31 August 2015

QYT KT8900 - digital noise on audio

One of the most noticeable defects of QYT KT8900 is a digital buzz that can be heard when the stock microphone is plugged in. This problem has been tracked down to the digital logic on the microphone communicating with the main radio board.

The picture shows how the loudspeaker signal looks like on an oscilloscope:


Spikes are about 25 mV peak, one every 3.5 ms or so. I couldn't get a stable trigger on the signal and the picture shows my best capture (100 MHz analog Tek, 100 MHz probe at 1x).

Any idea for a simple fix?

30 August 2015

AA to C cell adapter

[In Italian: "Adattatore da stilo a mezza torcia"]

So, your kid just received a new toys that requires batteries. Not plain simple AA cells, but C size. "Batteries not included" the box says. After the fist moments of unspoken words, this is a good chance to prove your audience your homebrew ability with a macgyverism!

Grab two ubiquitous AA cells and wrap them with cardboard until their diameter fits comfortably into the battery compartment. Fix the wrapping with a bit of adhesive tape. That's it!

AA on the left was partially extracted from the adapter.

How is it possible? AA and C cells share the same height. A C cell is an AA put on steroids. Theoretically it holds 2-3 times the capacity, but some people reported fake C batteries that were wrapped up AA's.

Have fun with the new toy!

26 August 2015

QYT KT8900 current consumption

Is it me, or the QYT KT8900 manual does not specify the current consumption? I measured it. The unit I got my hands on, performs as follows:

RX: 0.27 A (backlight on, stand-by)


Readings taken at 12.5 V out from the PSU.

I have no idea what the RF power output is. But I noticed that it will not transmit below 10 Vdc, while the current drain is constant between 10 and 13 Vdc supply.

24 August 2015

QYT KT8900 arrived

The parcel containing a QYT KT8900 has arrived today. It took less than 20 days and it was shipped through Germany to Italy. I haven't opened it yet...

22 August 2015

QYT KT8900 transceiver

Online reviews do not rate it at the top of the category, but QYT KT8900 VHF/UHF mobile transceiver has worthy features for less than 100 USD. And it is small, very small.

I have ordered one, that will come once it completes its long journey from CN/SG/HK. I am looking for two info that current reviews do not cover:
  • spectral purity of the transmitter
  • microphone sensitivity

My own tests will tell me how it performs. The challenge will be to build a probe that turns 25W RF into something acceptable by the spectrum analyzer...

14 August 2015

Insulated element support for ultralight VHF/UHF Yagi-Uda beams

Once upon a time, 10 years ago, I used to build VHF beams for high mountain contesting that could be carried in the backpack. My best result was a 5 element Yagi of DK7ZB design whose boom was a telescopic fiberglass fishing pole, elements attached with clothes pegs.

The system could not grow, though: longer poles have larger diameter, while there are no large clothes pegs.

I had built a 4 metre long aluminium square boom cut in two sections, that is OK for backpack transportation where there is no vegetation around. I broke it before the first contest.

Fast forward 6-7 years and I still haven't found a solution to the larger clothes pegs. But while hanging around "special offers" from Chinese online retailers I came across this product: Adhesive cable ties in packs of 6. I couldn't pass on them at the incredible price of 0.8€/pack, including shipping. Their original purpose is to keep cables (like USB) from falling off (behind) furniture.

But their base size matches my aluminium boom (6 or 8 mm per side) and the cable hole is perpendicular to the longest side: do you see a better, lightweight and cheap alternative to large clothes pegs?

The clip sitting on my old boom.

8 mm element into the clip.
The hole diameter is about 7.5 mm. An 8 mm dia. element fits perfectly with the right friction. A 6 mm dia. element needs to be embossed at the junction point with some rubber pipe.

While I could stick them to the fishing rod once fully extended, I will rebuild the Yagi beam with a square boom and element supports already in place.

Side view of the element inserted into the support.
Too bad I got only one color!

09 August 2015

biNixie Clock current consumption

The biNixie Clock deserves a box with an antique look. But the technology inside remains modern, so why not use a rechargeable battery supply?

Without any optimization, at 12 Vdc the biNixie Clock drains 82 mA with two digits ON and 65 mA in stand-by. This means a normal battery pack or 2000 mAh would last between 20 and 30 hours. I have to identify how much current each module drains so that I can plan the power supply and the box size accordingly.

I would like to keep it running off a 3S LiPO pack!

05 August 2015

Vacuum Flourescent Display from a Toshiba VCR

On the wave of my recently born passion for neon/vacuum displays I remembered I had a VFD (Vacuum Fluorescent Display) sitting in the "might come handy" box.

The board was removed from a Toshiba VCR and it was the front panel circuitry. Under the display there is a Toshiba TMP47C1670 chip. It is a pretty complex device, probably programmable like a recent microcontroller, plus it can drive VFD natively.

How to light it up? Two things helped tracing down connections: the TMP47C1670 (partial) datasheet and a DVM.

With DVM in continuity test mode I looked for the filament. Incidentally they are the outermost pins. The datasheet provided hints on which pins drove grids (output pin called Gn) and which were for anodes. With the help of DVM I mapped IC pins with display terminals.

That was it. I fed the filament with about 4Vdc from a battery. Then my 12V PSU ground went to one side of the filament and +V to grid and anodes. Moving around grid and anode touch points I could lit all segments:

This experiment was fun and rewarding. I learned a bit more about VFD technology. Unfortunately the display has too many pre-shaped symbols that make it unattractive for reuse. It is offered free + shipping to a good home (with a documented project).