This little turd blossom!!! Its blower relay inexplicably turned its cooling fan off while I was using it to shitcan 23,500 watts of RF power!!!
Looks like I’ve got some interlock logic to fix up! This should have opened an interlock upon the load’s air vane switch dropping and caused the transmitter to drop back to plate off. You know, instead of me hearing its blower stop from in the restroom and having to go running to prevent the meltdown. XD
That moment when you hit seek on your car radio, and it stops on a channel that should be vacant with a dead carrier with a strange warbling beep every second, followed by a voice screaming “¡DISFRUTA!” and then the carrier drops without a trace
“Hello, Derp Island Broadcasting, may I help you?”
“Hi, I’m with Papa November Network, and I’m interested in an LMA from your site.”
“Okay. We’ll give you 250 watts ERP from our expertly maintained state of the art transmitter site. However, you may not install remote monitoring here, and if you try to visit the site, you will be run off by our management. But rest assured you’re getting your money’s worth.”
“Hey, can you go check on our station on Derp Island?”
I have screamed internally all I can and now I must shitpost
Now that I have skin on my hands again and it doesn’t hurt to type this—
Proper acoustic treatment is vital in studio and performance spaces to avoid echoes that will prove destructive to audio quality. Sometimes it’s done right. Sometimes it’s done wrong. To me it’s utter black magic but I know what works for the most part.
The first thing you want to look out for is stray sound entering your studio space. This may be trickier to do unless you’re constructing a studio from scratch. The methods I’ve seen used to great success are either adding insulation batting inside walls as they’re being built, or using cinder block walls to enclose the space instead of hollow. Either way, avoiding the use of walls shared with noisy things like air conditioning units or bathroom plumbing are very good ideas. Don’t use the other side of the wall pictured here if you can avoid it 😉
The second stage in acoustic treatment is to manage reflections within the space. Any hard flat surface within the studio may cause echoes which can become unpleasant and generally make your recordings sound like you’re standing in the shower.
The typical method of dealing with this is to put up some kind of sound absorbing material on the walls. A number of criteria should be considered when selecting your sound absorbing material:
Fire retardant properties
Materials I’ve seen used:
Carpet. Can be obtained anywhere, extremely cheap and actually very durable, since it’s made to be walked on. Works pretty well, but double check that all materials you are using will meet fire code standards for use on a wall. (Carpets may only be rated for proper fire retardant action when installed on a floor, as the flame spread dynamics are a hell of a lot different. Consult the manufacturer and/or their documentation before use.)
You can glue and/or staple it to the wall or even use the self adhesive carpet tiles that peel and stick.
CAN BE CLEANED. This is a big advantage. Over time, airborne contaminants will settle on the wall material and should ideally be removable. I’ve found that in an old studio with textile walls, allergens will gather and cause some people to get sniffly if not removed. Your mileage may vary, but I’ve found that the ideal maintenance interval to vacuum the walls is about 2-6 months. That’s all it takes, actually – just use a vacuum cleaner with a hose and an upholstery attachment and suck the dust out. More major junk may be dealt with using carpet shampoo and a wet/dry vac.
Cloth and Fiberglass. I’ve seen this one in a couple of older studios and it works very well. Small wooden strips were attached to the wall followed by fiberglass batting being installed between them, with a fairly open weave upholstery cloth used to cover it all up. The cloth is finally secured by tacking a small wooden moulding to the wooden strips below it.
Performance is very good. Flame resistance will be identical to that of the upholstery fabric used. Durability isn’t as good; wheeled chairs, furniture, irresponsibly sharp spiked leather gauntlets (of course I say this from experience), and other things can tear the fabric. Repair is easy though.
Cleaning: Can be surface cleaned. I’ve always just used a vacuum – with a HEPA filter just in case glass fibres are released through the surface fabric as you’re sucking the schmutz off. When I’ve done it, I have never noticed glass fibers visible in the vacuum as I cleaned it out – just lots of fine dust and pollen!!
Foam. This is a very common acoustic treatment material and I kind of wish it wasn’t. Performance is good, yes, but durability is totally in the dumper. When it’s new, it’s easy to tear up, especially when…. SOMEONE… decides to get creative and try to hang decorations from it. By the way, don’t cover your acoustic treatments with posters, because the essentially airtight paper of the poster will just form a really nice REFLECTOR, completely defeating the treatment. Some materials have fire retardant ratings, check with the manufacturer.
When it gets old, some foams will just crumble and disintegrate horribly, especially if touched.
Cleaning: …. maaaaaaybe. If you’re lucky.
I’ve had okay luck with vacuuming the surface of Auralex StudioFoam products using a soft bristle brush. Work slowly and gently going along the ridges. In one studio I started doing this and the foam turned from dingy gray to its original burgundy red— I didn’t even know that was the color it was supposed to be! Yeeeechhhhh.
I wouldn’t even try this at all on the type of sound insulation where it has alternating deep, thin vertical and horizontal ridges.
Now here’s where I talk about strange things.
A while back I was in the studio of The Jeff Adams Show and as soon as I walked in, I saw his wall panels and couldn’t believe my eyes. They’re wood! They did not contribute to any unpleasant echoes, yet had a little bit of a warm reverberation. You can see one in this picture: I didn’t get a closeup picture of the panels when I was there, but they did not have a smooth planar surface. The different planks are overlapped forming a convoluted surface, which would contribute to diffusion instead of a clean surface reflection. Notice the mic he’s holding here – that’s a Heil PR 40 cardioid dynamic that is just INSANELY sensitive. You’d almost think it’s a condenser (I mean— it kind of even looks like one). The Heil PR 40 is NOT forgiving to bad room acoustics. It proves that these funky wooden wall panels work perfectly as an acoustic treatment!
In contrast, the Electro-Voice RE20 that’s used at close proximity by a lot of radio jocks couldn’t care less what your room acoustics are like as long as you aren’t in an all glass aquarium or something.
The same soft vacuum brush cleaning I mentioned above will work if they ever start to gather dust. Since he’s doing video on the same set, avoiding lighting glare is definitely a plus – they’re not shiny in any way.
Now, you may have a sick curiosity and wonder what the first sentence of this post is all about. Well…. I had to displace some very old carpet on a wall at work to hide a cable behind it, and I don’t know if it was the 20+ years of old crud built up in the carpet, the glue that held it to the wall, or a likely combination of both, but something caused a very bad reaction on the skin on my hands that caused it to blister, crack, and weep as if it had been severely burned.
If you’re really curious, I have a picture of the results here. Content warning: extremely gross. There’s good reason I put /nope/ in the URL. It’s mostly healed now and all that remains is a little redness. That week sucked, man.
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.
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:
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.
This one’s probably been around the block a few times. I was initially kinda worried because the fins looked roughed up but after some unscientific testing to make sure I could blow through them and fit a cable tie down between the fins to ensure there was a good pathway, I went ahead and installed it, and it came up just fine with a perfectly good stack temperature…
NO EXCUSES ON DA BOWL
The old tube went soft pretty quick. Don’t they have pills for that now?
And here’s what that goes in, a Harris/GatesAir 25,000 watt FM transmitter.
To swap it, you release a hose clamp holding a big collet on the movable top plate of the cavity so you can slide that plate blocker/chimney up, then release the hose clamp holding it to the tube (bottom), slide it up then clamp it again out of the way. Then you can carefully disconnect the plate voltage supply cap and somersault the tube upside down above the socket and remove it. I wish I’d taken a video of how bizarre this all looks.
Cooling air is forced both through holes in the socket and out of the cavity through the tube’s plate cooler after entering from the blower duct.
The two big fat orange wires – GatesAir / Gates / Harris Broadcast really likes that fat orange wire – are the filament supply, 10vdc Max at 150 amps. The HV, ~9500vdc at about 3 amps and change, comes in on a piece of RG-213 coax from this off board power supply.
A little dust but not too bad. The dust seemed to really like gathering on anything live with plate voltage. Eww. A more through cleaning will occur soon.