Nick
That sizzling sound is HT arcing somewhere and not good. I have the same radio and find the drive doesn't need to go much beyond 11 or 12 o'clock for full power out, so I have always left it at that point.
You need to take the covers off and give it a good visual inspection, with a strong light, in and around the PA box and driver valves. Look for burnt resistors.
WARNING!! the 900v HT remains for sometime even when the main power has been switched off. There are no bleeder resistors!! Also HT is present on the PA valves even when in RX mode and Heaters switch "OFF"
Check the 6146 anodes with a testmeter set at 1000V before touching anything.
If you're lucky it could just be a spider web shorting something. While you're in there look for any signs of disturbance of the crystal mixing board. Any hacking about in there will indicate it may have been "adjusted" for use on 27 MHz at some stage.
Did the power output vary when you heard the sizzling noise I wonder? If so it could have been flashover in the PA bottles.....once again, not good. However, once that happens its usually game over for the 6146's and the EHT tripler board.
Also worth knowing is the "PO" meter on the rig is only a relative power output indicator, basically a sniffer via a diode rectified to the meter and is adjustable via a pot on the rear apron, so needs checking with an external power meter.
Finally, my tip is to count up to 10 while tuning up.
