I have a 1972 Fender Twin Reverb with AA270 circuit and no MV. I just discovered that the "MIDRANGE" tone control has to be turned up past 1 just a tad or the amp will not produce sound. Once the midrange tone control is turned past 1 it will produce sound and the Volume control works as it should. This is the way it is on "both channels". I have several AA270 schematics and Layout diagrams and all the wiring, resistors, caps, optic reverb module , etc are wired according to the schematic. Is this normal for the Twin ?