Audio Systems: YSM1P Monitor Hiss, full wave rectifier, amplifier stage

Question
Hello,
I recently was given a pair of studio monitors due to a
strange problem they have, and it would be extremely
fortunate if I could find a way to remedy it.
The monitors are two Yorkville YSM1P monitors, and the
problem is that after a period of time, usually around 15
minutes, of the speakers being on and plugged in to an
output such as my laptop, a monitor headphone out, or my
ipod, both monitors begin a moderate hissing noise. This
doesn't happen simultaneously, but both start hissing within
a minute or two of the other. It's a shhhh noise that
persists until the speakers are turned off. Once the
speakers have been turned off, they can be turned back on
immediately and show no signs of hissing for another 15
minutes. I have tried multiple cables as well (2 different y
cables, and an assortment of 1/4" instrument cables).
The hiss is definitely loud enough to ruin casual listening
or mixing, but other than that, the speakers sound
wonderful. What can I do to make my monitors function
correctly?

Answer
It could be a power supply problem. It could be one of the opamps acting up.  There are several possibilities.  A little troubleshooting is in order.

ONE: Does the hiss level increase and decrease with the volume or loudness settings?  If so, that indicates the problem is in a front end amplifier stage (ahead of the volume control section). And does the hiss go away when no cables are connected and volume is at zero?

TWO: If the hiss level remains the same level independently of the loudness setting it could be the power supply.  It is a very straightforward power supply design and you might want to start by replacing the big fat electrolytics just off the full-wave rectifier.

You can find the schematic diagram here:

http://www.yorkville.com/downloads/servman/smysm1p.pdf

They are 2200 MFD at 40v.

THREE:  Since the hiss doesn't start up until after a while and repeats itself without warm up it is more likely a capacitor somewhere in the circuit.  

FOUR:  Also, don't rule out some kind of instability that causes a high frequency birdie in one of the opamps.  You can usually find them with a pocket radio as a wand moving it around close to the circuit board.

Maybe this will get you started on it.  

Good Luck.