Wednesday, September 14, 2016

Ameritron AL-80A - Fixed!

AL-80A re-assembled, needing just 3-500Z and cover.
As I wrote previously, my AL-80A needed a repair. The plate choke was burned, with several windings melted, the protective diode and capacitor on the B- rail to ground had gone open, and I'd found that the amp would not tune on 160m -- due to some melted switch contacts.

I thought it might be a while before I could get around to doing this -- considering the last repair took about a year. However, since I had some parts on hand, I put the amplifier on the bench and got started.

1N5308 diode next to glitch
resistor on rectifier board. And
the replaced capacitor below.
First order of business was to fix the protective circuits on the B- rail. I removed the 1N4007 I had installed earlier. I tested it and found it to be completely open. Which is pretty much what I expected, given that the capacitor had opened destructively....

I replaced the capacitor with a fresh part, and installed a 1N5308 diode, as recommended by W8JI. And I mounted it on top of the board this time.

Next part to arrive was the Ameritron Plate Choke (Part Number 10-15197). It was a thing of beauty, and despite the package having a nice accordion dent in one corner, was intact.

Replacing the choke was actually pretty easy. I figured out I had to unsolder the wire to the parasitic suppressor and plate cap connector, and then the two blocking capacitors. This was much easier with the tube removed. (I put the tube inside the plate choke's packing box). I then removed the mounting screw and unsoldered the high voltage and the bypass cap.

Beautiful new choke installed.
Oh! Did I mention that you need to unplug the amplifier and make sure the plate circuit is completely discharged before sticking your hands into the amplifier? No? Gosh, that would be dumb. Of course, you do that first.

The old choke had a teflon washer under the ceramic, but it completely disintegrated when I removed the choke. I decided it wasn't required, and mounted the new choke directly to the sheet metal base.

The new choke had a slightly different geometry with respect to the lugs at the base and top. I decided to loosen the lug on the top and rotate it about 100 degrees. Otherwise, the blocking caps were not going to reach. To move the lug, I first had to unwind the top end of the winding and re-wind it to the lug. I think the choke wire ends were unsoldered for this reason.

Once in position, the lugs were easy to solder.

The final task replaces the melted switch contacts. I must say I got a lot of positive answers from the Amps email reflector (amps@contesting.com). Their diagnosis of my issues was spot on, and I got a lot of helpful advice.

Vic Rosenthal 4X6GP sent a link to an article about band-switch arcing on the Heathkit SB-1000, which essentially was a clone of the AL-80A. He suggested I should add the corona washer, which would help prevent more melted switch contacts. From what I can tell, my AL-80A is a very early model, and it did not have this modification. George Hall N2CG also sent me an email with pictures of the washer.

A huge thank you to goes to Louis Parascondolas, who not only wrote to me with suggestions on how to replace the switch contacts, but mailed me six contact elements (I needed four), plus the brass 2-56 hardware needed to secure them. Thank you, Louis!

Switch wafers disassembled. This was the hard part.
On left is the wafer where the contacts were replaced,
shown with contacts removed, and glued.
Replacing a switch contact isn't a huge deal. The first step is to remove the switch wafer. That took some doing. I deduced I could accomplish this without doing a lot of desoldering and re-wiring. The rear bandswitch wafer is basically connected to the coils and a couple of padding capacitors. One end of each coil was fastened to the capacitor studs, which simply required removing a couple of nuts. The larger coil had two small phillips screws holding it to plastic mounting blocks. The padder capacitors could also be unscrewed from their lugs. Easy right?

Not so fast! a single wire needed to be unsoldered connecting to the loading cap, and the rear switch wafer slid off with both coils attached.

The second wafer just had three connections, easily desoldered: coax and coil end, coil tap, and the cap. Once those were removed, it slid off.

The third wafer is the one we are after. I unscrewed the connection to the plate padder cap, and slid it off with the coax connection in place. That made it easier to unsolder the coax.

Once you get these wafers off, you have to handle them very gingerly! They are delicate ceramic and can easily shatter. The tricky part is to remove the old contacts. I recommend a drill press and very small drill bits to drill out the rivets holding the contacts in place. Gradually go to larger bits until you have removed all of the rivet material. Go very slowly because you don't want to break the ceramic switch wafer.

This is what I told myself, and I managed to get the contacts off with no trouble. I still had the shell of the rivet, which I carefully drilled out of the first hole. The last hole, of course, was a problem. I broke the switch wafer.

New switch contacts engaged for 160m.
I apparently said something aloud which brought my younger daughter into the basement to see if I was OK. (I was) The wafer was now in three pieces. What to do? I decided that I could glue the three pieces together, and it should hold. Superglue to the rescue! The glue took easily to the ceramic, and soon I couldn't tell where the split was.

I cleaned as much of the burnt contact debris off the two switch wafers as I could. When I was done, you could not see any discoloration of the L network (second) wafer, but there was still a little in the contact well of the 160m padder (third) wafer. The contacted would cover that up.

Adding the four contacts to the switch wafer was really easy.

Getting all the switch wafers re-assembled was the hard bit. I added the coax ground connection to the 160m padder (third) wafer, but I held off on the padder capacitor connection until the very end, so I could place the corona washer.

The L network (second) wafer was a bit problematic. I couldn't remember the orientation of the switching portion of this wafer, as there are two ways to put it on. Then I couldn't remember which wires went where. More pictures before disassembly would have been helpful. Studying the schematic made it clear which way things went.

Corona washer in place.
The bandswitch (first) wafer was easier. The tricky bit was soldering the connections to the loading capacitor.

Last bit was to install the corona washer to the padder cap. A brass washer was specified, but I didn't have one in the junk box. I made do with a steel washer and a lot of solder. I verified the continuity of the connection with an ohmmeter. Perhaps I'll redo this the next time I have to fix the amp. (which I hope may be a while)

After any fix like this, there's always the moment of truth. No sparks turning it on, and a quick test on a dummy load proved it tuned up with no surprises on 160, 80, 40, 20, 15, 10, 12, and 17m.

Ten minutes later, I had replaced all 13 cover screws and was already using the amp to chase DX on 40, 20 and 17m.

Working great again!

2 comments:

  1. This post is just what I needed. I thought a plate choke from an AL-80B would fit the AL-80A, but I was not sure. I need to replace the one in my AL-80A. I got my amp about a year ago. I gave it a test on the bench when I brought it home and it worked fine. I did not open it until recently. That is when I noticed a previous owner must not have be careful replacing the final. The plate choke has a few damaged windings. Not yet burnt or over heated, but just the same an area where heat could build or open.
    TNX & 73

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