"ll" wrote in message
ups.com...
On Jul 3, 8:18 am, "Arfa Daily" wrote:
"Meat Plow" wrote in message

...

On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote:

"Eeyore" wrote in message
...

Arfa Daily wrote:

"Eeyore" wrote
ll wrote:

I recently installed a transformer (12VAC, 2Amp) in a musical
instrument amplifier (rated at 10 Watts RMS), and when I test it
with
a variety of input sources (guitars, keyboards, different
cables),
for
about 30 seconds, the amplifier produces a nice healthy volume
level,
and then, consistently, the volume just drops about 50%. For
some
reason, this problem sounds like it might be heat-related. Has
anyone
else experienced this?

Are you by any chance sine wave testing it for those 30 seconds ?

I took it he wasn't, as he says that "when I test it with a variety
of
input
*sources* ... " and then specifically mentions guitars and
keyboards.

I was a bit dozy at the time.

I've seen that effect though.

Graham

Likewise. I often deliberately test group amps with a 400Hz tone for a
few
minutes, just to get the output stages thoroughly thrashed up to
temperature.

I made a looped test CD with me playing some leads, licks and riffs on
my
Les Paul through my Boss GT-6 just to test guitar amps.

You should send me the files !!

Arfa


Yes, guitars, lots of guitars (a variety) were used to test the amp.
The tx was an exact replacement. It seems we have narrowed this down
to heat-related.


In what way had the original Tx failed ? Might have some relevance to the
problem, which although may be heat-related, might not be a 'heat' problem -
if you see what I mean ... For instance, it may be that one of the supply
rails is dropping due to a monolithic regulator IC going into heat foldback,
but that could be because something on the end of it is drawing too much
current. I have also had this occur due to open circuit decoupling caps on
the regulators, allowing them to hoot, and even open circuit decouplers
around the output IC, or open circuit Zobel networks on the output, either
of which can allow the stage to hoot ultrasonically.

Arfa