Thursday, November 24, 2016

Rethinking the Station Desk

Recently, the water supply line to my Gwinnett house had to be replaced, and that meant I had to clear out my station operating desk so the plumbers could work. With everything moved out from that end of the room, it was the perfect time to re-think the station desk.

Truth is, I had been thinking about this for the past year. Ever since I moved back from Floyd county, I had considered making some changes. Some of this was based on my experiences with the Micro-shack.

Let's start with the desk. In 1985, I bought a 30" solid-core door and placed it over a couple of two-drawer file cabinets. Over thirty years and two moves later, that's what I'm still using. The door has stood the test of time. I originally stained it and coated with polyurethane. When I moved to Gwinnett county eight years later, I sanded it down again and gave it a fresh coat of polyurethane. More than 20 years later, it was again time.

I also made one modification I should have done from the start. I took a router and a 1/4" radius bit and rounded off the top edge of the door. For years, the sharp edge of the door had been cutting into my arms as I rested them on the edge while contesting. The gentler radius prevents that entirely. With a fresh coat of polyurethane, it feels quite smooth.

The two-drawer file cabinets were cheap and provide ample support -- although one of them has to be pushed back pretty far to support the back end of the desk, and thus you can't really use the top drawer to store anything. But the biggest drawback is the height. The resulting tabletop is 30 and 1/4" above the floor -- just a little bit high. In contrast, the Micro-shack desk was closer to 29" above the floor. A much more comfortable height.

I've considered moving the filing cabinets out and replacing them with a 2x4 frame to support the weight of the desk to get a more optima height. I may still do that.

Back nearly thirty years ago, I built a shelving hutch that sits on the desk, holding the equipment on three levels. Made from 12x1 lumber, it's not quite as square as I'd like, but I wasn't much of a craftsman thirty years ago, either.

Back then, I purposely made a shelf about an inch off the desk, and that's where the equipment goes. The space underneath was intended to allow logbooks and other operating aides to be stored, out of the way. I haven't used paper logbooks in a decade now. I did just fine in Floyd County with radios right on the operating desk. If I did again, the shelving hutch would have the first compartments on the desk directly.

Another thing that has changed is space for a computer. Thirty years ago (and yes, I did have computers thirty years ago), the computer was off to the side, and 90 degrees to the operating desk. Today, I almost exclusively use laptop computers. Using a computer is integral to radio contesting, and a lot of my DX chasing. And one side effect of having bifocals is not to have the monitor too high off the desk -- otherwise I end up leaning my head back uncomfortably far.

Any new shelving hutch would have space front and center to slide in a 15-inch laptop or a keyboard and monitor. With room on the right (and left) for radios.

Certainly something to think about.

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 ( 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!

Tuesday, September 6, 2016

Fixing the Heil BM-10

Repaired Heil BM-10
Back in the early 1990s, I'd been contesting using a Kenwood MC-50 desktop microphone. It worked well enough. But long hours of contesting in one position became uncomfortable. So, I decided I need a headset with a boom mic.

Heil had recently come out with a lightweight headset, the Heil BM-10. For right at $100, I purchased one with the HC-4 element. The HC-4 was supposed to have a little bit more "punch", great for DX and contest work.

I was immediately pleased with how it performed. I used that headset for many years, many enjoyable contests. It was pretty comfortable, although after several hours, even the light touch on the  ears would cause a little irritation. The foam pads on the ears have been replaced a couple of times.

And then, in December of 2007, it quit. The microphone element stopped working. I disassembled it, determined that the element wasn't working, and it sat. I wanted to order a replacement HC-4 element. After all, Heil used to sell them. But no more.

After a while, I purchased a Heil Pro-set, perhaps one of the last ones made with the HC-4 element. It works spectacularly with the Elecraft K3.

So the BM-10 laid disassembled, for a suitable microphone element.

Electret microphone nestled in a bed
of foam.
This past January, I ordered an electret microphone element from Ramsey Electronics. It was probably one of the last kits they sold. The kit came with a resistor and coupling capacitor to isolate the bias. The capacitor was far too large to fit inside the 8-pin DIN connector, so used a smaller unit from the junk box.

I also replaced the boom mic clip. The original one had long ago cracked and was held together with a screw. Hell fortunately still sells them as a replacement part.

I had to do some hunting to find the microphone cover. The headset had been disassembled so long all the parts weren't in the same place.

Works like a champ. A bit of foam to keep the element from rattling around, and perhaps it is good for another dozen years or so.

Monday, September 5, 2016

Pursuit of 5BDXCC IV - 80m

Shunt-feed wires rise up the side of the tower.
For the last three years, near the end of summer, I have written about converging on 5BDXCC. I wasn't close when I only had thirty countries to confirm on 80m. The next year, I had inched a little closer, but taking the proposition a bit more seriously. And last year,  I made some serious progress, and was hopeful I was within striking distance.

Let's review the numbers, shall we? Last year I stood at 89/88, right? Well, turns out, this was slightly off. I discovered that a confirmation of CP1FF on 15m Phone was credited to 80m. This has since been corrected, so I was really at 88/87.

Today, I hold 91/90. Yes! At least down to the last ten confirmations. Not a lot of progress from last year, but at least progress.

I do have paper QSL confirmations for a couple more entities on 80m. According to ClubLog, I've worked 103 countries on 80m. Looking at the data, perhaps a couple of those have busted calls on one end or the other.

This leaves me with two methods of achieving this goal: I can pursue getting paper confirmations from contacts I've already made, or I can strive to make more contacts and get LoTW confirmations from them.

Let's look at the latter.

Last year, I ran into a few obstacles trying to work DX on 80m. Since moving from Floyd County to Walton County, I had to take down the 80/40m dipole and also the very effective 160/80/40m Inverted-L. The 80/40m dipole is up at the Walton County QTH, but the low height and local noise limits its usefulness. With the Gwinnett County QTH just minutes away, I've been more tempted to operate from there, when I can. The 160/80/40m Inverted-L and all the 30 lbs of radials are still sitting in a box. I don't really have a location to put it up right now.

That leaves me with the shunt-feed tower for 80m. I ran into a number of issues last fall with the shunt-feed matching network. The 80m cap had to be replaced. I also found that the shunt isn't as effective as it might have been. Certain times of the early evening, the 80m dipole was much more effective.

Plus, I'm seeing some RFI-related issues when using the shunt-fed tower. These didn't affect the Elecraft K2/100 or the Kenwood TS-430S, but they sure make a mess of the Elecraft K3/100.

I also wanted to put up the K9AY loops with a new loop controller. The new loop controller didn't work properly, and even using the old mechanical switch, I discovered I had other RFI feedback from the K9AY loop feed line.

So, It's clear to me I need to take care of a number of things before fall begins:
  • Fix the Shunt-feed network. To improve matching on 80m, I'll add a relay and an extra variable capacitor, which would give me settings for 160m, 80m and 75m. I'll also bypass the control lines to prevent RFI ingress. Decoupling on the feed line couldn't hurt, either. 
  • Add Radials. The 29 or so radials for the shunt-fed tower may have deteriorated in the last 15 years, and a few of them have been damaged. I have about 2,000 feet of surplus cloth-covered copper wire, and my plan is to put it ALL down as radials. 
  • 80m Dipole. I'm working on a design for a trap 80/40/20m dipole, which may double as a second-radio antenna. That should go back up where the old 80/40m was.
  • Amplifier. One benefit of the 80m dipole is I can use the amplifier. The shunt-feed network can't handle much more than 100 watts. 
  • K9AY Loops. Fix the controller. Re-route the feed line to avoid RFI pick-up. 
Once all that is done, there's the small matter of being there. I guess I should start operating more from Walton County. Perhaps plan to work a few DX contests on 80m. Perhaps the ARRL DX contest would be good.

How about the former? I've already used ClubLog and OQRS to request paper QSLs for a couple of the contacts for which I have matching information. Those should come eventually.

Plus, I need to go through my batch of QSLs and see if I have anything matching on 80m. I think I did this exercise last year, but I've gotten some new QSLs from the bureau that I haven't processed yet.

The final step is to use ClubLog to identify some of those contacts I've worked and not confirmed and request paper QSLs. Those may take a while to arrive, but at least I don't have to fix antennas or wait for propagation to get that done.

Saturday, September 3, 2016


There's that moment -- that moment of pure clarity. The pure clarity that occurs when you realize you've done something really stupid -- something stupid you can't take back.

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.
I switched the amp off and then back on, still dark. Clearly, I've blown a fuse. A quick check verified that one of the two 10A fuses had blown. It was replaced, and the amp came back to life. But as it turned on, there was a tiny arc coming from the plate choke. I switched it back off, turned the meter to the HV setting, and briefly switched it on again. The High Voltage looked fine, but I was going to have to get the amp on the workbench to see what was wrong with the choke.

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.
While I had the amp on the bench, I checked out a couple of other things. Looks like one of the protective caps I placed on the B- rail to ground failed. Probably helped to protect the meter movements.

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.
Another mystery of this AL-80A I solved -- why it wouldn't tune up on 160m. An inspection of the front switch wafer shows that the switch contacts for the additional plate tuning cap have completely melted away. Since I've never used the amp on 160m, this was not me -- this was an existing problem. The switch contacts, or perhaps the wafer will need to be replaced to restore service on that band.

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.

Friday, August 12, 2016


JeppGuide page for Stone Mountain Aiport
Twenty years ago, I lost a good friend. Well, not exactly a friend, but a place.

Stone Mountain Airport closed twenty years ago in June. 

Stone Mountain Airport was one of the older airports in Georgia. It had been in continuous operation since 1930 until it closed in 1996. It was a very special place, with lots of character. It originally had crossing grass runways in the early days. By the time I started frequenting the place in 1989, there was but one runway -- a 2800 foot stretch of asphalt oriented 170/350 degrees.

The runway had displaced thresholds at both ends -- 600 feet at the south end, and 1200 feet on the north. The displacement at the south was mainly due to tall trees 200 feet off the end. The displacement to the north was another story. Sometime in the 1980s, a fellow built a warehouse in the lot adjacent to the airport, and then immediately complained to the FAA about all of the airplanes flying over his building. (Did I mention the airport had been there since 1930?)

The FAA solved this problem by requiring pilots to fly a three degree profile and clear the building by a sufficient height, which dictated a 1200 foot displacement. This mean that takeoffs on Runway 35 or landings on Runway 17 effectively lost nearly half of the usable runway length.

Stone Mountain airport was an uncontrolled field -- meaning it did not have a control tower. For my flying buddies that learned to fly at Peachtree-DeKalb airport, Stone Mountain seemed like something out of the wild west. A scary place where only the most daring of pilots went -- a too short runway with no controllers to talk to. Frankly, those of us comfortable at Stone Mountain felt the same way about Peachtree-DeKalb -- that it was a scary place. It had jet traffic, and you had to talk to someone to get permission to taxi, take-off or land.

The FBO was in a small white building with three rooms. As you approached the door, you'd notice two white benches on either side, under an awning. These benches were a mere 100 feet from the center of the runway, so it was a great place to watch planes take off and land. Inside, there was a great big room a dilapidated sofa and several easy chairs.

At one end of the room was a huge stone fireplace. In the winter, there was always a wood fire going, as it was practically the only source of heat. Just behind the sofa was the counter, where you could pay for your fuel, buy maps or AF/Ds, or pick up an ice cream sandwich before going back outside to the white benches to critique the landings. Behind the counter was a small room that served as a classroom and office.

Landing at Stone Mountain wasn't all that terribly difficult, so long as you had reasonable control over your airspeed. The short runway left little room for excess speed, and pilots there had no qualms about going around.

Prevailing winds generally favored using runway 35, which was the longer of the two runways. Crosswinds made this more interesting. The presence of a 900 foot dome of solid rock a couple of miles away from the runway made crosswinds more common. Winds out of the west would wrap around the mountain and produce a burble on the approach to runway 35. Best technique was to crab on final and keep adjusting, as the crosswind usually disappeared as soon as you dropped below the tree line. Usually. If not, you'd kick into a slide slip just before touchdown. While it sounds nerve-wracking, it wasn't so bad once you knew what to expect.

When the winds favored runway 17, airspeed control was even more critical, as a delayed go-around faced the prospect of putting the airplane into the tall trees off the end of the runway.

Approaches were gauged with the Mark I eyeball and the "poor mans VASI." We had no proper VASI, but about the time the displaced thresholds were ordered by the FAA, a clever arrangement was set up. About 50 feet from the runway, three large boards were placed horizontally. Two were held spaced apart and painted white, while the third was further down the runway and painted orange. If you were on the three degree glideslope, the orange board would appear directly between the two white boards. Crude, perhaps, but it worked well. You could easily tell if you were too high (and thus carrying too much kinetic energy) or too low (and risked going into the trees). The front, white boards were right at the displaced threshold, too, which made it easy know where to land.

Landing at night generally favored runway 17 in any case. You'd fly over the bright parking lot of the Walmart across highway 78, and squint into the darkness for the LIRL (low-intensity runway lights) around the runway. Seriously. You'd never want to set up a landing for runway 35. You'd never see the trees on the approach end, and the lights of Walmart would prevent you from making out the runway as you got close.

Stone Mountain did have it's share of accidents -- some minor, some fatal. On one particularly windy day, I witnessed a fellow in an Ercoupe make three landings before mid-field on runway 35. The second bounce broke the nose gear, and a bunch of us had to push it off the runway. Clearly a case of pilot-induced oscillation trying to force the aircraft down.

My friend Forrest broke a beautiful Cessna 172 -- N7654G. He was out flying on a particularly blustery day, attempting to land on runway 17. He had already gone around once, and made the approach with a bit more determination. Unfortunately, he dropped it in so hard, that one wing hit the ground, then the plane rocked back the other way so hard the other wing hit. Nose gear collapsed also, but both wings clearly had a bit of extra dihedral. Forrest walked away, unhurt, other than his pride.

Summertime could be difficult, as the heat and humidity would cause the density altitude to rise. A pilot renting a 172 packed the plane very full with four adults on a summer afternoon, ended up putting the plane on the road beside the warehouse north of runway 35. His takeoff was very anemic, which he should have aborted. He was still in ground effect as he left the runway environment. While no one was hurt, it was too close.

While Stone Mountain had a few hangars, we mostly parked our planes out on the grass at tie-downs. Some were in the back forty where the old grass runway used to be. Mine was across the runway from the FBO, about 150 feet from the runway. Some folks might think this strange -- having pedestrians cross an active runway on foot. No, perfectly normal.

Yes, I'll never forget that feeling crossing the runway on foot on a crisp autumn morning, getting ready to preflight my Cessna 150D for a Saturday of flying. It would be chilly, the grass would be wet with dew, and the earth gave off a rich, wet smell.

For several years, Stone Mountain was host to a glider operation. A Cessna 305A / L-19 "Birddog" was the tow vehicle, and the pilot would waste no time after release getting back down to the ground. For the L-19, Stone Mountain seemed a copiously large airport. Glider rides were very popular over the mountain, and, in retrospect, were relatively cheap. $85 bought a ride from 5,000 feet, and I took one on my 30th birthday. It seemed over all too soon.

Stone Mountain pilots were a pretty social crowd. On July 4th, Stone Mountain Park would put on a pretty good fireworks show, and we could go up to the airport and watch. Now, we were on the wrong side of the mountain -- most of the action was in front of the carving. But we could see plenty.

In the warmer months, pilots would frequently gather for an impromptu "dawg killin'". This was a euphemism for a cookout with hotdogs and hamburgers, and plenty of aviation conversation.

I remember a couple of pilots that I haven't seen since the airport closed. There was "Jack" (not his name) who was a perpetual student pilot. You see, he'd never gotten his private pilot's license. He probably had a couple of thousand hours by the time I met him, likely most of them undocumented. He had a Cessna 172 and flew frequently -- but I never knew him to take any passengers.

There was another fellow who owned a DeHaviland Chipmunk. It was a joy to see that plane in the air, with it's inverted, in-line four-cylinder engine. It cut a handsome profile with it's unusually narrow and long nose. But we rarely saw it in the air. I generally remember this fellow sitting on one of the white benches on a beautiful Saturday, debating on whether or not to pull his airplane out of a hangar.

I do remember on fatal accident while at Stone Mountain. I arrived early one spring Saturday morning for a lesson, only to find the airport closed. An hour earlier, a Cherokee 140 had taken off on runway 17, and ended up in the trees off the end of the runway. The two occupants had perished in the impact. Two factors seemed prominent in this accident -- the aircraft was packed very full with camping gear, and might have been overloaded. The other was the wind. While the wind sock might have been favoring 17 near the ground, the winds above the tree line were clearly out of the northwest. They had made a very marginal takeoff only to encounter a tail wind above the tree line. While no flying that day, the lesson was one of reflection and study.

Whether tragedy or fun, all of it came to an end in June 1996. You see, the Atlanta Olympic Committee built the Tennis venue just a half mile from the end of runway 35. While the Olympics were in Atlanta, there would be no flying out of Stone Mountain, due to the Temporary Flight Restriction (TFR) around the Tennis venue.

Owners of the airport decided to rent out the space for parking. By June 1, all the aircraft had to be moved to other airports. They then came and removed all the runway lights, and the fencing, and put down a crush-and-run surface down for parking. At the time, their intention was to re-open after the Olympics.

Alas, it didn't happen. While closed, someone took an interest in the property and made the 30-odd owners an offer they couldn't refuse. They never re-opened.

There were rumors that someone was going to build a retirement community on the property, perhaps a shopping mall. None of that transpired. Years later, a local model-aircraft club (Stone Mountain Flyers) bought part of the property and operate model aircraft on what's left of the runway.

I drove down Bermuda road the other day. This road runs right past the airport, with about a half or quarter mile of trees in between. The south end of the airport property nearly runs into Bermuda. Years ago, there was a little spot on the road you could turn off and you'd have a clear view of the entire runway from the approach end of runway 35.

That whole area is now grown up with pine trees over 40 feet tall. You can no longer see the runway. It is a sad and ignominious end to a place where I learned to fly.

Other information and pictures, here.

Monday, May 30, 2016

Fixing the Ameritron AL-80A. Again.

If you remember from last August, I repaired my AL-80A. Well, that fix lasted little more than a month before I once again had an arc while tuning and it started acting weird. Grid meter would drift up when the amp was keyed, and would pin hard against the right stop when RF was applied.

Surprisingly, though, it was working -- only the metering seemed to be affected. Sadly, my lust for DX overcame my caution, and I occasionally used it even though the metering was weird. While the amplifier continued to work as designed, the grid current meter pounded so heavily against the right stop that eventually the needle broke off. 

Blown 1.5 ohm 3 W resistor on left.
Proper-looking spares on right.
I finally pulled the rig over to the workbench and took it apart. What I found wasn't pretty. Yes, the same 1.5 ohm 3 watt resistor had disintegrated. The metal foil had been blasted away from the resistor form. It was completely open. It was no wonder that the grid meter acted so strange -- it had no shunt!

I also found a article by Tom Rauch, W8JI on proper amplifier metering. It talked about a very simple circuit change that would likely prevent this sort of damage in the future. 

Indeed, after doing some research, I found that this protective circuit to be part of several Ameritron amplifiers, include the AL-811, AL-80B, AL-82 and AL-572. Unfortunately, it wasn't included in the AL-80A.
Small caps added across the B- rail.
Added ground lug visible on rectifier board.

No matter, very simple to add. I placed two small capacitors on the B- rail to ground, right on the capacitor board. The glitch diode took a little more doing. I already had a glitch resistor mounted to the rectifier board, and it seemed logical to place the diode there. I added a ground lug to give me a good ground connection, and placed the diode underneath the board. 

Tom recommends a 1N540x diode, but I didn't happen to have any. I did have plenty of 1N4007 diodes -- which don't offer nearly as much protection. However, I figured that some protection was better than none at all.

I still had the problem that the grid current meter had no needle. The broken needle was still in the meter face, so I used a tiny drop of superglue to re-bond it to the post. So far, it is holding, although I don't know how long that will last. 

1N4007 on bottom of rectifier board.
Note the picture has it installed backwards,
something I have since corrected.
Buttoning it all up, I thought this would be an easy victory. Well, the multi-function meter seemed to be operating correctly now, but the grid meter behavior hadn't changed. It was still pinning with the least bit of RF. Something was still not right. 

It was a few days before I had a chance to pull the amp back to the workbench. Off came the thirteen screws holding the cover. Then the eight screws holding on the front panel, plus all the knobs and indicators. Pulled the meters, and then the meter board out where I could see it.

Placing an ohmmeter across the grid current meter showed that the shunt wasn't in place, since it read about 450 ohms. The resistor itself was OK. Looking at the meter board, I found that there was a open in one of the traces. Apparently, the glitch was strong enough that part of the board acted like a fuse! A bit of solder and a piece of bare wire bridged this open.

Buttoning it up again, it once again works as it should. Now I can enjoy chasing DX without the guilt. Indeed, I worked XR0YS on Easter Island just as soon as the amplifier was repaired.