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Old 24th June 2012, 11:57 AM   #1
timwebb is offline timwebb  United States
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Default memphis st1500

im working on one of these amps and the negative voltage reg only has -4.5v. i cant find what dragging it down.
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Old 24th June 2012, 12:54 PM   #2
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Are any parts getting warm, op-amps? Otherwise pull it from the board and test it externally with your supply.
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Old 24th June 2012, 02:33 PM   #3
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Is the 7915 operating at a significantly higher temperature than the 7815?

Did you insert a 1 ohm resistor between the output leg of the 7915 and the board and measure the voltage across that resistor to determine if the current being drawn was more than the regulator is rated for? This is covered under item #26 of the audio troubleshooting page of the DVD.
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Old 24th June 2012, 06:06 PM   #4
timwebb is offline timwebb  United States
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ok i found what was pulling it down. now ive rebuilt the output driver cards and when i put one in the green and red light come on. the 1 ohm resistors check good. i replaced the 1027 and 1023 transistors and the diodes. what am i missing?
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Old 24th June 2012, 06:16 PM   #5
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Does it power up normally with no driver boards?

Does it power up and produce oscillation on the outputs with only one board? Try both boards, one at a time in each location.
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Old 24th June 2012, 06:47 PM   #6
timwebb is offline timwebb  United States
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ok it powered up but a resistor on the card r208. a 120ohm resistor instantly smoked. so im pulling the 2sa183 thats connected to it.
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Old 24th June 2012, 07:53 PM   #7
timwebb is offline timwebb  United States
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the part test good the resistor still burns.
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Old 24th June 2012, 08:09 PM   #8
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Does the resistor burn with the A1381 out of the circuit?

Does it burn with the 1381 in the circuit but the A1023 connected to it out of the circuit?
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Old 24th June 2012, 10:36 PM   #9
timwebb is offline timwebb  United States
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[QUOTE=Perry Babin;3071042]Does the resistor burn with the A1381 out of the circuit? no.

Does it burn with the 1381 in the circuit but the A1023 connected to it out of the circuit? yes
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Old 24th June 2012, 11:36 PM   #10
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That would likely indicate that you have a solder bridge somewhere. The current has to pass through a higher value resistor and the 1381 with the 1023 out of the circuit. That would cause the higher value resistor to burn (assuming that it's the same size) if the transistor was defective.
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