[Amps] Antenna Tuner Overheating

[Jim Brown]

I'm seeing some overheating that can only be in one of two possible 
components. One is a TenTec 238 antenna tuner, the other is the 
loading coils in the antenna that I'm feeding with it. I've also 
observed the same symptoms using a Ten Tec 229B tuner with the same 
antenna. The tuners are nearly identical. Here's the setup.

Antenna is an 80/40 dipole with no balun, fed with some vintage 75 
ohm KW twin lead, with both ends of the twin lead tied together and 
fed as a long wire on 160m. The tuner has a good counterpoise on it 
(roughly 140 feet of 6 ft high wrought iron fence that runs around 
the front of my house). This thing tunes nicely, and indicates a nice 
low VSWR at the beginning of transmission, but the SWR rises, and the 
lights start dimming when I key it (indicating more power supply 
current), about 15 seconds into the transmission. I'm seeing this now 
with a Ten Tec Herc II at only 500 w, but I've also seen it with my 
Titan. 

Any ideas what's going on?  The fixed capacitors in the tuner high 
voltage types, but with significant temperature coefficient. Are they 
heating and changing value?  

The other possibility is that the loading coils are arcing or 
drifting as they heat. They are pretty near each end of the antenna. 

Jim K9YC