Studiomaster mixer / amp fault

[Byron G. Jacquot]

>My father has a Studiomaster mixer/amp which seems to take a couple of
>minutes to 'warm up' after being switched on. During this short time, it is
>completely dead and nothing lights up. When it does finally come on, it
>works fine, no problems at all. I'm sorry I can't be very specific at this
>moment in time as to what model he's actually using - I can find this out.
>However, I just wondered if this might be a common kind of fault with
>amplifiers in general (or with Studiomaster equipment) and, if so, is there
>a fairly common cause for this?

>From the description, it sounds like it could be a thermal failure in the
power supply...maybe combined with a cold solder joint, or a flaky transformer.

Does the behaviour happen every time it's switched on, or only when turned
on after being unused for a while?

What can happen is that when it's off, the supply and surrounding areas are
cool, and they contract a slight bit...when they do this, they might break
some vital connection.

When you switch it on, there's enough jiuse in the right components to let
some current flow, which warms up the supply curcuits & the surrounding
area.  Eventually, they're just enough larger to fix whatever connection
what caused by contraction.  If there are cold solder joints on parts that
get warm, they could become intermittent like this.

The connection could be to one of the supply rails, or somewhere along the
audio path, or even somewhere else.  It might take some tedious
troubleshooting to track down...

If you were to look inside, you'll probably find some standard voltage
regulators and opamps.  If you can find the datasheet for the opamps, you
can test the continuity between the supply rails at the opamps and the
supply itself...or turn it on and make sure they're getitng reasonable voltages.

Another possibility is that it's got a time delay relay on the main
output...to keep it from making big noises when switched on/off, and it's
delay somehow drifetd from a few seconds to a couple of minutes...

Byron Jacquot