LM3886 Power Transistor help needed

[Roly]

I can't see anything abnormal, so here's hoping.

The BA6110 is a rather unusual type of "op-amp" which is a current-controlled gain block, the gain being set by the amount of current flowing into the CNT or control input via R19.  (the datasheet shows pin 4 as "no connection" so I don't know what is going on here, maybe a different package, however...)

The output is sensed for overdrive via U3b, and its output is used to reduce the gain of U6, making a form of automatic volume control, similar to a compressor.  This is a desirable characteristic in a PA amp but not really what you want in a guitar amp.

Since there is nothing obviously wrong with the preamp you can proceed to sort out the power amp chips as outlined above.

[jimmy74]

maybe a silly question, but isn't pin 3 on the BA6110 supposed to be positive?

[Enzo]

This data sheet shows pin 3 as an input.

http://pdf1.alldatasheet.com/datasheet-pdf/view/36051/ROHM/BA6110.html

[jimmy74]

Well I'm having a hard time trying to understand where exactly should I connect the meter clips for the output test, U5 in the schematic is connected directly to J2 and J6 but there's no continuity from pin 3 to any of the connector pins... another shorted jumper?

[jimmy74]

got it fixed it was a shorted jumper just like the other one!!

Anyways fitted in one of the amp chips in the U5 position, clipped the meter leads to the output pins for the tweeter, gave it a short burst through the limiter and the tester went above 2vDC, I changed the meter setting to 200v and the amp gave me around 31vDC... so it's a bummer?

thanks

[Roly]

shorted jumper

I think you mean "open".

the amp gave me around 31vDC

If that's output to ground, across the speaker, then yes it looks like it's boofed.  Mark it as sus and try another (but do check the actual chip legs to make sure that -ve supply is actually getting to the chip).

Repeat until you get to one that has less than +/-0.5V across the speaker.

[jimmy74]

I checked the voltages with this amp chip soldered in, -ve voltage to pins 4 & 6 are -37 but on pin 8 I'm getting -3 instead of -13, could this be indication of a fried chip?

thanks

[Roly]

Is the output pin within half a volt of ground?

[jimmy74]

No the output pin is giving me around 31vDC.

[J M Fahey]

I'm *quite* certain that some tracks or pads are either cracked open , missing, etc.

That circuit is quite simple, but if its pins are not connected to what they are supposed to, no LM3886 will work there, even the newest freshest one.

I suspect (heavily) that that PCB has been butchered beyond repair by now.

I would get/buy/make an LM3886 PCB, mount the chipamp there, bolt it to the heatsink "upside down" (no 2 solid bodies can occupy the same physical space, and the old PCB is still there) and connect proper +/-V rails, ground, in, out  with short wires to the approppriate points.

I do that all the time, specially to repair amps with unobtainable chipamps (TDA1514 , uPC1188, etc.).

Build what I suggest on the bench, "outside " the amp, just getting power and ground from it, obviously bolted to a heatsink, and test it with music.
Once it works, "transplant" it inside.

[jimmy74]

Ok I fitted in another amp chip and this time it behaved properly, going to zero on pin 3, however I'm still getting -3v on pin 8.

After pulling the first amp chip, I remeasured the voltage on pin 8 and was getting -13v, so the -3v once the chip is fitted in must mean that the amp chip is pulling the negative voltage down or is this normal?

thanks

[jimmy74]

Fitted in another chip amp in the U8 position, tested the voltage on the output pin, 0 volts and pin 8 is -2.5v.. The limiter lamp lights up very dimly (it's 40watts), is this normal? Should I proceed to placing the last chip amp in the U7 position?

[jimmy74]

Is the -2.5v on pin 8 indication that the chip(s) need to be replaced? If the 0v on pin 3 means the output is correct, then should I proceed to fitting in the thrid and last amp chip in the U7 position?

Also where do I find the correct length chip amp mounting screw isolation washer for this sort of amp chip?

thanks

[jimmy74]

went ahead and fitted in the thrid and last amp chip, the current limiter lights up at first then goes down to a dim glow, pin 3 for all amp chips is 0 and pin 8 is -2.3v.... what next?

opinions?

thanks

[J M Fahey]

After turning on and checking you still have 0V on pin 3 , connect a speaker first and drive it with music, repeat with the other, then with both.