Monthly Archives: January 2021

Photos of the new Tecsun GR-99 emergency radio

Many thanks to SWLing Post contributor, Mei Tao, who writes:

Hi Thomas:

I’m happy to tell you that yesterday [January 29, 2021] Tecsun released a new model Emergency Radio. I’ve included some photos:

The GR-99 includes FM, AM and SW bands. As most of today’s radios, it is also based on the DSP chip which offers good performance. It can be powered by the built-in Ni-MH battery and two AA batteries.

This radio also features hand crank power generator which can charge your device such as smartphone through the micro-USB cable. In case of emergency, GR-99 with flashlight and SOS alarm can give you a hand.

Nearly a month ago, I helped to test the prototype of this radio and gave them my advice. Now it’s great to see it on sale.

At last, provide you with a photo of me, almost two years ago. I took this selfie with my radios.

Oh I must admit that several radio in this photo were my friends’.


Mei Tao

Ha ha!!! I love the photo, Mei Tao–absolutely brilliant!

I’m happy to see that not only is Tecsun still producing an emergency radio with an analog dial (which requires less of the battery than a digital display), but also is still including the shortwave bands.

Thank you very much for reporting on this early production run Tecsun radios, Mei Tao!

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How to build a PC keyer and AM modulator for the EMTX emergency transmitter

Many thanks to SWLing Post contributor, Kostas (SV3ORA), for sharing the following guest post which originally appeared on his radio website. Note that this project builds on the EMTX emergency transmitter project:

PC keyer and AM modulator: A 15-components versatile keyer and powerful PSU modulator for the EMTX (Emergency Transmitter)

by Kostas (SV3ORA)

Schematic of the keyer and modulator (on the left) for the EMTX. The EMTX schematic is shown as well on the right, to determine the connections to the keyer/modulator.


My very successful emergency transmitter (EMTX) was only capable of CW or other slow speed ON/OFF keying modes. Then I thought, why not “give voice” to the design? CW is good, but it is half of the fun. If you could use your simple CW transmitter to send out your voice as well, this would be great. You could now chat comfortably on the nets or use any digital radio amateur mode and have much more fun. The simplest modulation you can apply to an existing CW transmitter, is the AM modulation. And whereas this is an old modulation, mostly abandoned by HAMs due to beeing inefficient, there are still AM nets on HF. But do not forget, AM can also be heard by SSB receivers by zero-beating the receiver to the AM carrier. So you could still use your simple AM transmitter to QSO with the SSB guys!

Along with the modulator, there is also a versatile keyer embedded to the circuit, so that the EMTX can be manually keyed with different ways or automatically keyed by audio tones from the PC. For more information on the keyer, keep reading.

The AM modulator

In the old days, the most common way to apply AM modulation was to modulate the high voltage to the plate of the tubes, using a transformer and a powerful audio amplifier. In low voltage solid state circuits, you can still do it using transformers, but you can also use series transistors instead of the transformer. All these things require many components and/or powerful AF amplifiers if one is to modulate higher power transmitters. This does not match the keep-it-simple design I am trying to achieve here.

So I thought of a simple trick with the use of the extremely common LM317 regulator, used as a modulated power supply. This modulator uses just a few common cheap components and it is able to achieve remarkably good modulation levels for it’s parts-count, just from line audio input. It juices every bit of the internal circuicity of the LM317, just look at where the base current of the 2N2222 comes from.

The AM modulator is a kind of novelty. Whereas there is nothing special in a modulated power supply, this circuit has some interesting properties. It is amazingly sensitive and it is able to provide lots of modulated current to any low power transmitter that it can feed. It can be easily driven by the line output of any laptop (around 20% volume) and provide a very good depth modulation to the transmitter. Charles Wenzel was kind enough to do a simulation on the circuit I developed, which is shown below.

His simulated circuit is a slight variation (for measurement purposes). The resistor to ground on the base stabilizes the bias and the ratio of R1 and R2 set the output voltage (0.6 volts across R2 gives about 8 volts across R1). He put in an emitter resistor just for good measure. Same for the series resistor from the source. Charles words, “I don’t know how believable these results are but it looks pretty darned good!”.

The circuit is being used as a current booster, the current being the supply to the transmitter and dependent on the voltage it produces. The LM317 always tries to keep 1.25V between it’s output pin and “adj” pin but where we benefit here is the current at the “adj” pin is very low, so it is easier to apply audio to it. Effectively, the error amplifier inside the voltage regulator is used as an additional amplifier stage. The output pin voltage varies according to the voltage on the “adj” pin so if we use it to bias the transistor we get negative feedback which improves the quality of the modulation. More output voltage = more bias current = lower output voltage. The result, is a very cheap, low components-count, very sensitive AM modulator that can supply lots of power to easily drive the transmitter and produce a clean and deep AM modulation!

The AM modulator bias is set with the 1M potentiometer. Depended on the bias level, the idle carrier on the EMTX can be set from about 0.5W all the way up to 8W. Needless to say that this modulator can modulate any similar power transmitter, not just the EMTX.

The keyer

If it is to modulate the EMTX from the PC, so as to use the different digital modes, there must be a way to key it also from the PC. This is why I decided to embed into the same circuit, a PC keyer which is triggered by the line audio of the PC, but also triggered manually (internal or external key). Keying by audio tones was decided, because modern PCs do not have LPT ports to trigger directly by DC. This keyer uses a reed relay to reliably, fastly and scilently key the EMTX, which is activated by a transistor. The base current for the transistor is derived from the audio signal after rectification. The incoming audio from the PC line passes through the mini audio transformer to increase its voltage, it is rectified and then charges the shunt capacitor to drive the base of the transistor. The keyer “speed” (decay) is determined by the shunt capacitor size. The circuit starts to trigger from about 50-60% of my sound card output signal level.

The relay used to key the EMTX, must be able to tolerate at least 1A of switching and carrying current. Note that the relay contacts switching current is not the same as the contacts carrying current. Reed relays are the best especially if you want long relay life, noiseless operation and very fast switching speeds, like the ones used in Hellshreiber. If you can’t find such a relay, you can use a reed switch capable of 1A of switching and carrying current and then place a suitable electromagnet close to it, so you can build the relay yourself. If you do so, find the best point where the reed switch responds to the electromagnet.

The keyer relay must be as close as possible to the emitter of the transistor used in the EMTX. The connectors at the back of the EMTX and the keyer/modulator have been physically placed so that when the two units are side by side, a very short link cable is required for this purpose. With the two devices placed close together, you can now use any length of cable for your manual external key, which is now connected to the “EXT” connector of the keyer/modulator.

The keyer does also have an internal mini straight key. I find this idea very nice, to avoid extra cables. It is not the most convenient key in the world, but it is there along with the transmitter every time you need it. By using a special panel switch from apem, I was able to triple this switch usage for the different modes of the keyer. The vinyl lever cap you see in the next picture, is the original part of the switch, to make it easier to key with your finger. But you may build such a part on your own, to fit on other switches types.

The switch is an ON-OFF-(ON momentary) switch type. In the default (middle) position, only the PC keying action is activated. In the top position (ON), the keyer is always active, which is useful for broadcasting audio (into a dummy load). The bottom (ON momentary) position, is the manual PTT action. This is used as a straight key on OOK operation, or as a PTT on AM voice operation. Simple and effective!

Initially, I used one channel of the PC sound card for triggering the keyer and also as an AF signal for the AM modulator, but this caused several problems of unreliable keying or distortion. So I decided to use a second separate AF input (KAF) to key the keyer. This second input, uses the other channel of the stereo sound card. With the addition of this input, there is no interaction between the keyer and the modulator. The AF levels that the keyer and the modulator require, can be set independently. Instead of adding more hardware for the purpose, I have chosen to set these levels by adjusting the volume and the balance of the sound card, which works great. Also, programs like Fldigi, have options for using one of the two channels of the stereo sound card as a keying interface (PTT channel), which makes the keying efen more reliable. When the program is in transmit mode, a continuous tone is heard on the PTT channel. This steady tone, is used by the keyer as a reliable keying signal, independent of the audio signal of the digital mode that modulates the modulator. This solution works very reliably for any mode. But if the program you are using does not have an option for a PTT channel, that is ok, as the keyer works reliably even without this feature. For voice communication or broadcasting music (into a dummy load) you just use the internal key switch as a PTT to handle these modes.


Prior to building the keyer and the modulator in the same device, I had tested the circuits independently quite a few times, to ensure the results can be reproduced. The modulation quality and depth out of the AM modulator have to be listenned to be believed. I have not made any linearity measurements, I just trust my ears on this one. It works great on music as well as on voice. Apart from that, this is the most sensitive AM modulator I have ever built, requiring only a small fraction of the line level output of the PC sound card.

When modulated by this modulator, the EMTX shows no audible signs of FM modulation. I switched my receiver to SSB and I could perfectly zero beat the AM modulated music signal which stayed on frequency and it’s tone did not change during loud audio signal music. Switching back and forth from SSB to AM modulation on the receiver, I did not notice any difference in the audio quality, apart of course from the narrower bandwidth on SSB modulation, due to the narrower IF filter inside the receiver on SSB.

The AM/OOK switch is used to select the modulation applied to the EMTX. When the keyer is set to be triggered by audio from the PC, at the OOK position, the EMTX is just switched on and off by the audio tones applied to the keyer, or by the manual key, internal or external (connected to the “EXT” connector). At AM position, the EMTX is switched on by the audio signal applied to the KAF connector and at the same time AM modulated by whatever audio signal is applied to the AF connector. On voice communications, the momentary position of the internal key is used as a PTT. On music broadcasting (into a dummy load) the non-momentary position of the internal key is used to keep the keyer always active.


Back connections to the EMTX.

Pictures of the finished keyer/modulator. You don’t have to build it that nice-looking if you don’t care.

Modulator prototype and EMTX built on a breadboard. Yes it worked just fine onto a piece of wood.

Thank you so much for sharing this brilliant and simple project with us, Kostas. Your handiwork is absolutely brilliant too!

Click here to check out Kostas’ website.

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“Our Little Rendezvous” has a shortwave reference

Many thanks to SWLing Post contributor, Gregg Freeby, who writes:

Heres another one for you. I know we are always watching for short wave radios in popular movies but what about pop music? Turns out Chuck Berry’s song, “Our Little Rendezvous” recorded in 1960 has a reference to a short wave radio. You can listen to it here:

How very cool! The grandfather of rock’n’roll never disappoints.  Thank you for sharing, Gregg!

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KMRT: “Your one stop shortwave shopping experience”

Many thanks to SWLing Post contributor, Fastradioburst23, who shares the following announcement (originally posted on Reddit):

Please join us for the grand opening of KMRT, your one stop shortwave shopping experience! If frequency hopping has got you down, then park your dial in the aisles of 9395 kHz at 00:00 UTC Monday 1st February 2021. On KMRT we dare to sell you the muzak you’re already listening to, but just don’t know it yet, because these subliminal tunes have been beaming to you from our corporate headquarters for years! We also have the specially priced radio culture you’ll need to survive in the years to come.

A cavalcade of special guest hosts, including the Store Manager, alongside KMRT regulars breathlessly parade the goods you need and instant-compose new modern music classics as the show progresses from the meat market to the candy aisle, on down to the home goods section. And if you bring your kiddies along, we have a FREE toy they can download over the airwaves, if you get our special proprietary KMRT radio.

As the folks who installed our audio system told us, “Something as simple as a mallsofted music selection, and a bit of retro in-style vaporwave, along with choice library cuts, muzak and ‘happy buyer’ spatial oddities within a store can completely change a listeners radio shopping experience.”

We have ensured your that the sound levels will all be uniform and consistent, because a soundtrack not only creates a mood and experience within a store, but also creates a culture around the KMRT sound.

Love it! Thanks for sharing and we look forward to tuning in!

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Sangean HDR-16: Benjamin pleased to see re-mapped front panel buttons

Many thanks to SWLing Post contributor, Benjamin, who writes:

I recently started looking into smaller HD radios and most recently have been looking at the HDR 14 and 16. In the process of doing my homework it looks like there was a very recent change to the HDR 16 that is a major pro.

The new version now has the MENU and INFO keys sharing a single button and the former INFO button now being remapped as a PAGE button.

Just like the HDR 14, the new 16s have a total of 40 presets 20 on AM and 20 on FM. It looks like they may have also changed the back light but it is hard to tell on the video that I saw. It makes me wonder if there were any other changes made under the hood that could affect either positively or negatively the HD reception as well as analog.

Here is a link to the YouTube video that I found the change through.

Thank you as always for all of the wonderful information you provide on your blog. I will be curious to see if there are indeed any other differences to this radio and maybe somebody else already has one that can comment pro or con about it.

Thanks, Benjamin for sharing your findings. If you’ve recently purchased an HDR-16, please comment!

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Ron notes that “Purple Power” is not the same product or formula across the globe

We’re shared a number of posts here about how well the product “Purple Power” cleans the sticky residue off of portable radios than once had a rubberized coating (the coating breaks down over time and becomes incredibly tacky).

Many thanks to SWLing Post contributor, RonF, who writes from Australia with this important note:

Tip for non-USonians: what’s sold as “Purple Power” (and “Simple Green”, and several other frequently-recommended cleaners for this sort of thing) are not necessarily the same products around the world.

For example, in the US “Purple Power” is an ethylene glycol based cleaner/degreaser; here in Aus, if you ask for “Purple Power” you’ll get a sodium hydroxide based degreaser.

One will clean the gunk off your radio; the other will clean the gunk *and most of the labelling* off…

Wow–that is an important distinction! Thank you for sharing, Ron!

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Getting the Grundig G6 out of a sticky situation

This has been a busy week, but Wednesday evening I took a few minutes to finally remove the sticky residue on my Grundig G6.

In case you’re not familiar, back in the day (roughly 2009 to 2013) Eton/Grundig covered a number of their radios models with a rubberized coating that unfortunately breaks down over time and becomes tacky or sticky to the touch. The Grundig G6 was one of those radios.

If you’ve been an SWLing Post reader for long, you’ve also no doubt read our numerous posts about cleaning off this mess. There are a number of solutions, but it seems the most positive long-term results by employing a de-greasing product called Purple Power (click here to read archived posts). Indeed, it’s the solution Eton Corporation recommends and the one I used to clean my Eton E1 XM.

Pre-cleaning, the G6 was incredibly sticky. It’s hard to see in the photos, but it was so sticky, it was challenging to remove it from its OEM pouch where it had been stored.

The Purple Power solution is effective, though. It requires only a few minutes to clean off the residue, then another few minutes to do a final polishing (I use a simple window cleaning solution).

The results are so impressive.

When I pulled the G6 from its pouch before cleaning, the back stand fell off. I believe it actually stuck to the inside of the pouch.

I’ll likely follow in Troy’s footsteps and eventually replace it with a 3D printed one. I’m not in a hurry at this point, though, as I’ve so many other things on my plate at present.

It’s so great to enjoy the G6 once again. It is a gem of a compact portable. One thing that surprised me? I forgot how fluidly the tuning works with no muting between frequency changes and how quickly (immediately) it switches into SSB mode. In the day an age of DSP portables, we’ve forgotten that these legacy receivers are actually better at both of these tasks.

Next up is my Grundig G3 which is quite sticky. I need to pull it from its storage bin.

Have you rescued a sticky radio recently? Please comment!

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