The little matter of the Variac.

A lot of FM broadcast transmitters (and who knows, maybe even TV, AM….?) use a tetrode or pentode tube based power amplifier. Control of the final output power is achieved by metering the output and adjusting the voltage to the screen of the tube, thus adjusting the amplifier’s gain.

A woman loves a man with a great big old Johnson.*

On a lot of transmitters this gets done electromechanically. In this case this is done using a Variac or variable transformer, but in some smaller rigs (2.5, 2KW, and below?) a rheostat voltage divider may be used.

In the above picture, the screen voltage is all the way up for maximum gain. This occurred as the result of the tube wearing out and its cathode emission going soft, so the automatic power control kept trying to run the gain up higher and higher to maintain the desired power output until the poor little motor tanked.

Now a word on folksonomies and genericized trademarks. You may notice I capitalized Variac. This was originally a trademark held by the General Radio Company with first use in 1933. It was allowed to expire, probably by the end of 1994. Variac, however, kind of became a household word (if you could call a variable autotransformer a household item?) like Band-Aid, Xerox copy, Jello, and the like. Within the folksonomy of electronics there are a few things that have become genericized like this and it may be easier to categorize information involving them by an old brand name. Other examples would include “Black Beauty” capacitors which were a Bakelite encased oil and paper cap, Vactrol for light variable resistor type optocouplers usable for audio and analog signals…. Sorry for geeking out worse than usual here. Anyway, with the Variac / variable autotransformer…

Shown here, a Harris HT 25 FM. This is very very similar to the FM 25 K series and was made like this for years, because once Harris had a design that worked very well, there was no reason to make big changes. I bet they made thousands of these transmitters and many of them are still happily thrumming along to this day.

The mechanism is simple but prone to issues, especially if the automatic power control is used. Here’s the problem: Every change in line voltage, even a change in output impedance caused by rain or ice hitting the antenna, may cause the power to fluctuate. The controls compensate by moving the mechanism, and eventually something gives.

The original mechanism used this unusual bidirectional synchronous motor geared down to 0.5 RPM, running on 120 volts AC. This motor is discontinued by Hurst, though they’re still around and still have a somewhat similar product. However, it has to be custom manufactured with a lead time of 14 weeks, and nobody just has stock of ’em.

The solution: get to hackin’. First, a control system to step this down to a commonly available DC gear motor:

Constructed:

 

And now comes the fun part. The DC motor’s shaft is fatter and shorter, but it’ll work! Here’s the original shaft coupling it goes into and the Variac. Off to the right is the transformer that steps 240VAC up to 1000VAC. Or maybe it’s 707VAC if it just uses the peaks. I dunno, man. It Just Works.

There’s also a silvery looking band wrapped around it with one of the two end stop screws. More on that later. I removed all the grubscrews for safe keeping.

 

The bronze coupling drills like a hot knife going through butter, albeit with a disturbing squeaky sound.

This is how the stop screws are installed from the factory. They hit the micro switches under the mounting plate to limit the motor’s travel.

Now, you’ll notice the new motor’s shaft is shorter. Under the silver band there’s another pair of set screws that clamped the old motor’s shaft. However, the hole the stop screw is in is tapped exactly the same as that set screw hole.

Flippity flop ’em.

The silver band will just cover up the set screw down the upper hole once it’s all assembled.

Trust me— it works fine.

And no, of course the silly thing didn’t spin in the vise, what ever made you think that? No, of course not! DURRRRRRRRR

Here’s the mounting plate. I expanded the hole where the original shaft went down to 1/2 inch, and wound up just drilling two new mounting holes and using the side mounting holes in the gearbox.

This motor also provided three tapped screw holes in the bottom, but I didn’t wind up using them. If I had this to do over I’d probably do it though. They’re Metric and I couldn’t tell you off the top of my head what the screw sizes are. M2? M3?

And it’s up and running.

If I had it to do over I might slightly revise the motor mounting as I don’t entirely like the amount of slop the rubber grommets induce in the system, but it works– and it doesn’t hunt back and forth. The Variac requires a very small amount of torque to turn it (I could grab that axle with two fingers gently and turn it throughout its full range). The DC motor I used is impossible to turn by hand so I brought a 9v battery to hold against the terminals as an assembly aid to reach all the set screws. 😉

 

And now I need to redo my nails.

*

So I’m trying to get SSL certificates working for kg4cyx.net and am currently having little luck. I’ll revisit that later. For now you can get here by https but get a huge certificate mismatch error all up in yo’ grill and I don’t even know– probably has something to do with the fact certbot can’t figure out my vhosts. According to all known laws of aviation, there is no way a bee should be able to fly. Its wings are too small to get its fat little body off the ground. The bee, of course, flies anyway because bees don’t care what humans think is impossible.

HackerBoxes

Just got my first package from HackerBoxes and I’d originally planned to make an unboxing video but that just wasn’t gonna happen unfortunately–

So I’ll spill the secrets of what was inside…

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Waaait up is that

Is that…. A PENGUIN?

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Clearly I have made a very good decision here.

Holy moly I love this already.

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This is the “Cellular Metal” box. Next to the breadboard is a small GSM(?) cellular modem. I haven’t looked up the info just yet. The black cable is a u.fl to SMA pigtail to connect it to the small antenna. There’s also a SIM card.

A pack of breadboard jumpers and components are present, as well as an Arduino compatible board, a USB to TTL level serial adapter, an Atmega328 (as used on the Arduino) and an ATTiny.

The card, meanwhile, does double duty. Flip it over and…

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Pinouts!

This is just as cool as the business cards from Marlin P. Jones Associates that have an electronics color code/resistor code guide on the back.

Soon I shall be looking at their online documentation to see what possibilities are in store for all this.

And…. Finding a suitably awesome spot to put that penguin in. I mean— it’s Tux with “Hack The Planet” written on it—!!!!! Soooo perfect.

The fix for easily moved radio knobs

First off, our radio knobs were too easy to turn, causing them to get unexpectedly muted or knocked off channel.

Second, I fail so hard at shitposting. I always want to just fire up WordPress and drop a useless shitpost on here then I think of something actually useful and informative. What follows is a failure to shitpost.

I’m still not exactly calling this a great post because I’m too lazy to edit the images.

Step one: pull the knobs off. Pull straight up. The knob may be tight on the shaft, just don’t apply excessive force in any direction if it is. Be patient. On this Hytera it was pretty easy to pop off.

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The recess here is what we’ll be modifying. Cut two little circles out of craft foam, mouse pad, inner tube… Whatever rubbery thing you have handy… Or use rubber o rings. It don’t matter.

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If there’s no hole in them yet, fold in half and cut a slit.

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Press it down the shaft and all the way into the recess.

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Reinstall the knob. Test to see that it moves and has more resistance. If there’s no effect, add another layer. If it doesn’t fit back on there, remove it and try a thinner material.

This took me about two minutes per radio I did it on and eliminates annoyance like nobody’s business.

The first time I did this mod was on a Baofeng, so I’m gonna add the shitpost tag. You’re welcome.

Flip dots, or, what nonsense am I getting into now?

Flip dot signs are kinda awesome. They’re sunlight readable, and noisy. You’ve probably seen them in a bus somewhere or other. They usually have neon green or yellow pixels.

Let’s take a look. This is a Luminator GTi series circa 1990.

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Here are the dots. They’re swiveled by the thin metal prongs on the ends. Note that this sign has a test message left on it, and retains this without power…

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Each dot is a small, magnetized sandwich of metal. Note the small black dot visible through the cutout. This is significant.

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Each pixel sits on a U shaped yoke of metal with a low magnetic coercivity. One side of that yoke is the dot you saw above.

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I’ve seen different variants of how the dots are electrically driven. On this one, there is one coil per dot. Other variants I’ve seen used two, one for black and one for yellow (green? white?…) On this one, what you get is a function of the polarity of current applied.

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Um, okay, it was 1990, we can forgive this uninspired light source for nighttime use.

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Here’s the driver. Bottom section converts 24vdc to 12 and 5 and also has a switch for that florescent lamp’s power. Top section takes data over RS485 and makes pretty pointillist pictures.

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The Micrel chips are shift registers with integrated source and sink drivers. Each row and column appears to be connected, respectively, to a pair of their outputs allowing “push-pull” drive.

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Socketed 8 pin ic: rs-485 line driver. To its right: 80c51 CPU, GAL chip that interfaces it to the shift registers, RAM, ROM, other logic bits…

So what can I do with this sign? Right now, nothing. It was made to work with a Luminator master controller that contained all the data to be displayed.

My next step is to try to figure out how to drive the matrix. I’m a little nervous with this one though as it is possible to short +12 to ground by turning on the respective outputs of two of the shift registers that are connected to the same output… However, to flip dots, you have to turn on two shift registers – one for row, one for column. I don’t know which is which and it’ll require analyzing the board layout more to figure it out. My plan is to put about a 25 ohm resistor in series with the +12 line while testing – the dots may not flip like that, but neither will the tops of those cute little driver arrays either. Oh, how I wish they were TPIC6C595 with their short protection. 😉

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But this thing can take a flying leap. LED tape shall be applied.

Wearing a little more paint than usual

So a week or so ago I found a really nice leather jacket at a thrift store. It was kind of weathered soft, and fits me like a glove.. I mean, perfect… But just one thing…

It was brown.

I’m too old school goth for brown.

Therefore, it needed to look like rusty old metal.

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Initial test
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The back of the jacket. Please excuse the cluster foxtrot workshop.
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The front. It's not asymmetrical like a biker jacket; it's just hanging all derpy because I needed to get behind the collar.
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Inside the collar.

I was originally just going for a rusty appearance but when Dominik of Slaughterhouse Apparel told me the red looked bloody I went with it. It’s got the look of something worn by a post apocalyptic mechanic; the red bits kind of reminding us that in an apocalyptic world, Shit Gets Real sometimes.

The funny part of all this? One of my coworkers told me this will probably look best with brown pants. Thankfully my wardrobe isn’t entirely black otherwise.