"Tweetldee" wrote in message
news:OP4wd.266447$R05.42256@attbi_s53...
"Dean" wrote in message
om...
Hi,

I have done a quick search but couldn't find a problem that was very
similar to mine in previous posts, so here goes.....

I have just bought a 30 year old amp (Rogers Ravensbourne Stereo) and
it's got a bit of a problem. There is a continuous static noise coming
from the speakers (one channel more than the other). You can also see
the cones in the speakers moving in and out slowly along with the
noise almost like applying a DC voltage.

The amp works OK otherwise, the noise seems to be from the output
stage as volume, bass, treble, input selection etc make no difference.
I have whipped the top off and can't see any obviously fried
components. There are some thermistors on the power supply that look
pretty tatty but I shouldn't think these would introduce a static type
noise.

Thanks

Dean

You didn't say wherther the amp is tube or solid state, but given the age,
I'm assiming solid state. You also didn't state whether the transistors
are
in sockets or hard-wired. If they are in sockets, start there by removing
the transistors and renew the contact surfaces by removing/inserting the
transistors a few times to break up the contamination.
If that doesn't solve the problem, there are a number of possibilities for
the cause of the static. Electrolytic capacitors are always suspect in
equipment that is old as your amp, especially if it has been in storage
for
more than a few years. Paper capacitors are also suspect. Resistors,
especially carbon composition types are not immune from causing this type
of
problem. Even the transistors can become noisy and cause the static.

Thermistors in the power supply?? Don't think I've seen that very
often...
more likely the thermistors are near the output transistors for bias
stabilization.

Other things to look for include weak or stressed solder joints and
components that have gold plated leads. The plating is known to separate
from the lead and cause a myriad of problems, including the type you're
experiencing.


Rather than thermistors they're probably posistors, they have a higher
resistance when cold which drops much lower as they self heat. Common in
larger audio amplifiers to reduce the inrush current when first turned on.