[Amps] Ten Tec 425 Repair

[Ken Claerbout]

A friend asked me to look at his 425. First thing I noticed was the plate choke was burned. I think it came from trying to operate on 12 meters. I replaced that choke with the Ameritron model, which I understand is not resonant on 12 meters. The amp also had a burned padder cap on the 160A position. That's replaced. The amp is working now on every band except for 75 & 80 meters. It tunes fine to about 500 - 600 watts. Advancing the power further, the grid current goes from zero to pegging the meter, with no in-between. 

I've replaced the padder caps for those segments with no change. The band-switch and contacts have been checked. As far as I can see they look fine. I thought I was on to something, when I found the connection from the tap on L4 had a cold solder joint on the band-switch. L4 is used on 160, 80 , and 75 meters. I Re-soldered that but same thing. I'm beginning to think that the toroid L4 is wound on is fried although, I don't see any obvious signs. I haven't taken it apart and removed the tape. After several tests on 80 & 160, I took the cover off and felt the core. It was very warm, almost to the point of being hot. I'm mindful that L4 also comes into play on 160. But something appears to be heating up, or arching, once power is advanced much past 500 watts output. Also when I first turn on the amp, it's cold, I can get close to legal limit on 80 & 75. But after several attempts the grid current problem pops up. Seems to indicate the toroid is heating up? 

I should also mention the input SWR on 80 is quite high, 3.5:1 but much better, 1.6:1 on 75. I suspect that's a separate issue and will look into that once I get the output side squared away.

So the question, especially for those who have experience with this amp, I don't, have you seen the core in L4 go bad? Is there something I'm overlooking? I did try a spare set of tubes, nothing changes. 

73
Ken K4ZW