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?

Thanks
Louis

"ll" wrote in message
oups.com...
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?

Thanks
Louis


Several questions. Why were you having to 'install' the Tx in the first
place ? Was it a replacement for one that had failed ? If so, in what way
had it failed, and was there any apparent reason for the failure ? If this
Tx is a replacement, is it an exact manufacturer's original, or a 'ringer'
that you've sourced yourself ? If a ringer, is the output voltage exactly
the same as the original ? Have you checked if any of the power supply rails
are dropping as the output diminishes ? If the power supply uses monolithic
regulator ICs ( 78xx / 79xx series ) then if these are subjected to
excessive input voltage, compared to the amount of heatsinking that they
have, they will rapidly overheat, which will cause them to retreat into
their SOA, by reducing their output voltage. Also, if the mounting bolts to
the heatsink have come loose, or the heatsink compound behind them has dried
out and powdered, the same thing will happen, even with the correct level of
input voltage.

Does anything mounted on a heatsink ie regulators, output IC or transistors,
'feel' or even smell like it's getting too hot ? Very often in small or
cheap amps, the heatsinking is only just about adequate for the job, and the
devices will run very hot even in normal circumstances. Never-the-less, they
should still not be so hot after 30 seconds, that they are so uncomfortable
to touch that you can't keep your finger tip on for more than a couple of
seconds.

If you suspect that you have found a device that's getting too hot, if you
can find enough exposed metal on the heatsink, you can try clipping on an
office bulldog clip to temporarily increase the amount of heatsinking to se
if that makes it stay on longer.

Arfa

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 ?

Graham

"Eeyore" wrote in message
...


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 ?

Graham


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.

Arfa

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

"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.

Arfa