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 Post subject: Richfair's am transmitter build
PostPosted: Mar Sat 06, 2021 8:36 pm 
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Location: Equinunk PA 18417
Attachment:
1_Front panel label mockup sml.jpg
1_Front panel label mockup sml.jpg [ 252.86 KiB | Viewed 1752 times ]

This build emerged from two schematics I posted last year for a part-15 broadcast transmitter and for an audio processor front end. They work well together but mine remained just ugly prototype boards. No enclosures, no connectors, no conveniences. So, a couple of months ago while thinking about an enclosure I decided to add a few conveniences. Along the way I updated last year’s documents with a few revisions from this build. Because those documents are the foundations of this build, without clutter from cpu and connective glue, I hope to avoid uploading a big mess of stuff that may be of little interest to all, and instead link to them.

Transmitter docs: https://antiqueradios.com/forums/viewtopic.php?f=12&t=354147
Audio Processor docs: https://antiqueradios.com/forums/viewtopic.php?f=12&t=354297

This build includes:
1. All metal enclosure (to withstand Fibber McGee’s Closet)
2. Microprocessor interface and control
3. Carrier range of 400kHz to 1800kHz. Currently my software accepts only exact channel frequencies (10kHz or 9kHz spacing) but since the AD9850 DDS can generate practically any in-between frequency, I may eventually work the interface to allow it
4. Inductor resonance is adjusted automatically by servomotor under cpu or user control. A traditional air variable capacitor is present and typically remains at its minimum setting but can be adjusted (by hand) at any time. It is most useful to extend the lower resonant frequency to 450kHz
5. An on-board whip or a 1-3 meter straight wire serve as antenna. I have no intention of installing a “permanent” or remote antenna and thus have not brought out a direct connection from the pre-tank modulated signal
6. The LCD display’s top line becomes a bar meter to show relative antenna signal strength or peak-reading audio modulation. There is a modulation pinch-off warning indicator (you can see a remnant of it in the top pix, LCD top line far right
7. The modulating mosfet is protected against static discharge into the antenna
8. Several audio connector styles feed a mono combining amplifier ahead of the processor
9. A “feed through” type ¼ inch TRS jack provides a breakable insertion point between the processed audio signal and transmitter’s modulation input

By this point you may wonder why? Why put so much into so little? This is built to be a piece of gear for my bench and for fun. Some mistakes were made and lessons learned (or re-learned), and it was a grand distraction from the winter of 2020-21. I am especially pleased with the inductor mechanism’s operation. With no prior servomotor experience, it was gratifying to see this homebrew contraption work so well!

Attachment:
2_inductor assembly ready for test sml.jpg
2_inductor assembly ready for test sml.jpg [ 205.28 KiB | Viewed 1752 times ]

Attachment:
3_inductor rewound sml.jpg
3_inductor rewound sml.jpg [ 200.36 KiB | Viewed 1752 times ]

Aluminum angle, acrylic tube, 6mm ribbed belt, 12 volt gear reduction motor, optical limit sensors, moving ferrite bar. The inductor was eventually rewound for wider adjustment range

Attachment:
4_mainpcb top sml.jpg
4_mainpcb top sml.jpg [ 318.44 KiB | Viewed 1752 times ]

Attachment:
5_mainpcb bottom.jpg
5_mainpcb bottom.jpg [ 299.72 KiB | Viewed 1752 times ]

While the main pcb is new construction that holds most of the components, I decided to re-use my functional but messy prototype audio processor from last year. For some reason It developed an instability that was not seen last year or during early testing of this build. Tiny fluctuations of the VCA control voltage, like a jitter, would appear when it rose past a certain value. I still don’t know why but I have two theories. This is described in better detail in the processor’s updated documentation. Anyway, once that problem was quenched (I dare not say “fixed”) there was also carrier leakage upsetting the sensitive VCA controls. By mounting the processor board in an improvised tin metal box and wrapping its panel controls and their wiring with aluminum foil, processor operation is once again good. Plopping a bendy tin box into an already tight space did cause some annoyance.

I’m still tweaking software for niceties like display sleep timing, “in between” frequency settings, and so on, but the transmitter is working as intended. What could I have done better? For one thing, a larger resonant range from the inductor would have been nice. Due to practical considerations and space limitation, the inductor’s length had to be reduced from an early prototype and in so doing its tunable range was reduced. I have chosen to compromise lower frequencies the most since they can be easily resonated by adding capacitance, necessary from 450-650kHz. Above that no extra capacitance is needed and peaking is typically done by the inductor alone. Capacitance is easily added by turning the 10-100pF air variable. Turning its knob is not an egregious compromise but I’m certain it could have been avoided with a more efficient inductor design. Another thing I should have included is an automatic audio mute that would kill all modulation when the cpu goes into its inductor peaking routine. I thought about it during the design phase and kick myself now for not adding it. Also, I should have completely isolated the electronics 12v supply from the motor’s 12 volts supply. The motor is never used while broadcasting, of course, so it’s not a show stopper but emi noise from the motor is strong. Much was tamed by adding bypass caps at the motor’s brushes and tacking in an extra-large electrolytic across the main board’s 12v distribution point, where the two 12v supply rails come together.


Attachments:
File comment: All together and tested without the audio processor board
6_assembled before processor sml.jpg
6_assembled before processor sml.jpg [ 372.55 KiB | Viewed 1752 times ]
File comment: RCA and XLR audio inputs, TRS audio insert, 12v DC input, ground, antenna, BNC oscillator output.
7_case rear mounted sml.jpg
7_case rear mounted sml.jpg [ 273.67 KiB | Viewed 1752 times ]
File comment: Processor board installed in tin box. Blue tape protects my fingers and wires.
8_assembled top view with case open sml.jpg
8_assembled top view with case open sml.jpg [ 291.87 KiB | Viewed 1752 times ]

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sat 06, 2021 11:11 pm 
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Pretty impressive. That may be the ultimate transmitter project.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sat 06, 2021 11:35 pm 
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Location: Auburn, AL
Very nice. Respect that you were willing to leave the cigar box parts in there for good luck :)

I’m trying to up my game in the enclosure department. It seems to take 10x as long to make functional enclosures (or to adapt one) than it does to build something in the first place. But the results are so satisfying.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sat 06, 2021 11:37 pm 
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Location: Auburn, AL
What did you do for the labels/lettering on the case?

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 12:54 am 
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Very impressive. Very nice build quality. It's especially nice to see that someone has finally automated the antenna tuning for these Part 15 homebrews.


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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 5:44 pm 
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Location: Auburn, AL
Just realized the tin box was olive oil, and not cigars :)

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 6:11 pm 
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Location: Omak,wa,usa
Hello Richard,
What a great build it looks great So have you figured how much money did take to build this unit ?

Richard like Bob said "It's especially nice to see that someone has finally automated the antenna tuning ."

So for The Labels Did you use one those Label maker because I use one all the time .

Sincerely Richard


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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 6:12 pm 
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Wow. You had the multilayer PCB custom made? Did you design the PCB? The project looks pretty impressive and professional. Are you planning to sell similar units?


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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 6:14 pm 
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Location: Equinunk PA 18417
anchorman wrote:
What did you do for the labels/lettering on the case?
The photo is a mockup (stated in file name but I admit not mentioned elsewhere). I am considering options still and any advice you all may have for me. I originally planned to paint the case and to print something fancy on waterproof label paper to cover the entire front panel, and would be sprayed with clear acrylic for protection. I've done this in the past and the stuff sticks well for a long time. This enclosure's brushed aluminum is nice looking already so I did a mockup with text using Paint to evaluate the approach. I like it. So now, what do I do? I no longer have a color laser printer, just an ink jet printer, so toner transfer is not possible. Avery makes clear glossy label material for ink jets that is large enough to cover the entire front panel, not outrageously expensive and available is small quantities. I would think it could be given several coats of clear acrylic for better wear protection. I wonder if it will yellow over time as their transparency film does.

Another option, albeit the one for lazy people such as myself, is to just stick on strips of printed white labels cut from weatherproof paper. Wouldn't look amazing but now that I've posted here, who else am I trying to impress? Very few people will ever see my work bench and fewer still will give a hoot what anything looks like.

Radiorich, what is the label maker you use?

edit: Yes the tin box is from a gallon sized olive oil container

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 6:18 pm 
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Dare4444 wrote:
Wow. You had the multilayer PCB custom made? Did you design the PCB? The project looks pretty impressive and professional. Are you planning to sell similar units?

Hi Dare, thanks, yes I did. Years ago I used to etch my own pcb’s and miss that convenience. We live in a watershed area now and etchant disposal requires taking it somewhere it can be legally dumped. Besides, I could never consistently achieve high enough resolution for smaller surface mount packages. These boards were fabricated by a US-based board house and were by far the largest single expense of this build. Many of the parts came from my stash, which may explain why there is a mixture of leaded parts alongside smt.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 6:39 pm 
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Hello Richard,
Far the label maker that I use I have two different brands one is my trusty DYMO brand and the other one is made by Brother and it's called P-touch .
I use the Dymo in the classroom almost everyday to label my students Lego kits and at home to even label wires on a radio restoration .
Sincerely Richard


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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 8:17 pm 
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Outstanding!


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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 8:50 pm 
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Location: Auburn, AL
I’ve been contemplating silkscreen. Never tried on metal, though, just cloth/paper. I’ve got a few sets of letter/number punches, and have considered building a little jig to keep everything lined up. Fill with wax crayon/paint for better contrast. Laser engraving can be an easy option, but works best on anodized surfaces. I’ve had some success painting things and using laser engraved to burn through the paint. Would could conceivably paint a small patch, and use the laser engraver to burn the paint off around it, leaving bare metal with black remaining.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Sun 07, 2021 8:51 pm 
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Another option would be acid etch into the surface using a wax or photo resist.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Mon 08, 2021 2:38 am 
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Clearly you have a better junk box than do I. :lol:

Outstanding!

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Mon 08, 2021 5:05 am 
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Colin, Tom, Richard, black85vett, you guys are always so supportive and kind. I appreciate you all very much, thanks!

Yes there were some expenses but not as tall as they might have been. Hands-free adjustment was my goal from the start but I had nothing remotely useable for a motor. I looked around for something used and eventually just bought from Pololu.com. That wasn't too bad. The gears and ribbed belt were auction site deals, another $23 for the motor controller and DDS sub boards. The circuit boards were costly for sure. One reason I didn't finish this build last year was that I waited for a suitable metal box to show up. An olive oil can did but nothing else so I bought one from Circuit Specialists. Oh, and I anted up for new knobs at a few bucks each. Most everything else including most parts, display, switches and pots, wire and connectors, hardware, most components, is left over from previous projects. I have a questionable habit of buying two of something even when I only need one, and there have been a lot of projects over the years. I use an existing 12v supply.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Mon 08, 2021 5:08 am 
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anchorman wrote:
I’ve been contemplating silkscreen. Never tried on metal, though, just cloth/paper. I’ve got a few sets of letter/number punches, and have considered building a little jig to keep everything lined up. Fill with wax crayon/paint for better contrast. Laser engraving can be an easy option, but works best on anodized surfaces. I’ve had some success painting things and using laser engraved to burn through the paint. Would could conceivably paint a small patch, and use the laser engraver to burn the paint off around it, leaving bare metal with black remaining.

I hadn't thought about laser engraving. Interesting idea

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Mon 08, 2021 4:12 pm 
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A trophy shop may be able to etch a flat panel.

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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Mon 08, 2021 5:29 pm 
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Lovely work. Magnificently thought out.

Below is a photo of what the P-Touch clear labels look like on brushed aluminum. Please disregard where I had taped my circuit diagram onto the exterior of the case. I know that that tape detracts from the presentation.. But it's not where I can see it anyway.

Attachment:
20210308_104032.jpg
20210308_104032.jpg [ 441.23 KiB | Viewed 1550 times ]


The clear labels are not as invisible as I would like for them to be. But they do look better than some sort of hand lettering even dymo type labels. IMHO your outstanding project deserves something that looks as good as laser etched. As stated above, a trophy shop can do a lovely job, but it would probably need to cover up your brushed aluminum. I have a trophy shop do my brushed brass plates for my Tannerins. I wouldn't be surprised if they have brushed nickel or brushed stainless steel or brushed aluminum looking material.

Attachment:
Image2-5.jpg
Image2-5.jpg [ 119.86 KiB | Viewed 1546 times ]


I've also used laser transparency film and it looks better than the tape because there's no borders of the tape. However it's hard to secure without having visible securing points.

I also used a color printer to print the complete panel of a regenerative radio I built, and took it to a laminating machine. It produces a glossy finish, albeit somewhat mottled in that the lamination is not a completely flat surface.

Image

I have a color laser printer if you or anyone else ever needs it. Just PM me. That way I can play a part in your success!


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 Post subject: Re: Richfair's am transmitter build
PostPosted: Mar Tue 09, 2021 2:27 am 
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Hello Richard,
That is so funny you talk about buying extra I am the same way like I just bought some mini amplifier kits that use the LM386 anyway i bought 4 kits .
Back in the day Radio shack had some great labels that you could buy they came in black .
Sincerely Richard


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