Hello Fred et all.
I was on the right track.
The RG-58 cable does indeed connect to pin 8 of V3, this is the plate of the 1st IF amp.
The 6.3VAC filament was a no brainer, I tapped it off V2. As for the B+, I found 205VDC on the HV supply.
I replaced the 12AX7 with a new one, and did the normal smoke test, all the smoke remained inside their respective components.
The QF-1 is a very nice addition to the SX-99. All that's left to complete my vintage Novice station is to make a circuit for the DowKey and rec muting.
73
Dan, N8IE