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EA8/LA3ZA April 2023

✇LA3ZA Radio & Electronics
By: Sverre Holm
21 May 2023 at 18:36

This was a fun holiday operation from the island of Tenerife with 2.5 - 4 Watts running digital modes, mostly FT8 and some FT4 using a low-band and a high-band QDX.

The best bands were 30 m (29%) and in particular 10 m (65%) with a lot of contacts across the Atlantic ocean as the picture shows. In total 62 different entities/countries were contacted. 

QSL via Logbook of the World.


3 tips for not blowing the finals of the QDX transceiver

✇LA3ZA Radio & Electronics
By: Sverre Holm
28 April 2023 at 16:49

I have now used both the low- and the high-band QDXes daily as EA8/LA3ZA for a period of two weeks without destroying the four BS170 final transistors. Here are some procedures and tips.

But first, I do actually have experience in blowing the finals. That happended under testing prior to leaving, and all it took was 9.5 Volts for my 9 V build and what I thought was a dummy load, but which might have been an open circuit load. One BS170 developed a short between drain and gate with the result that 9.5 Volts was passed directly into the outputs of the driver IC5, 74ACT08, so IC5 blew as well.

My three tips for avoiding such failures are:

1. Use a reduced power supply voltage for tuning

I reduce the voltage from 9 to 7 Volts during tuning. as shown in the first image. That greatly reduces the risk of getting too high voltages over the BS170s.


2. Use a current limited power supply

The image shows a limit set at 1.3 Amperes. That reduces the risk of overheating, should the current for some reason rise more than expected.




3. Use Zener diodes to protect the final transistors

This tip comes from many of the transmitters designed by KD1JV over the years and consists in connecting Zener diodes from drain to source of each pair of PA transistors. The Zener diodes will conduct if the voltage exceeds a voltage somewhat less than what the transistor is rated for, 60 Volts, and protect the BS170s. 

I use 1N4756A, 47 V, 1W. I  measured power output before and after fitting them and could not detect any change from 80 m to 10 m. 

I have accidentally transmitted with full power into an open-circuit load after I fitted the diodes, and the QDX was just fine afterwards. I doubt that that would have been the case without the Zener diodes.

I cannot guarantee any adverse side effects of the Zener diodes, but my experience is that both the QDXes have worked flawlessly over the last few weeks, with plenty of contacts in South and North-America as well as in Europe. Those contacts have primarily been on 10 m (high-band QDX) and secondarily on 30 m (low-band QDX).

The image shows how the Zener diodes are fitted on the underside of the printed circuit board of the Rev 4 PCB of the QDX.

QDX Twins 80-10 m

✇LA3ZA Radio & Electronics
By: Sverre Holm
18 February 2023 at 22:15
My QDX twins from QRPLabs: 

  1. On top, the high-band version, 20-10 m, with a revision 4 PCB
  2. In the bottom, the original 80-20 m version with a revision 3a PCB
Both have been assembled for 9 Volts operation nominally.

My wife and I have identical twins in real life and at times one of the ways to distinguish between them was by different colors. Here it is the same, so the high-band QDX has a yellow LED and the low-band one has a green LED.

QDX with voltage regulator

✇LA3ZA Radio & Electronics
By: Sverre Holm
31 December 2022 at 16:02

Here's my low-band (80m - 20 m) 9 Volt QDX with a voltage regulator. Its only modification is a green rather than a red LED, as I don't like red LEDs to indicate anything but error conditions. 

The power amplifier of the QDX has hardly any built-in protection and can be ruined if run at full power into a poorly matched antenna. It will also easily be ruined if run at a higher voltage than the 9 or 12 Volts one may choose for at build-time.

A recurring theme on the QRPLabs discussion list is how to feed it with the right supply voltage. My solution is an "ZK-4KX CNC DC DC Buck Boost Converter CC CV 0.5-30V 4A Power Module Adjustable Regulated power supply" from AliExpress as shown in the image. It can take any DC voltage between 4.8 and 30 Volts and convert it up or down to the desired value.

I turn it down to 7 Volts for manual tuning of the antenna and up to 9-9.5 Volts in order to achieve near 5 Watts output. If I exceed 10 Volts, the power module is set to turn itself off.

QDX on 17 and 15 m: Receiver

✇LA3ZA Radio & Electronics
By: Sverre Holm
26 August 2022 at 11:46

I got myself one of the marvellous digital transceivers from QRPLabs recently: The QDX - QRPLabs Digital Transceiver. It is set up with receiver bandpass filters and transmitter lowpass filters for 80, 40, 30, and 20 m. It has also been found that the 40 m filters work well for 60 m. 

The latest firmware, version 1_04, has a new "Band Configuration" screen where one may configure the list of supported bands. It is the intention that higher bands will eventually be supported.

I am not so interested in 80 m, so I wanted to see if I already now could get the receiver to work on 17 m and 15 m. With reference to the schematics, I modified the input filter's inductor, L12. 

It is tapped at 19, 30, 36, and 41 turns for 20, 30, 40, and 80 m respectively. It resonates with input capacitors C28-C31, with 22, 30, 56, and 220 pF respectively. 

I gave the inductor 36 turns, omitting the extra turns for 80 m, and then tapped L12 after 13 turns. I also also changed C30 to 15 pF. That give me the  bandpass characteristics shown below, as analyzed with the built-in analysis function of the QDX:


The filter peaks at about 19.5 MHz and is about 2 dB down at 18.1 MHz and about 7 dB down at 21 MHz. It seems to work well, and in my first tests, it receives FT8 well on both bands. 

Here's a new sweep with firmware 1_05_002 (beta) which uses colors which are easier to read:




And now the next thing to do is the transmitter's lowpass filter for 17/15m, which is a bit harder.

QDX Revision 3 Build Begins in REAL TIME

✇MiscDotGeek
By: Ryan Flowers
1 July 2022 at 02:16

Hey everyone- been a busy few months lately. Between bad health, travelling, and then working hard just take care of Live Stuff, I’ve hardly had time for the Ye Olde Blog! I’ll try to write more regularly. A lot of my attention has been going to YouTube and in this series, starting with the video …

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The post QDX Revision 3 Build Begins in REAL TIME appeared first on MiscDotGeek.

QDX Down! Understanding and Preventing QDX Revision 1 Failure

✇MiscDotGeek
By: Ryan Flowers
19 March 2022 at 21:16

The QRP Labs QDX is an incredible piece of radio engineering, but the first revision had a flaw that ultimately destroyed a some of the Revision 1 radios. To be clear: Unless you bought out of the first batch of 500, this does not apply to you. The second and upcoming third batches do not …

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The post QDX Down! Understanding and Preventing QDX Revision 1 Failure appeared first on MiscDotGeek.

FT8CN and the QDX

✇Steve G1KQH's Amateur Radio Blog "The Font of all Knowledge"
By: G1KQH
21 February 2023 at 10:56

 

 

It is being reported on the QRP Labs forum, that some have got the little smart QDX digital transceiver communicating with the FT8CN app.

The rig is noticed straight away by FT8CN using an OTG USB lead and a minimum of configuring. 

The QDX and a small phone or tablet, allow true FT8 portable operating with minimum hardware to cart around.

Pick up the thread here

Details about the QDX https://qrp-labs.com/qdx.html

   

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