High Freq Sound at Output

[Danlampton]

I'm working on a Long Ranger III portable PA and I am left with on annoying issue. When I turn the unit on a high pitch whinning sounding begins to ramp up. It is much lower in level than the source but clearly present. The volume contral does slightly affect the volume of the tone.

I am just not finding this issue. Can anyone point me in the right (or any) direction?

There is an LED (D9) that is lighting up on the circuit board - I have no idea what that means. There is also a bunch of tiny surface mounted components with which i have no expierience.

Thanks for any help. lriii schematics.pdf

[Mark Black]

Have you tried contacting Lectrosonics tech support? Lectrosonics - Repair Services

I notice numerous revisions and reference to ECO's (Engineering Change Orders) on the schematics, so it's possible they may have encountered the problem before & created a fix. Or they may simply have encountered it before and can point you to the likely culprit.

I think D9 should be lit under normal operation - it's part of the biasing for Q17, which looks like part of a circuit to monitor & adjust for VCC changes.

[Danlampton]

Thanks Mark. I have tried that and I am trying again.

[Jazz P Bass]

D9 LED indicates the presence of VCC. Positive power supply amplifier voltage.
I like the note:"cut trace & install R84 on revs A-F for blown U1 & U7."
See if you have any Vdc on the output pins (1 & 7) of the IC's.