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Old 5th September 2008, 09:49 PM   #1
rawadia is offline rawadia  United States
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Default PPI 4100 front input leaking to rear ouput

Intro: Front channels go to a pair component speakers, rear channels are bridged to one sub.

So I was messing with stuff in my trunk and unplugged my rear input rca cables and to my suprise my sub kept playing. I thought at first the 'combine' switch was pressed which sends the front signal to the rear but it wasn't, and when I did press it in, the output to my sub seemed to double.

Any ideas on what might be messed up to allow the front signal to leak to the rear?

Also I was messing with the LP/HP switch for the rear channel, I could see the woofer moving in and out as I pressed the switch, when it is set to HP I measure 2mV of DC on the output, but on LP it is like 0.2mV, is that a problem? I have it set to LP right now since it is going to a sub.
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Old 11th September 2008, 10:41 PM   #2
rawadia is offline rawadia  United States
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Default further clarification of problem

Just to add to this, if I engage the hp filter for the rear channels,only sound from the rear inputs is used, but if the lp filter is engaged, somehow sound from the front inputs gets through to the rear channels.
I'll take any suggestions no matter how far fetched!
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Old 12th September 2008, 05:31 AM   #3
ppia600 is offline ppia600  United States
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Let me go find the pic of the end controls on one of those. Can't remember how many buttons... That is very odd that a noticeable amount is leaking, ppi have great channel seperation. Could something have been spilled into the amp?
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Old 12th September 2008, 05:34 AM   #4
ppia600 is offline ppia600  United States
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Are you sure you're not plugging one of the rca sets into the amp's output rca jacks? Is it a pc4100, the really long one?
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Old 12th September 2008, 10:42 AM   #5
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Are all of the large 10 ohm resistors near the RCA jacks within tolerance?

With no RCA cables plugged into the amp, do you read ~20 ohms between the shields of all of the input jacks?

Are there any open traces in the preamp section of the amp?
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Old 19th September 2008, 09:52 PM   #6
rawadia is offline rawadia  United States
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Okay, finally got the amp out of my car.
All large 10 ohm resistors are good (10.5 ohms usually)
All Shields have about 20.8 ohms between them
No open traces that I can see

This is the really long amp, all rca's were plugged into the proper input, I've never spilled anything on it but since I bought the amp used, it could have had something spilled on it, but judging by the inside, doesn't look like it.

Not sure if this is related but might as well through it out there, when I was repairing the amp previously for a broken leg on the large inductor, I somehow fried the 108a muting transistor on the left rca output, maybe by misaligning the pre-amp board. So I pulled up the 'gate' leg on the 108a for both the R and L rca output and the amp started to play again. The drain and source of the transistors are still soldered to the board, so maybe the faulty transistor is doing something?

My other idea is that the hp/lp circuit is messed up since there is good channel separation when in hp mode, but in lp mode the front signal is getting to the rear output.
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Old 19th September 2008, 10:23 PM   #7
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Pull the 2 muting transistors near the output RCAs to see if it changes the symptoms.

You should check the switches in both positions to confirm that all of the contacts are in good condition. You should read very near 0 ohms for each set of contacts. Let me know if you don't know how to check them and need more information on the internals of the switches.
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Old 29th November 2008, 11:27 PM   #8
rawadia is offline rawadia  United States
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I apologize for the long break, but I got busy with other things and didn't get a chance to look at the amp in a while.

I replaced the muting transistors, so the rca outputs now work. I checked the connections on the hp/lp switch. I've attached a pic of the connections. Let me know if I need to check it another way.

Also its seems that a bit of signal leaks to the rear channels from any input (not just fronts) when the switch is in low pass mode. So even if I connect a source to the right rear input, I will get a bit of sound coming out of the left and right rear output.
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File Type: jpg hplp switch.jpg (17.0 KB, 140 views)
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Old 1st December 2008, 01:12 AM   #9
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If you read 0 ohms between those points, the switch looks like it's OK.

Use the following image as a guide when checking the switches. You only have to confirm that the sliding contacts are making a good connection between each of the two fixed contacts. This should save some time.


Click the image to open in full size.
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Old 1st December 2008, 04:22 AM   #10
rawadia is offline rawadia  United States
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Thanks for the diagram.
When I touch the two probes of my meter together I get .4 ohms. I got that same measurement for the pins the diagram said to check, so I think the switch is good.
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