This happened yesterday. You see, I got off work early and thought I'd spend the afternoon chasing a little DX. I managed to work VP6J on 12m CW, and later they moved down to 17m RTTY. I've never worked Pitcairn using RTTY, so I was watching their frequency closely. They weren't strong, but I went ahead and set up the amplifier on a clear nearby frequency. The day was waning, and I figured if they ever came up a bit so I could get clear print, I'd want to give it my best shot.
In the meantime, I moved down to 30m and worked some other stations. Of course, I switched the AL-80A to standby, and cranked up to 100 watts.
Everything was fine. Until later I went back to 17m to see how VP6J was doing. Their signal had come up a bit, and I could actually read the print well enough to try for a contact. I reached over and flipped the amplifier to operate, and clicked the button to send my call.
That's when I had my moment of clarity.
There was a loud noise from the RF compartment of the AL-80A, along with several sparks, and abruptly, it went dark. An acrid burning smell hung loosely in the air. Certainly, something had gone wrong.
I knew right away what I had done. I had transmitted 100 watts into an amplifier designed to take only 50 watts input.. An amplifier whose original design had not included 17m, and was happier with 40 watts on that band.
|Roasted plate choke in AL-80A.|
Once I got the cover off, it was easy to see the problem. The plate choke has a black burnt mark. Looking closer, the turns of the choke had melted and separated. The choke would need to be replaced. From the location of the burn, it looks like it had arced from the end of a screw on the plate tuning capacitor to the choke.
I took the tube out and gave it a quick check with an ohmmeter. No grid to cathode or grid to plate shorts, which was good. The Taylor 3-500Z is pretty darn tough, and likely shrugged off the abuse I had just put it through.
|Low voltage cap blown to protect meter.|
I'll be replacing that cap, along with the 1N4007 rectifier, since it must have gone open for the capacitor to fail. I'll replace it with the recommend 1N5408 rectifier, which can handle a whole lot more surge current.
|Melted switch contacts.|
I ordered a replacement plate choke from Ameritron. They don't carry the original AL-80A part, but instead use a single choke (Part number 10-15197) for virtually all their amplifiers, including the more modern AL-80B. I've read this part avoids resonances on 17m which may be present in the original single-winding choke.
In the meantime, I'll have to chase DX without an amplifier for a while. Perhaps there won't be as many fireworks.
Update: I've been told that part of the problem with the 160m switch contacts is the lack of a corona washer. The picture clearly shows an absence of this part. I'll be adding one when I make the repair.