Roland AC60 schematic needed

[HDSarge]

Looking for a schematic for a Roland AC60 amp. There are several BC60 schematics around listed as AC60, but they are different amps. The AC60 is an acoustic amp with a LM4766T stereo power amp chip.
The one I am working on had an open main fuse (Repair Tag said "plugged into a bad socket"). Replace the fuse and bring up with a Variac and all seemed well, no big current draw, all lights on, correct signal to power amp inputs, but no output. The voltage rails are there, the Mute voltage, which is switchable, is -8.8v and drops to -.8v across the 4.7K mute resistor. Speakers test good.
I would like to see the schematic, or draw on the forum, before ordering a LM4766T. I do have LM4765's, and it looks like that could work, but the Mute pins would have to float. The Lm4766 uses a negative voltage and sinks 0.5ma from the mute circuit while the lm4765 is muted when 2.5v is applied to the Mute pins.
Any help appreciated.

[The Dude]

I don't have the schematic, but here's the datasheet for the 4766. You ought to be able to determine if the chip's bad from there. Do you have a scope? Check supply, check signal in, check signal out, etc.

http://www.ti.com/lit/ds/symlink/lm4766.pdf

[HDSarge]

Thanks Dude,
I started with the datasheet to identify the 4766 pinout and operation. Tracing with a scope shows the signal at the two input pins of the 4766, but the output is not there (like both channels are still in Mute). Voltage rails and pcb mounted fuses all test okay.
I am concerned that although the Mute pins have conditions that appear correct, current should be greater than .05ma, that the chip is still muted. The -8.8v coming from the main board to the mute resistors (4.7k) switches on/off via a smt transistor that is controlled by the panel mute button, but is that the design voltage? Without the schematic, tracing the mute voltage will involve pulling the main board....not a big deal, but a hassle if the existing conditions are correct.
The negative rail is -32V and I was tempted to hook that up to the mute thru a pot as a divider, but a schematic would be a help.

[HDSarge]

I talked to the owner and the amp was hooked to 220VAC. Replaced the LM4766 and the amp now works.
I would still like to see a schematic to verify that the rest of the circuit is correct.

[Jazz P Bass]

With any of those all in one ic amps, if you have proper rail voltage, mute off signal & a signal going in and it still doesn't work.....

Then the IC itself is highly suspect.

[HDSarge]

I agree Jazz Bass, the symptoms all pointed that way. I do not have a LM4766 and was hoping for a Mute issue to get this out the door. I still have not found a schematic, and would like to see one. Also, I am not done with this Roland, as I installed a 4765 to verify the problem. Had to be careful(use a Variac) as the max rail to rail voltage of the 4765 is 66V, the amp is at 68V.
At least in my experience a bad power amp chip usually fails in a short, often blowing a hunk of the encapsulation off. Both hard to miss. This amp must have shorted(main fuse was blown) and then the chip went open, as no excessive current was being drawn. Actually, I bet the current draw was too low...I typically watch, but not read the analog meter on the Variac during power up. There is always more to learn.
Thanks for the help and best for the Holidays.

[paleeman]

Hello HDSarge

If you still need it, I can e-mail it to you.

[Drewline]

Here you go:

SM_AC-60_17058134E0.pdf

AC-60-M-AC-60-RWM_SM_17058832E0.pdf

[HDSarge]

Thanks for making this schematic available to all here Drewline. Paleeman emailed the schematic to me, allowing the repair to be completed.
Open power amp IC, and failed IC18, 4556, the headphone driver. This little amp uses 9volt supplies on the IC's/mute circuits.
The owner plugged the AC60 into an outlet with 220VAC where 120VAC was expected. He then proceeded to plug a Roland KC350 and a Crate amp in before questioning the, "recently repaired", outlet. The other amps suffered power supply filter cap damage and have been repaired.
Thanks again for the schematics.