ppi pc4100 no output, help!

rawadia

Member
2008-03-12 7:44 pm
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.
 

Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
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.
 

rawadia

Member
2008-03-12 7:44 pm
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?
 

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Perry Babin

Member
Paid Member
2003-11-20 11:01 am
Louisiana
www.bcae1.com
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.
 

rawadia

Member
2008-03-12 7:44 pm
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
 

rawadia

Member
2008-03-12 7:44 pm
Alright, I'll order that part, hopefully it is the reason I'm not getting the -12V. I still have been trying to trace where those jfets get the -12 from.
Haven't found it yet. Thanks for your help and being so available. I'll keep posting with progress.
Regards,
Raiyomand
 

rawadia

Member
2008-03-12 7:44 pm
Replaced that transistor. I still get only -80mv at the gate of the j108a transistors. Still no output.
I also decided to the check out pin 3 of the ceramic board. It sounds like when I turn on the amp, that sound gets to the pin for a split second, but then gets cut off.
 

rawadia

Member
2008-03-12 7:44 pm
Its base is connected to the emitter of the 2N6488 voltage regulator transistor. But I'm getting good voltages off of that, b,c,e =+14.7,+41.1,+14.23
It might still be the culprit, I pulled it, and got good forward drop voltages from base to emiiter and base to collecter. When I did the diode check the other way collector to base it showed voltage building then went to 0, and when I did emitter to base , the voltage started building but stopped at .98 Volts. Not sure if that has something to do with my meter or if it is bad, but it still puts out the correct voltage on the emitter.