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Old 20th June 2007, 03:41 AM   #1
Shaft is offline Shaft  United States
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Default Ppi Pc450

I have a PPI PC450 that I want to repair. Please help me if you can.

Here's what happened to it. I had this in my ex-gf's truck and her stupid boyfriend <not me; this was after we broke up> removed the speaker enclosure and didn't tape off the ends of the speaker leads. When I later removed the amp out of her truck I found that the fuse was blown.

Several months later I tried to install the amp in one of my cars and the amp comes on, but only plays for about half a second before going into protection. That last for about 2 seconds and then it plays for half a second and the pattern continues.

Common sense tells me that some of the transistors are blown, but I don't know how to begin testing them. Is there anyone that can help me out?

FYI. The sub box was running 4 ohm mono, bridged on channels 3 and 4 of the amp.

Thanks
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Old 20th June 2007, 06:38 AM   #2
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With no power applied to the amplifier...

Measure the resistance between legs 2 and 3 of the output transistors. None should read near 0 ohms. I think they were using 2N6488s and 2N6491s for the outputs.

You should not apply power to the amp without the bottom cover in place and tightened. If you do, you need to monitor the temperture of the heatsink mounted semiconductors VERY closely. They can overheat and fail in seconds if there is a problem.

Try to find the original threads when replacing the bottom cover screws. They can be stripped relatively easily if you don't find the original threads and cut new ones each time you replace the screws.
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Old 20th June 2007, 06:40 AM   #3
Shaft is offline Shaft  United States
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So which legs are two and 3?
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Old 20th June 2007, 07:01 AM   #4
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http://www.onsemi.com/pub/Collateral/2N6487-D.PDF
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Old 20th June 2007, 07:19 AM   #5
Shaft is offline Shaft  United States
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OK, I tested them and it seems like the 2 2N6491's at Q4 and Q5 are bad. So I will try to replace them. Anyone know of a good source for picking a couple of those up?
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Old 20th June 2007, 07:26 AM   #6
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Try digikey.com or mouser.com.

If the amp uses two 88s and two 91s per channel, you should replace both the 88s and 91s in the defective channel.

There's a good chance that only one of the 91s is damaged. If you pull one and find that only one is defective, you can reinstall the good one and test it at low power to see if there are any other problems. If it plays cleanly, you can replace the 88s and 91s and likely be done. If it's still distorted or goes into protection when the defective transistor is removed, there may be defective driver transistors.

Remember, you need to replace the bottom cover before applying power.
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Old 20th June 2007, 10:55 PM   #7
Shaft is offline Shaft  United States
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I took the 2 2N6491's out and went up to the local TV repair shop. They tested them and agreed they were bad. They don't keep those up there so they had to order them for me. They told me that the alternative transistor was a TIP42 NTE332. I did not know there was more than one model number for the basically the same transistors.
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Old 20th June 2007, 11:10 PM   #8
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The TIP42 is not a good sub. I'd avoid using NTE or similar generic replacements.

Order the correct replacements. Replace all 4 outputs in that channel (88s and 91s).

Pick up some heatsink compound from Radio Shack (276-1372) and apply it between the transistors and the insulators. Be sure to clean the area before applying the new compound.
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Old 21st June 2007, 12:02 AM   #9
Shaft is offline Shaft  United States
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Are TIP42's known for poor sound quality?
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Old 21st June 2007, 12:30 AM   #10
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The reason it's not a suitable replacement is because it's rated for only 6 amps vs 15 amps for the 88 and 91.
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