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I don't know what make & model of microphone or clip light you're using, but try unplugging the microphone cable from the preamp. That will tell you if the problem is in the microphone or the preamp/amplifier.
If you still get noise with microphone unplugged, try checking the power supply. You'll probably hear any hum from power supply. Make sure you're using one that's recommended or sold by the manufacturer.
Finally, try replacing the output cable. Any open circuits in the output cable could cause hum & noise.
This problem is usually dur to some corrosion in the speaker relay. If the relay can be opened, burnish the contacts and you should be OK. The higher volume creates a higher voltage that can "jump the gap" so to speak. Once the connection is made it will remain. That is why lowering the volume will work.
To test speakers get a 1.5 volt battery and put the (-)wire of the speaker to the negative end of the battery. Then take the + wire and for a split second touch the positive terminal of the battery. You should hear the speaker pop! Listen to each unit inside the speaker, any that don't pop are not working.
If you look inside the amp you may see that something has burnt up. Look carefully at whatever is on the heat sink.
It's a muting problem, not a power amp problem. The muting circuit on this amp has a very high impedance node, which makes it very sensitive. Right next to the area of the board that contains the muting circuit is a large-ish capacitor that is held in place by a blob of glue. The glue has usually spread out enough to reach some of the muting components. The glue decays over time and becomes slightly conductive. This is enough to keep the amplifier in the muted state, permanently, instead of releasing it about two seconds after power-on.
The picture shows the amplifier with the top removed. The box is where the old glue used to be. The glue is to hold the purple tubular capacitor steady. It is not really necessary unless you have the amp sat on top of your speakers (not recommended).
To remove the glue, unplug the amp from the mains, remove the lid (six screws), locate the area in the picture and scratch the glue away with a large needle or tiny screwdriver. The most important bit is to remove the glue from the bare wires at the ends of the resistors (the brown stripy things).
If this doesn't solve the problem, or it returns, the capacitor that determines the length of the muting period may have failed. This is a slightly less diy repair as you will need a soldering iron and a new capacitor.
It sounds like a source slecton issue. Try the following and if that does not help I would start with a simple stereo setup and source and work up from there.
The RV-8 is powered on, but there is no audio.
1. Make sure volume level is audible. Volume level can be
increased with the front-panel volume knob or the remote
control VOL and buttons.
2. Make sure audio has not been muted. The message “MUTE
ON” or “FULL MUTE ON” will appear in the on-screen and
front-panel displays when audio has been muted. To deactivate
mute, press the Mute button or adjust the volume level.
3. Check the INPUT SETUP menu DIGITAL IN and ANALOG IN
parameters to ensure the appropriate audio connector is assigned
to the selected input.
4. Make sure the RV-8 is receiving an audio signal. To do this,
follow the instructions that begin on page 2-29 to open the
STATUS menu for the current input source. Hint: (Use the "STAT2" Button on the remote.)