Hi Group: Any advice for the following? I'm a guitar player, so bear
with me! (and a very amateur amp repairman) I'm repairing a Behringer
Ultatwin amp---without a schematic. I'm pretty sure the output amp--a
National Semiconductor LM3886T--is shot. I'm going to replace it, but
I'm wondering why it totalled out in the first place. Everything seems
to check ok---any suggestions as to what to look for?
Thanks in advance.
Arthur

Arthur:
I would replace the output amp chip and that will probably be all that is
wrong..... obviously you should first do some component testing and look for
other faulty parts, but unlike most audio power output stage discrete
designs with individual transistors, the "power pack" chip IC modules
usually don't have catastrophic failures because of faults in preceding
amplifier stages.... they do sometimes but not usually. It could have just
been it's time to fail or maybe too much volume well into distortion, too
many speakers, etc. Make certain that you use plenty of heat sink compound
when mounting the replacement chip.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
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wrote in message
oups.com...
Hi Group: Any advice for the following? I'm a guitar player, so bear
with me! (and a very amateur amp repairman) I'm repairing a Behringer
Ultatwin amp---without a schematic. I'm pretty sure the output amp--a
National Semiconductor LM3886T--is shot. I'm going to replace it, but
I'm wondering why it totalled out in the first place. Everything seems
to check ok---any suggestions as to what to look for?
Thanks in advance.
Arthur

wrote in message
oups.com...
Hi Group: Any advice for the following? I'm a guitar player, so bear
with me! (and a very amateur amp repairman) I'm repairing a Behringer
Ultatwin amp---without a schematic. I'm pretty sure the output amp--a
National Semiconductor LM3886T--is shot. I'm going to replace it, but
I'm wondering why it totalled out in the first place. Everything seems
to check ok---any suggestions as to what to look for?

Some reasons why power devices fail:
1/ Voltage transients caused by lightning, etc.
2/ Too much signal (volume turned all the way up).
3/ Inadequate thermal design of the heatsink.
4/ Improper installation (no thermal compound).
5/ Environment (unit was operated in an enclosure and thus overheated)
6/ Shorted output.
7/ Murphy's Law

Could have been a normal failiure or poor thermal connection to the
heatsink. Some Behringer Eurodesk PSU's, for example, suffered from poorly
machined heatsinks which meant the voltage regulators didn't have full
contact with it and so could not dissipate the heat quick enough, and
failed.

When you buy the new device, buy also a good quality thermal pad if there
was one fitted originally. The more expensive ones (grey or white rubber
sheets) conduct heat better and don't require heatsink compound. Also make
sure the mounting area is clean and there are no burrs or any other debris
between the heatsink and the device, and tighten the mounting nut well.
Make sure there is no possibility of speaker wiring faults.

Having said all the above, I would expect there to be at least rudimentary
protection built in to this device (check the manufacturers data sheet) to
prevent pverheating or damage by short circuits. But reality and theory are
sometimes two different beasts.

Gareth.


wrote in message
oups.com...
Hi Group: Any advice for the following? I'm a guitar player, so bear
with me! (and a very amateur amp repairman) I'm repairing a Behringer
Ultatwin amp---without a schematic. I'm pretty sure the output amp--a
National Semiconductor LM3886T--is shot. I'm going to replace it, but
I'm wondering why it totalled out in the first place. Everything seems
to check ok---any suggestions as to what to look for?
Thanks in advance.
Arthur

Thank you!
Arthur

Thank you!
Arthur

Thank you!
Arthur

I am presently repairing a guitar amp of much older vintage, in which
the previous repair person tore up some of the printed circuit traces
by removing parts the wrong way. It's worth being very careful pulling
a multi-pin part. One method that sometimes works is to clip all of the
leads, throw the part away, and then you can work on one lead at a
time.

My technique, once the pins are cut away from the chip, is to melt the
solder and pull the lead out with a tweezers, then once the leads are
gone, clean out the holes with the desoldering pump. This seems to
inflict minimal damage on the board.

wrote:
Hi Group: Any advice for the following? I'm a guitar player, so bear
with me! (and a very amateur amp repairman) I'm repairing a Behringer
Ultatwin amp---without a schematic. I'm pretty sure the output amp--a
National Semiconductor LM3886T--is shot. I'm going to replace it, but
I'm wondering why it totalled out in the first place. Everything
seems
to check ok---any suggestions as to what to look for?
Thanks in advance.
Arthur