I noticed the following during a recent QC test. I've only seen this with the CHAN/ZON brand power supplies.
PROBLEM:
When you have one phone plugged into the 1A2 KSU board and
pick up the intercom line to listen, you may hear a weird
repetitive "squeaking sound", like someone rubbing on a balloon.
The pattern of sound may follow the blinking of the KSU's
"CPU STATUS" LEDs.
The sound is typically low volume, but loud enough to be annoying.
This does not happen if there's 2 or more extensions plugged in.
The noise only happens on the ICM line, not the other CO lines.
ANALYSIS
Apparently under the very low load of a single phone plugged in,
the CHAN/ZON switching power supplies have a small voltage ripple
that the ICM VOICE BATTERY passes through, and the phone's hybrid
picks up this small ripple as audible noise in the voice frequency range.
The tiny load of the CPU status LEDs affects the ripple,
which is why the noise follows their flashing.
It's not unusual for switching power supplies to exhibit noise
under low load. Some larger switching power supplies even
have a minimum load requirement.
The voltage ripple is quite small; less than 1/10th of a volt;
it can barely be perceived on an analog scope at full scale.
SOLUTION
The sound goes away completely if you simply add more phones.
Just one phone is enough.
Apparently the 40mA load of just one extra ICM lamp in a 2nd phone
is enough load on the supply to make the problem go away completely.
NOTES
Since it's not practical to use the intercom line with just one phone,
I'm calling this a non-issue. With two or more phones present,
the problem doesn't exist.
But the issue is at least note-worthy.
I found with one phone attached, just adding a 1W / 480 ohm resistor
across the supply was just enough to make the problem go away, and the
issue doesn't reappear as more load is applied. This issue only happens
under very low load. So in practice, one shouldn't add a resistor to
solve this problem; simply having 2 or more phones solves it.
This issue doesn't show up on the other lines, since the KSU doesn't
introduce power into those lines; their voice battery is provided
by the CO. Only the intercom's voice system is powered by the KSU.
I'm treating this as a non-issue, since it's impractical to make use of the intercom line with only one phone;
when there's two or more phones, the problem goes away.
I didn't notice this in testing, since I do the QC tests with 4 phones plugged into the board, which hides
the problem.