21 June 2018

A cheap hands-free solution: first test

Just a quick update on the hands-free solution.

The stock earpiece-microphone of the Baofeng UV-82 is quite lousy with its muffled sound and insensitive microphone. At least that's how my specimen behaves. So it is not a big deal to sacrifice it.

I removed the microphone capsule and used it to feed the audio coming out of the wireless receiver. I added a 470nF capacitor in series to block DC. Initially I chose 10nF but the resulting signal was too low (checked on the oscilloscope). Also remember that this C makes a high-pass network together with the input Resistance, so your voice could be attenuated further.

The first on-air test resulted in low audio. Then I hit twice the [+] button on the transmitter and the transmitted volume was back to normal. An unknowing HAM did not report a different sound of my voice, so the whole thing works.
VOX can work but it is insensitive and on UV-82 there is no setting for the hold time.

Next step will be to build everything into a comfortable box, add a lever PTT and possibly a way to power up the receiver together with the RTX. It has to work until I don't receive the external antenna and check if it can handle the RF power of the mobile transceiver I still own.

19 June 2018

A cheap hands-free solution: the idea

One of the many drawbacks of driving and talking on the radio is that the microphone is usually physically wired to the transceiver. Let alone that it is illegal in many parts of the world. Very few RTX's have a hands-free solution out-of-the-box and even in 2018 too few support some form of [Bluetooth] detached earphone-microphone. Last but not least, all off-the-shelf solutions can be expensive.

Obviously I looked for a DIY alternative. To simplify things a bit I assumed that an RTX for mobile use has enough audio power in the speaker so that an earphone or an audio amplifier is not needed. This means we need to transmit audio one-way only: from the mouth to the TX.

Now our feared Chinese fellows come very handy. Head to any of their portal and look for "Wireless microphone 2.4 GHz" (they have some that operate in UHF too). Be prepared to dig through many products until you land on something advertised for touristic guides. These products have a boom mic with transmitter and a simple receiver that is supposed to plug into an amplified speaker or other form of audio amplification.

I bought the product from NEWGOOD for 19€ (in 2017). It's still a bit more expensive that my desiderata since I hoped for 10-12€, but definitely cheaper than other solutions.


Both parts have an on-off switch, a microUSB socket for charging and the transmitter has volume control buttons. They are not Bluetooth so there's no need of pairing. I have no idea whether two similar units would interfere or could be swapped, but it's not a popular product I'd say and the risk is low even if the advertised range is 50 metres!

In its intended use, so with an amplified speaker on the other side, it works right. The audio quality is also very good.
How to interface with the transceiver then? Just feed the receiver output into the microphone input through a bypass capacitor, think of a way to handle the PTT: pushbutton, lever switch, VOX, .... and hope that signal levels & impedances match!

My current mobile RTX is the Baofeng UV-82 handheld, so it will be my test platform.

14 June 2018

HP 9403A System Control Panel

The purchase at Friedrichshafen 2018 with the best €/kg was the HP 9403A device. And I think it will have a very high fun/€ ratio too!

Let's see how it looks:

HP 9403A in FN, near the beer kiosk.
There are three display windows, a numerical keypad, other buttons, flip switches and a key operated switch. Let's not forget the two handles for carrying. On the back side there are three large barrel connectors with a high count of contacts (40 or so), AC power, fuse, power switch.

Me and the HP 9403A.
A preliminary search on the Net before the purchase did not return meaningful data. It turns out HP has "recycled" the 9403 code with inkjet printer cartridge. The challenge is set!

The seller held the device until the afternoon, when I picked it up on the way to Hall A (with new stuff) and the car.

I had to be careful not to hit someone's legs with it. I took few breaks along the way and I even sat on it. Hans G0UPL wanted to get one too because it had Nixies and all the driving circuitry. Yeah, I know, that's why I got it in the first place.

So the mysterious device came home with me, silently opening a deep dive into 1970's documentation.

07 June 2018

Friedrichshafen 2018

Friedrichshafen 2018 Ham Radio fair is history now.

I've been there, Friday and Saturday. My 3rd time, with 2015 first and 2016 second. You will read everywhere that there was a low both on visitors and exhibitors (on the "commercial" side), and that's true. It was extraordinarily quiet on Saturday morning! On the other hand everyone was more relaxed and easier to interact with. I finally met Hans Summers G0UPL from qrp-labs.com!

What did I bring home? For the future-me:

  • QCX kit for 20m, 42€
  • New Nixies from elecments13, 26€
  • Assorted Nixies used from LY2MQ, 10 pieces, 20€
  • Ammeter with calibration sheet, CCCP-made, 5€
  • Elka 135 calculator from a Polish seller with lots of vintage stuff, 3€
  • 7-segments LEDs MAN4640A in brown case, 8 pieces, 0,50€
  • HP 9403A Control Panel with fun discussion, the best €/kg!

I tested used Nixies with my portable HV generator. At least they are not dull at the origin.

The calculator was too cheap to be left there anyway, so I didn't try to fit my 4xAA batteries in the compartment full of dry leak.

The mysterious HP 9403A was simply too full of Nixie tubes that could be seen through the glass. It doubled as a (heavy) portable chair on the way to the car.

What I didn't buy:

  • IV-17 at 3€ ea.
  • Elka 101 calculator from the same Polish seller
  • Other instruments with Nixie inside (quoted at 50€)
  • Nice looking valves