Saturday, July 25, 2015

Fixed the AL-80A (Sorta)

My AL-80A amplifier has been busted for a while now. I think it was back in April of 2014 that I was chasing some DX, trying to quickly tune up the amplifier and something went ZAP! inside the box.

After that, the grid meter was reading all funny, like registering about 60 milliamps even with no RF. That's not right.

I put it on the workbench, and at some point disassembled it. I had pulled the front faceplate off, then pulled the meters and meter switch board out through the meter holes. But I never got around to fixing it. It sat like that, taking up much space on the workbench for months. Until today.

I had determined that the 3-500Z tube was likely OK. No indication of shorts or other damage. I figured the problem must be on the meter switch board -- one of the components there must have gotten damaged. Sure enough, a little probing around found that the 1.5 ohm 3w 1% resistor in the grid current network was showing as completely open.

I also figured out that the 10 ohm 20w "glitch" resistor I put into my amp was wired in incorrectly. The resistor needs to go in the B- circuit on the same side as the capacitor bank. I had placed it before, were it wasn't doing any good.

It's quite possible a glitch took out the 1.5 ohm 3w 1% resistor. This is not a typical value that I have lying around in the junk box. However, I did have a 1.5 ohm 1w 10% resistor. That would do at least for a proof of concept fix.

Wired the resistor in, put the switch board, meters back in place and put the front panel and covers back on.

Moved it over to the operating position, and behold, it amplifies! Well mostly. I got it to work on 80-10m, but I wasn't getting much of any output on 160m. Not sure what's wrong there. That fix will have to wait a bit.

However, I don't have any antennas for 160m that can take more than 100 watts anyway. Seems like the AL-80A should be good to use for a while, at least until I can get the proper resistor and troubleshoot 160m.