Nyqusit5 3/2/2017 1:17 AM
Looking for JBL PRX712 schematic
I have PRX712 making a lot of hiss and a big noise when switching the "Mix" switch. There clearly is an anormal voltage on the switch but this thing is all SMDs. Does anyone have the schematic? The only document on the web is a spec sheet...

doctor 3/2/2017 7:37 AM
Schematic will not help you much.
Replace on the preamp board dual op-amps MC33078 consequently and examine noise after each replacement.
If after the step it will be found that this replacement did not help, the desoldered chip is good and can be used for the next replacement.
Thus you need only one new MC33078 chip.
Nyqusit5 3/2/2017 8:18 AM
Thank you for your input, but for me the point of having the schematic is finding first the faulty op-amp. I find that pretty helpful to save time (which is money...)!

Still looking for the schematic...
Enzo 3/2/2017 12:19 PM
Sure, but until you do, scope the output pins of each op amp, and see if you find a big noise signal on one.
mellowamoya 9/4/2017 4:22 PM
hi did you solve this problem i have a prx715 with the same symptoms would be interested if you found a schimatic and a cure.
Nyqusit5 9/5/2017 12:48 AM
I didn't find the right schematic but repaired the PRX none the less. I can't remember exactly which one it was but one of the SMD dual opamp of the preamp was faulty. Probably the one just before or after the switch. Be aware that it is not located near the switch but about 10cm away from it: I had to check continuity around the circuit board to find it.
Best of l
drewl 10/9/2017 4:21 PM
I have a PRX715 with bad tweeter amp.
Tons of white noise, after the volume pots.

Anyone know if the "basket" (chassis) comes out?
Seems like it's glued in.
Can't pop it out with screwdriver.

If not I'll have to scope it best I can with just rear panel removed more of a pia.

I take it the MC33078's are just standard dual opamps.
drewl 10/12/2017 5:14 PM
Luckily I found the bad opamp(s) in the preamp and was able to fix it.
g1 10/12/2017 6:32 PM
Did the chassis come out?
drewl 10/13/2017 5:54 AM
I couldn't get it out.
It looks like you could remove the power amp/power supply board if you needed to work on it.
J M Fahey 10/13/2017 8:52 AM
It has a large back mounted "plate amp", meaning a large black aluminum extrusion, with "everything" mounted on it.
Removing all screws around it lets it loose to be removed.
In fact, itīs available as a spare for full replacement:

paleeman 10/13/2017 10:03 AM
Some info about the amp board. Made by Pascal Audio - Denmark

Jazz P Bass 10/13/2017 10:32 AM
Even with the screws removed, often times you still have to pry the panel assembly loose from the cabinet.