[Amps] TL922 failure

[Victor Rosenthal 4X6GP]

I'm about to take apart my TL922, which has worked perfectly for at 
least 6 years -- until this morning. I would appreciate suggestions as 
to what could be wrong.

Initial symptoms:

1) Low power output, about 700w instead of normal 1300.
2) Loading cap needs to be set to less capacity than normal.
3) Arc in loading cap.
4) Happens on multiple bands

Current symptom:

1) Very low RF output.

Test conditions:

1) Dummy load or antenna the same.
2) Normal plate voltage.
3) Normal grid current.

The amplifier has several mods, made a long time ago.

1) Grids directly grounded.
2) QSK relays.
3) Soft start.

Now I am about to take it to the bench, hopefully to find something 
obvious. Does anyone have any ideas of what to expect?
-- 
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/