There are FOUR controls that affect the volume of signals going through plus the tone controlling ones.
and your
You used the word "volume". Based on the schematic, and assuming you are talking about the control labeled "Volume" and not reverb, master or drive, I can give some hints.
OFTEN the resistance element in pots MAY have a bit of overtravel which can act like your unit, but USUALLY not as far upscale as yours appears..
If the control you are refering to is not the "Volume" but one of the others, then there are other things to look at.
The action you describe would be one of probablly an inverse signal leaking through and when you go up to 1.5 you are actually cancelling the leakage.
I SUSPECT you may have a cracked circuit board in the vicinity of the input jacks that is leaking the inverse phase of the input past the input triode tube stage.
Board cracks OFTEN occur near input jacks because people trip over cords... (save yourself by running cord through a handle before plugging in the plug)
This should be easy to find. Connect an audio generator to the input and use a scope to find the problem. Without a scope, use a capacitor to "ground out" suspected signal paths.
On the schematic they show a resistor (10 ohms) from ground near the input to the pot bracket. I have personally SEEN capacitive coupling to pot brackets feeding into the amps from this source sending in stray signals.
If this is connected to other equipment, a ground loop could cause problem, so test with nothing but the guitar connected.
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is this on both channels or one and which channels are not working??? sounds like a channel switching relay is bad. you can get one on ebay.. type in search for fender hot rod deluxe switch and you will see one that is black and rectangular has 8 pins, omron brand, 24vdc, 1521N1 0.6a @110vdc
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