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Thread: Peavey Prosys 112 - Has hum and low signal output - Need Advise

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    Peavey Prosys 112 - Has hum and low signal output - Need Advise

    Hello everyone! I have a Peavey Prosys 112 powered PA speaker. It has a slight hum and very low signal output. When I connect the output to another amp,the signal is a little louder, but the hum is still there. I contacted Peavey, and they emailed me the schematic. I done a lot of voltage testing, but all seems good. I'm kinda stumped at this point. I haven't check any of crossover network yet. It's so hard to access it. It's mounted on the inside top of the cabinet. And I would have to remove the woofer to get to it. Any advise will be greatly appreciated!

    I can email the schematic to anyone that wants it.
    Coop
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    Last edited by CoopDaKill; 04-13-2017 at 01:56 AM.
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  2. #2
    Supporting Member The Dude's Avatar
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    If you can just post the schematic here, you'll likely get more help.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Right on. Thanks for chiming in. I just uploaded the PDF, above. This unit has the same power amp section as the Peavey 300 CH. I'm going to double check my voltage readings tomorrow.

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    Supporting Member The Dude's Avatar
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    My first thought would be a problem with +&-16V. Have you checked those supplies?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Finally got a chance to test voltages again. On the input board, on C1, I got +0.4v (should be +15v), C2 is -16.9v. I changed these two caps and got basically the same readings. Should I go back and check power supply again? Thanks

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    On the power board, I checked voltages at the connector that goes to the input board. I got +15.5v on the red wire (normal), but I got -26.6v on the white wire (should be -16v) ???

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    Senior Member Enzo's Avatar
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    Fix the power supply.

    Your power supply is on the power amp. UNplug the cable to the preamp. Your layout identifies the pins for ground and +/-16v on the end of the board. The two supplies are derived from plain old three terminal voltage regulators, 7815 and 7915. Apparently the +/-26v is OK, so if the -16 is now -26, then either the 7915 is shorted OR the ground connection to it through CR26 is open. fix that before looking any further.
    Education is what you're left with after you have forgotten what you have learned.

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    Hi Enzo. Thanks for the advise. I removed the 7915 from the circuit. With my DMM set to diode test mode. I get a reading in all directions. No matter which two of the three legs I touch, I get a reading. Is this regulator faulty? I also removed CR26 and it tests normal. I get a reading with black meter probe to black end of diode, red prove to positive end.

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    Senior Member Enzo's Avatar
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    The 7915 and 7815 are not transistors, they are integrated circuits. If the input to output pins show a low resistance, it is shorted.
    Education is what you're left with after you have forgotten what you have learned.

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    I see. I did confuse them with transistors. I'll order the 7915 & 7815 Monday. And let you know after installing them. I really appreciate your time and knowledge. Us shade tree mechanics need more guys like you. lol

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    Got the parts from Peavey today. I replaced VR1, VR2, C21, C22, CR25, CR26. The hum is gone, but still no sound. It's dead quiet. I had previously pulled and checked each output transistor and they all checked good. I'll take some voltage readings this weekend. Any other advise is certainly welcomed. Thanks

  12. #12
    Senior Member Enzo's Avatar
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    "No sound" is not often due to output transistors.

    You had hum, so I expect the power amp works. Look at the cable to the preamp from the power amp, on the end of the power amp board. The layout is labelled for the +/-16, ground and IN. If you pull that cable, and touch the IN pin with a small screwriver or even your finger, you should hear hum out the speaker. If you do, then the amplifier is functioning. If not, then the power amp has the issue - and I would suspect the front end of it first. (compressor chip, mutes)

    You had power supply problems for the preamp supply. So it is not suspicious that some IC may be damaged.
    Education is what you're left with after you have forgotten what you have learned.

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    Enzo, I done as you instructed and pulled the plug from the power board to the preamp board. I did get a slight hum with my meter probe, but none when touching those pins with a screwdriver. With same plug pulled, I get +15.6v and -15.7v on same pins. So I'm assuming the power supply is good now. Would you please point out the compressor chips and mutes? Thanks again.

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    Senior Member Enzo's Avatar
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    Screwdriver should be louder than the meter probe, especially if you put your finger on the metal shaft. MAke sure we are touching the input pin, only one of those pins is input. Touching the others will do nothing. Pin closest to the corner of the board.

    On the power amp board, U2 is the compressor. You can remove it for testing, the amp will work without it. It is the only 87478 on the board.

    Q1 is the mute transistor, likewise it can be removed.
    Education is what you're left with after you have forgotten what you have learned.

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    I removed U2, and Q1. Whats next? Powered on the unit, still no sound output

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    Senior Member Enzo's Avatar
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    OK, those were simple things. APply a signal to the input pin and ground pin. Folow the signal through the amp. U1 has two stages at the input. Got signal at pin1 and at pin 7? Look at the red wires from the transformer, they connect on the other end of the board. the center of the three is red with yellow stripe. Is there signal on that pin?

    And don't overlook the speakers, the output of teh amp is on the molex connector in the center of the board, that is wired to the crossover, which it then wired to the speakers. A loose or missing connector anywhere along that chain will kill the sound.
    Education is what you're left with after you have forgotten what you have learned.

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    I don't have a signal tracer. Could I find the signal with my meter set to AC? I guess I could make some probes with female RCA plugs and run into another amp? Should put a small capacitor in series with the positive probe/plug?
    Last edited by CoopDaKill; 04-23-2017 at 10:07 PM.

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    Senior Member Enzo's Avatar
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    Yes, that is exactly how to make a signal tracer.

    Sure, set your meter to AC volts. A scope would be ideal, but those other solutions work. If I am doing quick checks I often use my meter rather than wait 30 seconds for my sscope to wake up.
    Education is what you're left with after you have forgotten what you have learned.

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    Ok. I've made my signal tracer probe, and input cables. Using some desktop computer speakers for tracing amplification. What connectors on the power board need to be unconnected? Signal injection is coming from a CD player (RCA jack, left output). Which is full line level signal, of course. The mute and compressor are still removed from circuit. I turned on the Prosys and I got this loud garbled audio. Startled me, so I quickly shut it off. Guess I should've unhooked the Prosys speaker, huh? Lol. And I should probably use an audio source with a volume control?

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    Enzo ?

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    Enzo where did you go my friend? I'm guessing that I should unhook the crossover and speakers before signal tracing?

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    Senior Member Enzo's Avatar
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    I only have so much energy to put into these posts anymore. That is why I don't post as much as I used to do.

    You mentioned putting a control on your signal source to prevent overdrives, so did you?

    A line level input to the power amp board should be a good match anyway.
    Education is what you're left with after you have forgotten what you have learned.

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    Enzo, I completely understand. And I think you tremendously for your time.

    I picked up a Walkman CD player with a volume control. But I realized that I should've unhooked the crossover and speakers before tracing the signal.

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    I understand completely. I greatly appreciate your time and help.

    I picked up a Walkman CD player with a volume control. So that should help me. I'll unhook the speakers and crossover, and trace the signal again. My wife and I recently became foster parents to a 14 month old . With three grandchildren, I'm with the you on the energy thing. Lol.
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    Tracing the signal now. On U1, there is no signal on pins 1 or 7. I have signal on pin 5.

    No signal on the red/yellow transformer pin.

    No signal on the speaker pin either.

  26. #26
    Senior Member Enzo's Avatar
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    If there is no signal on pin 7 of the IC, there won;t be signal any further either.

    Did you try a new IC there?

    Remove the 87478 IC U2, noting which way it sits in the socket. Now swap U1 and U3. Does that wake up the amp?
    Education is what you're left with after you have forgotten what you have learned.

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    I've had 87478 U2 removed. I swapped U1 and U3, still no signal on pins 1 and 7 of U1. No signal and where on U3. On U1 , I have signal on pin 5 only.

  28. #28
    Senior Member Enzo's Avatar
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    Unless both 4558s are blown...

    You have +15 on pin 8 and -15 on pin 4 of U1? Signal on pin 5, OK, but is ther DC there too, like 15v? And is pin 7 at zerovDC? Are all three ICs have the pin 1 end towards teh power transistors? So the pin 4 end is towards the small heat sink 15v regulators?
    Education is what you're left with after you have forgotten what you have learned.

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    I get absolute zero AC or DC voltage on pins 4, 8, & 7 of U1. And pin 4 is situated towards the regulators. That is, unless someone before me, turned them all around. I've been careful to put all IC's back in the way they came out. Here's a pic of U1. The round indention on the casing indicates pin 1? Correct?
    Attached Thumbnails Attached Thumbnails img_5702.jpg  

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    Supporting Member The Dude's Avatar
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    The IC pictured looks to be in correctly. There is a corresponding notch in the socket for sake of finding orientation. That said, if you don't have supply on pins 4 & 8 the IC isn't going to do anything. You'll need to find out where your supply voltages went. Measure resistance from those pins to ground and make sure something isn't shorted to start with.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Senior Member Enzo's Avatar
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    Yep, that lack of power supplies to the IC is why it isn't doing anything.

    Remember, the two voltage regulators are ONLY for the preamp supplies. They are wired directly to the cable pins to the preamp. The ICs on the power amp have their own 15v supplies from the power amp circuit, CR4 CR5. WHich makes me ask, are the +/-45v rails present? Those 15v supplies derive from the 45v supplies.

    Do check resistance to ground from pins 4 and 8, but also check resistance from pin 4 to pin 8 directly. A shorted IC might not even have a ground connection.
    Education is what you're left with after you have forgotten what you have learned.

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    Resistance of U1 is
    Pin 4 / 2.8M
    Pin 8 / 2.9M
    Pin 4 to pin 8 / 1M to 3k

    I rechecked the power amp voltages and all are good. So, I injected audio signal directly into the poweramp board, hooked up the speaker of the unit, and got good clear sound. So, this leads me back to the preamp board. ???

  33. #33
    Supporting Member The Dude's Avatar
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    Something is funky here. First you said no supply to IC1. Then, the power amp works. This can't be. Did you do something to restore supply voltage to the IC? Or maybe did you inject signal after the IC? Can you clarify? Do you now have supply voltages on pins 4 & 8 of the IC?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Tell me about it! I have 15v on pins 4 and 8. And the power amp passed good signal. Thinking about taking the shotgun to it! Lol

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