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

[CoopDaKill]

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

[The Dude]

If you can just post the schematic here, you'll likely get more help.

[CoopDaKill]

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.

[The Dude]

My first thought would be a problem with +&-16V. Have you checked those supplies?

[CoopDaKill]

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

[CoopDaKill]

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) ???

[Enzo]

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.

[CoopDaKill]

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.

[Enzo]

The 7915 and 7815 are not transistors, they are integrated circuits. If the input to output pins show a low resistance, it is shorted.

[CoopDaKill]

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

[CoopDaKill]

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

[Enzo]

"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.

[CoopDaKill]

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.

[Enzo]

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.

[CoopDaKill]

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