Good day all.
i purchased a 12 channel mixer and when i tested it, 4 channels were not passing any audio signal to the master out.
i opened it and tested the opamps using a multimeter and found that on the 4 defective channels, the side of the opamp that faces the mic and line sockets showed 15v on the +\-input pins and the output while the other side showed 0v. let me also state that i tested power supply and its fine.
all other opamps on these channels are working fine.
i switched out an opamp on one defective channel for a new one and tested. still no signal and the 15v was still there on the opamp pins.
i am seeking further help on repairing it.
thank you in advance
i purchased a 12 channel mixer and when i tested it, 4 channels were not passing any audio signal to the master out.
i opened it and tested the opamps using a multimeter and found that on the 4 defective channels, the side of the opamp that faces the mic and line sockets showed 15v on the +\-input pins and the output while the other side showed 0v. let me also state that i tested power supply and its fine.
all other opamps on these channels are working fine.
i switched out an opamp on one defective channel for a new one and tested. still no signal and the 15v was still there on the opamp pins.
i am seeking further help on repairing it.
thank you in advance
Without schematics, not even brand and model, not easy to help.
Just to start with *something* , please state which Op Amps, voltage at all pins on a "non working" one, and same on one which does.
Hopefully, you are missing +15V or -15V rail in the non working ones, but awaiting for your confirmation.
Just to start with *something* , please state which Op Amps, voltage at all pins on a "non working" one, and same on one which does.
Hopefully, you are missing +15V or -15V rail in the non working ones, but awaiting for your confirmation.
Shelah, are the four non-functioning channels together in a group? There may be a cracked PCB track on the -15V rail feeding all the four channels.
Gentlemen,
I've removed a number of unheplful and rude posts.
Please do not be unnecessarily rude.
Do not presume folks asking for help already know what you need to have on hand to help, at least not those who are asking for the first time. We were all newbies once, let's not forget where we came from.
If you can't be helpful, please do not post.
With no input, op amps tend to idle with the inputs and outputs all at the same voltage, somewhere in the middle of the two power supplies.
Op amps usually don't fail except the line driver outputs that can have been overloaded.
Solder joints fail. PCB tracks fail, especially the multi-level boards. These could be bypassed by a wire. Circuit board vias fail. These are plated through holes that are supposed to carry signal from top to bottom of the pcb, but don't. I've had a Peavey PV8 and a Sanyo 30" TV have this bad via problem.
I fixed the Peavey PV8 by tracing the signal from a radio input through the circuits until it stopped. I stuck a wire through the hole & soldered both sides, every thing was fine. I did not have a schematic diagram, only a datasheet for the 4565 op amps used. I used an analog VOM costing about $40, with a .047 uf cap series the negative probe to power supply. You do not need a $200 scope full of electrolytic caps that expire in 10 years.
Pamona grabbers or Q-ball clips are useful for tracing music through micro-electronics. You are less likely to short an output pin to a power supply and blow the op amp you are examining.
A coupling cap, especially electrolytic, could have shorted across to cause the 15 v.
Look through some threads on analog line level for schematics of op amp mixers. After 1985 when they went to op amps, they are all about the same.
Pots go bad, especially the master volume pot. I replaced that on the PV8 also.
The PA type mixers have a 30 to 200 watt amplifier at the output to drive 8 or 4 ohm speakers. This has its own problems, usually shorted cables or not enough heat sink. The ordinary type mixers only have 600 ohm capable op amps to drive twisted pair cables out- line out, monitor out, effect out. All mixers do tend to have a 5 watt amplifier inside for driving headphones. That gets blown up a lot by shorted phono plugs. It can also suffer from inadequate heat sink.
Happy hunting.
Op amps usually don't fail except the line driver outputs that can have been overloaded.
Solder joints fail. PCB tracks fail, especially the multi-level boards. These could be bypassed by a wire. Circuit board vias fail. These are plated through holes that are supposed to carry signal from top to bottom of the pcb, but don't. I've had a Peavey PV8 and a Sanyo 30" TV have this bad via problem.
I fixed the Peavey PV8 by tracing the signal from a radio input through the circuits until it stopped. I stuck a wire through the hole & soldered both sides, every thing was fine. I did not have a schematic diagram, only a datasheet for the 4565 op amps used. I used an analog VOM costing about $40, with a .047 uf cap series the negative probe to power supply. You do not need a $200 scope full of electrolytic caps that expire in 10 years.
Pamona grabbers or Q-ball clips are useful for tracing music through micro-electronics. You are less likely to short an output pin to a power supply and blow the op amp you are examining.
A coupling cap, especially electrolytic, could have shorted across to cause the 15 v.
Look through some threads on analog line level for schematics of op amp mixers. After 1985 when they went to op amps, they are all about the same.
Pots go bad, especially the master volume pot. I replaced that on the PV8 also.
The PA type mixers have a 30 to 200 watt amplifier at the output to drive 8 or 4 ohm speakers. This has its own problems, usually shorted cables or not enough heat sink. The ordinary type mixers only have 600 ohm capable op amps to drive twisted pair cables out- line out, monitor out, effect out. All mixers do tend to have a 5 watt amplifier inside for driving headphones. That gets blown up a lot by shorted phono plugs. It can also suffer from inadequate heat sink.
Happy hunting.
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