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Right, the 12at7 normally is used as the reverb driver and if this one's microphonic it feedbacks when you push it. You already did swap the AT7 as you said in the first post?
One Thing you could check: Sometimes the reverb tank ist attached via 2 small round plugs, a red and a black one here they are called cinch-cable.
Check if there's a marker to show where the red one has to be plugged in - fender doesnt mark this, but a tech sould do that before unplugging when he takes the amp out - maybe someone didn't take care and swapped the two cables - I'm not shure what the effect would be but this could be it.