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Old 12th March 2008, 09:14 PM   #1
rawadia is offline rawadia  United States
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Default ppi pc4100 no output, help!

I need some help debugging this amp. It all started with the output level dropping and distorting. I put a sine wave through each channel and saw that half the signal was being clipped. Opened up the amp and poked around, checking transisters, rectifiers, etc. Finally found my problem, a leg of the inductor to the neg rail had broken. I re-soldered it, and checked my rails, and got both my neg rail back. I thought this was the end of it, but when I screwed everything back together, I now have no output on any of the channels. I must have messed something up when I was poking around.

I've traced my input signal all the way from the input boards to a big blue rectangular caps in the 4 similar sections which contain caps,resistors, transistors, and a vertical mounted board. My power supply seems good, with rails at +35,-35. I just don't get a signal on the base of my transistors.

I don't see anything blown, and all channels are equally effected.
Any ideas on the problem? Is there a 'mute' or protect signal that is stuck?
There are 4 voltage regulator circuits, 2 make my +15,-15 rails which go to the input board opamps, the other 2 make +38, -38, which go to the vertical boards and some transistors. What kind of voltage should the vertical boards be getting?

Any ideas/help will really be appreciated, I'm kind of stuck right now.
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Old 13th March 2008, 03:48 AM   #2
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Did you remove any boards like towards the end plate ?? If so when you put the board back did you realign the connector properly ??

try to post a pic if possible its been a while since I looked inside my PC4100....
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Old 13th March 2008, 04:17 AM   #3
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Do you have audio on pin 3 of the driver boards?

Are there any transistors with the part number J108 near the driver boards?

If so, they are probably for muting. When the voltage goes to negative 12v on leg 3 of the muting transistors, the transistors release the audio. If there is no negative voltage on the third leg of the muting transistors, the problem is likely in the muting or protection circuit.

As long as you have significant voltage on the outer pins of the driver board, it should produce audio.
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Old 13th March 2008, 05:34 PM   #4
rawadia is offline rawadia  United States
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Here is a pic of the amp. The connector is aligned correctly.

I haven't powered the amp up yet. But there are two J108A transistors in each of the 4 sections. All 8 have the drain, source, and gate connected to each other.

Pin 3 of the board goes to a A06 which then sends a signal to an A56, not sure where the signal goes from there.

If I power up the amp without an input signal, will I still be able to see the -12V on the muting transistor?
Attached Images
File Type: jpg pc4100small.jpg (73.7 KB, 135 views)
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Old 13th March 2008, 05:48 PM   #5
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You don't need an input signal to see the mute drive. There's a 2-3 second delay when you power it up. After the delay, the voltage will drop to -12.

After the delay, you should see signal on the first leg of the muting transistor.

If you attempt to check the muting transistor, they will appear shorted (less than 10 ohms) when no power is applied.
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Old 13th March 2008, 07:52 PM   #6
rawadia is offline rawadia  United States
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With no input, I powered up and got -80mV on pin 3 of the J108A, so looks like we found the problem. Where is this signal generated?
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Old 13th March 2008, 08:04 PM   #7
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http://bcae1.com/temp/ppipc420002.swf
Right-click to zoom in.

This is from a different amp (PC4400.2) but maybe it's close enough to help. It's probably got a few mistakes.

The upper left of the diagram is the muting circuit. You can see the jfet.
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Old 13th March 2008, 09:07 PM   #8
rawadia is offline rawadia  United States
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Sorry, could you give a little more explaination into how the -12v is created and where.
I was able to find the optocoupler and trace up to the transistors above it, but was unable to find the jfet.
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Old 13th March 2008, 09:14 PM   #9
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J108 is the jfet.


I'm not sure where the voltage is produced. It's likely taken from the -15v regs. There will be loss through the various series-connected devices. That may be why there was only -12 at the gate of the jfet.

If you can't track the problem down, I may be able to look into it more closely. I have a similar amp here.
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Old 13th March 2008, 09:32 PM   #10
rawadia is offline rawadia  United States
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oh ok I see, but I still can't find how the jfets connect to the transistor in your diagram.
What I did find out is that before a resistor (R176 in your pic) goes to the +15 regulator voltage, it is also attached to the base of another transistor A06, and that transistor is displaying only an 80 ohm resistance from base to collector. The A56 that mirrors it on the -15V regulator circuit, does not have such a low resistance.

I know when I had the amp open, I powered it up a couple times without clamping the transistors to the heatsink, so it may be that either that A06, or the 2N6488 in the +15 regulator circuit got messed up. Although, I'm still getting +15V to my input boards
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