Friday, January 10, 2020

Ingenious Fan Holder for K2/100

Binder clip. I should have thought of this earlier!
The K2/100 is a nice little radio, but when running RTTY or other full-duty cycle modes, it lacks sufficient cooling. I couldn't run more than about 30-35 watts without the KPA100 amplifier heat sink becoming alarmingly hot.

I solved this problem a years ago with a small muffin fan on top of the heat sink. The fan is mounted on tiny rubber feet. It blows air upward, resulting in airflow across the fins. Placed just above the finals, it does a good job of keeping everything cool.

This fan is noisy at full speed, so I used a small resistor in series. The resulting half speed airflow is more than sufficient.

This worked great at my Gwinnett QTH for years, where the radio was raised above the desk a couple of inches by a shelf. The K2/100 sat almost flat, and the fan stayed in place. However, when I moved the K2/100 to the parsonage QTH, the radio rested on the desk, and I used the KAT100 tilt bail. Because of the tilt, the fan had a tendency to drift to the back. Every once in a while, it would fall off.

While I was readying the K2/100 for the 2020 ARRL RTTY Roundup, I wanted to make sure the fan stayed put. I came up with a very inexpensive solution. A small binder clip attaches easily to the heat sink fins. The ears of the clip rest against the fan body and prevent it from drifting.

Worked about eight hours over two days in the RTTY Roundup, and the fan never budged. I really wish I had thought of it sooner!

Saturday, December 21, 2019

Replacing the AL-80A Bandswitch

The AL-80B bandswitch, compared to the existing switch shaft.
With my wallet being $115 lighter, the AL-80B bandswitch arrived within a week. Now, it isn't the same as the original AL-80A bandswitch.

There are two key differences. First, as you can see from the picture, the shaft layout is different. In the original AL-80A bandswitch has the input network switch wafter in front of the bandswitch, whereas the AL-80B has it behind the bandswitch.

This means you can't just replace the AL-80A switch with the AL-80B switch. The best you can do is to unstack the wafers and put them on the original AL-80A switch shaft.

AL-80B switch with AL-80A wafers.
The second difference has to do with the switch wafers. Electrically, they are identical. The AL-80B switch has the 160m padder cap and the Pi-L network wafers reverse. Since we're going to re-stack the wafers anyway, this doesn't matter.

The Pi-L wafer is positioned differently. The contacts are two positions counter clockwise (as viewed from the front of the shaft) from the AL-80A. This means that a bit of re-wiring is necessary to make the connections.

Wafer-less switch.
Removing the switch wafers isn't a big deal, since I have done this earlier. The rear wafer comes off with all the coils attached, and the padding caps unscrewed, there being only one wire that needs to be unsoldered, and it slips off. The middle wafer has a similar wire to be unsoldered, and it comes off as well. The front wafer has one wire to the padder cap, and it comes off as well.

With all the wafers off, the new switch wafers are stacked on. Because the Pi-L wafer has contacts rotated two positions, they are a little harder to access between the switch and the loading capacitor.

New wafers in place.
Once all the wafters are on and screwed down, the next step is to re-solder all of the connections. I took lots of pictures before unsoldering the old wafers, so I had a pretty good idea of what goes where.

Front switch wafer anchors the coax shield, and adds the 160m padder cap. The Pi-L switch wafer was the only one that gave me trouble. Since the contacts are rotated from the original, this required a bit of extra wire to make the required connections. The rear switch wafter has five connections tot he coil assembly. It took a bit of work to unsolder. This sort of thing is best not rushed and done with great care, to avoid damaging the coils. Everything goes in the same place on this last switch wafer, which was easy once I got all the coil wires into the right positions in the narrow switch contact lugs.
The finished product. Reassembled with new switch wafers.

Sounds easy, eh? But realistically, it was nearly three hours on the workbench.

Moved it back to the operating desk and tested for full output into a dummy load on all bands. No problem on 160, 80, 40, 20, 15 and 10m.

For 17m and 12m, I proceeded more cautiously. The AL-80A is designed to operate on 12m using the 10m switch position. It works on 17m using the 20m switch position. But it has often been on 17 or 12m that I've had the most issues with arcing in the PA circuit.

For this reason, I've decided to reduce drive to about 30 watts for 17 and 12m. It's a little less output power, but also less of a chance of burning up a $115 bandswitch.

With the testing done, it was time to button up the amp. Took me about 20 minutes to find 10 of the 13 screws that hold the cabinet top on. I had removed them a year and a half before, but finally found them in a neat pile on the workbench.

The AL-80A is back up and running on all bands!

Friday, December 20, 2019

Digging back into the AL-80A....

Front switch wafer - showing contacts burned
completely away.
Back in July of 2018, I was chasing the KH1/KH7Z expedition on 17m, and the AL-80A stopped producing any output. I dug into enough to find that the Pi-L output switch contacts had arced and burned. So, I jumpered the switch so the amp was only usable on 40m and above. And it has worked OK since then - but not on 80 or 160m.

It's way past time to fix this, so I recently got started. I figured that I just needed to replace a switch contact or two like I've done before.

Back side front switch wafer showing more
contacts completely burned away.
Once I got the lid off and removed the switch wafers, well, the photographs tell the sad story.

Middle switch wafer. You can see my jumper
between the to contacts on right. There was not
enough material on the switch wedge to make
contact with the 40m contact.
Replacing the contacts won't be enough. Plenty of the switch wiper was gone on the Pi-L (middle) switch wafer -- especially the corner that used to short out the Pi-L coil on 40m. And the 160m padder capacitor (front) switch wafer isn't any better.

No, this means I'll have to replace the switch entirely. I've ordered one from Ameritron, with shipping it came to $115! This is the AL-80B switch, which is similar, but not identical.

Saturday, December 14, 2019

Sorry, I've been a bad blogger.

I just noticed that I had a number of comments awaiting moderation and I just published them all. Sorry about that folks.

Used to be Blogger would send me an email when I had comments, and I could moderate them then. Apparently, they stopped doing that about a year and a half ago.

So, if you left me a comment on one of my articles, it's published now. And I may have even answered it....

I'll try to keep on top of this in the future.

Thursday, November 28, 2019

Schematic for the "Novice" Transmitter

Schematic for the "Novice" Transmitter
I guess it has taken a while. I wrote about my Novice transmitter over ten years ago in January of 2019 as one of my first articles in this blog. But I never published a schematic.

Well, the wait is over. This summer I took some time to hand-draw a schematic reasonably neatly. At least neatly enough for someone else to make heads or tails out of it.

The result is in the image on the right. The circuit is pretty simple.

A couple of notes on the tank circuit. The coil is 1 3/4" diameter, 12 turns, about 7/8" long, which computes to be about 6.4 uH. A typical pi network for a 6146 at 7 MHz should be L = 6.2 uH, C1 = 90 pF, C2 = 500 pF. So it seems my design is pretty close.

The keying arrangement is unique. The oscillator is cathode keyed, but the PA is biased beyond cutoff for Class C operation. When properly the grid tuned circuit is properly adjusted, there is little to no chirp.

This radio works pretty well on 30m, too. The crystals I have for 30m are HC-49U crystals soldered in FT-243 housings. These drift a little bit key down, likely due to the high crystal current. I added a 100 ohm resistor in series with the crystal to limit the current.

Wednesday, September 4, 2019

E. U. N.

Some time ago, I read a blog article on how to prioritize requirements. The author called it N-U-N analysis. The letters stood for Needed, Useful, Nice. I thought this was a brilliant idea, but the acronym was awkward, because there were two "N"s. I decided to traded the first for a better synonym: Essential. Essential, Useful, Nice. E. U. N. This is a powerful organizing principle.

I recently was dealing with a ham radio project list that had gotten too long. I had a number of great ideas which I had added to the project list, so it crossed several pages of my station log. This made actually finding something I could work on difficult -- especially when dealing with projects that could actually add value.

Mulling this over, I decided I needed to apply the same principle to my project list. But what was Essential, and what was merely Nice? and how about the Useful projects in between?

I decided to break it down this way:
  • Essential - fixing what is broken
  • Useful - allowing new capabilities
  • Nice - anything that might be a nice addition
I organized my long list, and low and behold it was very helpful. I only had a few projects in the Essential section, but one of them I had basically forgotten (fixing a broken switch contact on the Ameritron AL-80A).

Several of the projects were in the Useful category, and I'll be getting those done sooner. Everything in the Nice category can wait a bit.

I can't say that this exercised has helped me get anything done sooner, but now I know my efforts will be focused on the most valuable projects.

Sunday, August 25, 2019

Rebuilding the HF4B

It's been a long time since I picked up the Butternut HF4B project from a friend. I decided it would be a good fit at the Fulton County QTH. It's small enough to be supported by a mast, and will turn with a tiny rotator. I took all of the pieces to Fulton County, disassembled and cleaned everything, and took stock of what I needed.

The basic components seem to be OK. One of the 1/2" tubes is damaged with a gouge near the end, nearly ran over by a lawnmower no doubt, but will be serviceable. One of the 1 1/8" element support tubes has a crack. I've looked for a replacement locally without luck. I think for the moment I'll use a clamp to hold it together.

Two of the four doorknob capacitors have cracked. I managed to find five 75 pF caps at a hamfest a couple of years ago. I bought five in order to have a spare. These things are fragile.

All of the stainless hardware is serviceable despite being out in the weather for over 25 years. The U-bolts, however, are another story.

I don't know what genius thought it was a good idea to sell an antenna with zinc-plated U-bolts, lock washers and nuts. After 25 years of weather, they are rusted junk. Some of the legs broke off when trying to undo the nuts. Those that didn't are so rusty as to be unusable for any purpose.

I ordered replacement U-bolts from McMaster-Carr, who had exact replacements for a reasonable price. Except these are stainless steel.

With replacement hardware in hand, I started to re-assemble the antenna. The wires of the bowtie elements needed replacement. This antenna has eight of these, but only four survived to make it home with me. Of these, only one is intact. I used it to make measurements and make eight identical replacements.

I hit the a snag assembling the first element. The clamps that hold the vertical tubes aren't tight enough to keep them from rotating on the support tubes. Butternut replaced these sheet metal clamps with U-bolts in a later release, which would work better. This requires sleeving about four inches of the vertical tubes.

I guess that's what I'll have to do.