Thank you for your reply, Phil!

I was testing this amp with an electric guitar. It belongs to a friend and I am working on it for him. It came to me for non functioning reverb, which ended up a bad connection on the connector inside of the reverb tank that connects to the output coil.

I thought that it was a feedback problem created by my guitar, but it still squeals without anything plugged in to the input jack. That is, if you depress the "master phase" switch and turn the master reverb past 4. I might drive myself crazy with this issue.  :loco

I've attached the service manual/schematic. Any suggestions would be greatly appreciated!

Hello everybody,

I'm having an issue with a Fender Acoustasonic Jr (pre-DSP). For some reason, this thing squeals terribly if I have the master reverb over 4 AND press the "Master phase" switch.
The reverb sounds fine at max, as long as I don't press the "Master phase" switch. Any ideas what could cause this?