PPI 2x150AM blowing fuses - diyAudio
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Old 4th May 2007, 12:15 AM   #1
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Default PPI 2x150AM blowing fuses

I had this amp for over 15 years , turned on the radio today and my mids weren't playing, went into the trunk and found the fuse to the 2x150 blown, replaced it and it happened again.
Unplugged everything to the amp had the radio on plugged in the remote turn on and it blew a fuse again.I just got around to taking the amp out the car opened it up and it looks and smells fine.Can anyone give me direction on what to look for ?
Thanks
Al
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Old 4th May 2007, 02:05 AM   #2
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I am not familiar with that particular amp, but it sounds very much like shorted output transistors.
Chances are they are using 2n6488 and 2n6491s.

Do you have an ohm meter?
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Old 4th May 2007, 02:36 AM   #3
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yep, I have a meter
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Old 4th May 2007, 05:22 PM   #4
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Set your meter to ohms and check each of the 3-legged devices that are mounted to the heatsink. If you find any that have anything near zero ohms between the center leg and either outer leg, it's likely defective.

I'm not familiar with the amp so photos would be helpful if you need additional help.
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Old 4th May 2007, 05:29 PM   #5
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Well measure across the three legs of each output looking for shorted condition. On those reading short lift the middle leg off its trace and recheck. Remove all that are shorted. The amp should stop blowing fuses. These amps also on rare occurrence have shorted power diodes.

I always opt to replace entire banks of outputs with new sets. its cheap enough, and always seems to just work better, with little to very rare rework required.

Good luck
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Old 5th May 2007, 06:22 PM   #6
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Pict attached,Some values dont hold , they gradually change higher or lower and some are negative readings,so would these be bad ?
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Old 5th May 2007, 09:11 PM   #7
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If the meter is 'counting' up or down, the transistors aren't shorted. The negative readings are due to charged capacitors. Have you tried powering the amp up without speakers or RCA cables connected to it?

Whenever you power up this amp, you should have the bottom cover in place and the screws tightened enougn to insure that the transistors are held tightly to the heatsink. Don't over-tighten them and find the original threads when replacing the screws. If you don't find the original threads, the heatsink's screw holes can become stripped easily. Until you find the problem, power it up through a 10 amp fuse.

For future reference, the output transistors are the ones with the third leg connected to the large gray resistors (orange, orange silver, gold bands? - difficult to see in the photo).
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Old 5th May 2007, 09:38 PM   #8
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How do I check for shorted power diodes ? I alos got readings of .400 m ohms on a few points checked.

Yes I went through every senario with RCA unplugged,no speakers etc...I blew 5 fuses trying..lol It happens almost instantly once I give the amp remote turn on.
If you want I can email you a better quality picture, I had to downsize to post ..thanks for your help
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Old 5th May 2007, 10:45 PM   #9
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The rectifier diodes are the 2 legged thingys in the top left of your picture. Just check from leg 1 to leg 2.

If none of these are shorted you really need to recheck the outputs.
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