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Old 22nd January 2009, 11:31 PM   #1
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Question PPI A600 problems

Hi guys,

I have an A600 (not the .2) that when powered up squeals. Not through the ouputs, but the amp itself squeals. The low impedance light stays on even when a load isn't connected, and when I connect the amp to subs and hook up the RCA's, the subs pop.

Anyway, I've never repaired an amp myself, but would like to give it a go. I opened it up and found that one of the caps had vented, so I know that will need to be replaced. Could that be the cause of all my problems? Or would you suspect more damaged components? Where do I start?

Thx in advance and please go gentle. I am here to learn!

Here is a link to some pics of the guts: A600

If you need something else, let me know.
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Old 23rd January 2009, 12:43 AM   #2
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Location: Louisiana
Has the enamel overheated on one of the green windings of the larger toroid?

It appears that one winding is rougher than the others. Can you scrape the green insulation off with your fingernails?


You also need to measure the resistance from leg 2 to leg 3 of the transistors adjacent to the large gray resistors. Check all of them along that row. Do any read near 0 ohms?

Check them with no power applied to the amp.


As a side note, when you get the amp working, you MUST continually monitor the temperature of ALL of the heatsink mounted components if you're operating the amp without the back cover in place. When you initially power it up, do so for only a few seconds and check the temperature of the components. If any are hot, you need to determine why they are getting hot. Nothing should get hot in less than 5 seconds. You also need to install a 10 amp fuse in the B+ line. This will 'help' protect the components.
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Old 23rd January 2009, 02:10 PM   #3
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Thanks for replying Perry. I don't think the enamel has overheated, but where the glue "trailed off" it seems to have overheated and made a bit of a mess. The green insulation won't scrape off.

I'm assuming you're saying to test all of the BDT 81's and BDT 82's? Did that and one is measuring .3 ohms and a couple others are at 1.2 ohms (on the bottom side in the pics) Most I get OL.
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Old 23rd January 2009, 02:47 PM   #4
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The one that's 0.3 is shorted. Remove it from the circuit and recheck the ones that were reading 1.2. If they're reading OL like the rest, replace and tighten the bottom cover and power the amp up with a 10 amp fuse in the B+ line. If the amp powers up amp works normally, you'll need to replace all of the output transistors in the channel that failed.

If you can't find the original parts, 2N6488s and 2N6491s should work as substitutes.

When you replace the screws in the bottom cover, start them by hand and make sure that you don't crossthread them. They should thread in easily until they make contact with the cover.
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Old 29th January 2009, 08:23 PM   #5
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Thanks again for taking the time to help out a noob Perry. I will try to work on it a bit this weekend if I can find the time.

One question though, when you say replace all of the output transistors on a channel, do you mean all of the ones along that side, or just the ones in the same "grouping"? The one that is bad is in a group w/a green transistor.... what is that? I can't see a number on it. If you look in the pictures from photobucket it's on the bottom middle group.
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Old 29th January 2009, 08:35 PM   #6
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There are 4 BDT81s and 4 BDT82s in each channel. You would replace those 8 transistors.
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