of that transistor.
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Old 29th April 2012, 07:06 PM   #5
diyAudio Member
Join Date: Sep 2006
I replaced the transistor and amp was working great so i gave it back..

Not 2 hours later he brought it back and said something blew up in it..

I tested all outputs They all test fine I also checked the rca shield ground legs where they solder to the board they look fine..\

With only power ground and remote hooked up NO RCA'S if i hook a test speaker to it I get a Very loud noise from the spaeker like its fluttering back and fourth like crazy...

The is no DC across the speaker terminals..

Any ideas ?
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Old 30th April 2012, 04:50 AM   #6
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
Are any of the potentiometers or switches broken?

Does operating them through their entire range make any difference?
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