Hi everyone I’m trying to fix this mixer. The output is heavily distorted from the main output but ok on monitor….any ideas please?
also have old schematics and no service manual. Help!
also have old schematics and no service manual. Help!
Note to moderators: Mixers are neither musical instruments, or guitar amps. They are PA equipment. They could also be considered analog line level equipment. You cannot plug your electric guitar into a mixer directly. Mixers will only drive a headphone or a power amp. Mixers don't have a speaker. Instruments forum is a weird place to move this thread.
I've fixed a couple of PV8 mixers. No schematics are available. They were simple op amp circuits with variable gain in the feedback. My unit had 4565 op amps, surface mount. I didn't have to replace any ICs. One unit the signal got lost at the master volume slide pot. Both the pot were bad, and a plated through hole via were bad just before the master volume pot. After the summing buss on the bottom. I soldered a 30 ga wire through the via.
Using a 5000 ohms/volt 2 vac scale of a simpson analog meter, and a FM radio with earphone (1/8" dual phono plug) to dual RCA plug adapter cable, I was able to trace the music in from the inputs. Also required 1/4" RCA jack to 1/4 phone plug adapters. Music came down from the input jack through the first op amp, then through the filter op amp, then through two horizontal mixing busses at the bottom. I was able to access the mixing buss through the volume pots. Then over to the master and monitor volume pots at lower the right. Then up through master + monitor + tape drive op amps, and out the jacks at top right. On the meter I used both tip probes, and pamona grabbers to banana plug adapters. Put a .047 uf cap between the negative meter probe and signal ground of a 1/4 phone plug, using alligator clip leads, This prevents the meter from reading DC volts on the AC scale. Of course, if you're a pro, you can use an oscilloscope. The used scopes I've bought have been full of expired e-caps and not worth fixing. Also scope probes are $40-50. Be careful probing op amps, don't short the output to the power supply pin. 4565 datasheet on datasheetcatalog.com . My simpson 266XLPM meter is 36 years old and has no e-caps to expire. Simpson still sells a 277 meter. $30 analog import meters with a 10 vac scale could be useful. Be sure to check the FM radio occasionally with a earphone to prove the tuning is on station and the battery is still up. You could also use a $100 signal generator if you do this for a living.
I replaced the master volume pot with a bourns unit from newark or digikey, which was a little shorter than the Peavey unit. I was able to run short wires from the pot to the pcb. I was able to secure the pot to the SS top plate with 2 mm flat head screws from mcmaster.com. My unit used a standard 10 k resistance.
The second mixer had a power supply problem. Old electrolytic capacitors and a missing 16 vac transformer. I bought a triad transformer from newark, but when it was stolen I substituted a 12 vac unit for $2 from salvation army resale. I don't need that much headroom for my signals.
Happy hunting.
I've fixed a couple of PV8 mixers. No schematics are available. They were simple op amp circuits with variable gain in the feedback. My unit had 4565 op amps, surface mount. I didn't have to replace any ICs. One unit the signal got lost at the master volume slide pot. Both the pot were bad, and a plated through hole via were bad just before the master volume pot. After the summing buss on the bottom. I soldered a 30 ga wire through the via.
Using a 5000 ohms/volt 2 vac scale of a simpson analog meter, and a FM radio with earphone (1/8" dual phono plug) to dual RCA plug adapter cable, I was able to trace the music in from the inputs. Also required 1/4" RCA jack to 1/4 phone plug adapters. Music came down from the input jack through the first op amp, then through the filter op amp, then through two horizontal mixing busses at the bottom. I was able to access the mixing buss through the volume pots. Then over to the master and monitor volume pots at lower the right. Then up through master + monitor + tape drive op amps, and out the jacks at top right. On the meter I used both tip probes, and pamona grabbers to banana plug adapters. Put a .047 uf cap between the negative meter probe and signal ground of a 1/4 phone plug, using alligator clip leads, This prevents the meter from reading DC volts on the AC scale. Of course, if you're a pro, you can use an oscilloscope. The used scopes I've bought have been full of expired e-caps and not worth fixing. Also scope probes are $40-50. Be careful probing op amps, don't short the output to the power supply pin. 4565 datasheet on datasheetcatalog.com . My simpson 266XLPM meter is 36 years old and has no e-caps to expire. Simpson still sells a 277 meter. $30 analog import meters with a 10 vac scale could be useful. Be sure to check the FM radio occasionally with a earphone to prove the tuning is on station and the battery is still up. You could also use a $100 signal generator if you do this for a living.
I replaced the master volume pot with a bourns unit from newark or digikey, which was a little shorter than the Peavey unit. I was able to run short wires from the pot to the pcb. I was able to secure the pot to the SS top plate with 2 mm flat head screws from mcmaster.com. My unit used a standard 10 k resistance.
The second mixer had a power supply problem. Old electrolytic capacitors and a missing 16 vac transformer. I bought a triad transformer from newark, but when it was stolen I substituted a 12 vac unit for $2 from salvation army resale. I don't need that much headroom for my signals.
Happy hunting.
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Thank you for the reply. its an odd one as the master volume pot works and so does the monitor volume pot. The distortion occurs at the main speaker output when using a tape and/or any other input. It does not occur when using the monitor output.
I'm not sure if this is the same service document that you have, but I'll attach it here in case it helps.
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Mixers come in two types - plain old mixers and powered mixers. Powered mixers are a mixer with a power amp or two built in. Like this PV unit.
SO your mixer part works and one of your power amps distorts, yes? You will have to troubleshoot that power amap section.
SO your mixer part works and one of your power amps distorts, yes? You will have to troubleshoot that power amap section.
Nope, I just searched for the service information in case it may had helped.
Circuits are circuits. All these things are similar inside. I ran an authorized Peavey repair center for decades. I never saw that particular model, but have serviced countless other ones.
The mixer works ok it’s the main power output that distorts but the monitor power output doesn’t. the 2 amps are mirror image and I done static tests and they both match so not sure what’s causing the distortion
It is pretty easy to spot the power output parts. They will be big. 2 or 3 cm per transistor or IC. The op amps are tiny. 4 mm.
Just trace the signal to the point where it goes bad and start replacing things. The way stage wiring goes, whatever touched the shorted speaker wire or send cable probably got shorted. 1/4 phone plugs pulled part way out are endemic. Bad coupling caps are less likely but usual suspects. Bad solder joints can happen, too.
The way you check an op amp, the output input & feedback input are not all the same DC voltage. Same result on a amp IC. All mixer circuits since transistor days have op amps used with signal going in one input and feedback going through a resistor or pot to the other one.
Just trace the signal to the point where it goes bad and start replacing things. The way stage wiring goes, whatever touched the shorted speaker wire or send cable probably got shorted. 1/4 phone plugs pulled part way out are endemic. Bad coupling caps are less likely but usual suspects. Bad solder joints can happen, too.
The way you check an op amp, the output input & feedback input are not all the same DC voltage. Same result on a amp IC. All mixer circuits since transistor days have op amps used with signal going in one input and feedback going through a resistor or pot to the other one.