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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.

Coordinated WSPR band hopping with the QRPLabs U3S

✇LA3ZA Radio & Electronics
By: Sverre Holm
10 July 2021 at 14:09

There is a recommended sequence for multi-band transmission on WSPR. It is given in the K1JT WSPR documentation. If enough receivers follow this sequence, then presumably letting WSPR transmissions also follow it, will increase the chance of detection. 

The recommended band hopping sequence cycles through 10 bands over a 20 minute period according to this table:

Band (m)   160  80  60  40  30  20  17  15  12  10
UTC Minute  00  02  04  06  08  10  12  14  16  18
            20  22  24  26  28  30  32  34  36  38
            40  42  44  46  48  50  52  54  56  58

The multi-band U3S is capable of adhering to such a sequence. There are however three considerations that need to be taken into account. 

First, for frequency stability reasons, it has been recommended to start with transmission at high frequencies and move to lower frequencies, i.e. the opposite of the recommended band hopping sequence. See recommendation #5d in QRPLabs Application Note AN001. My experience with a U3S with a 25 MHz TCXO driving the Si5351A is that it makes no difference for stability if the order is reversed according to the table above. The same applies to my other U3S which has the standard crystal with the Si5351A, but enclosed inside the QRPLabs OCXO box. Despite the oven being unpowered, just the thermal shielding seems to be enough to ensure stability no matter which order the frequencies come in.

Second, the U3S needs time for GPS calibration of the frequency from time to time. Typically the calibration interval is 60 seconds. There is no time in the above sequence for that. A calibration interval can however be squeezed in in these ways:

  • Skipping bands. By skipping some bands, initially or at the end. I typically transmit on the higher bands, from 17 m and up, in order to detect when there are openings. Then there is time from 00 to 12 for calibration every 20 minute. The 'frame start' in that case is 12. 
  • Longer frame. If transmission on all 10 bands is desired, one can transmit every 40 or 60 minutes instead of every 20 minutes, leaving a 20 or 40 minute break. This is done by transmitting with a 'frame length' of 40 or 60 rather than 20 minutes, and with a 'frame start' of either 00, 20, or 40.
Third, if one should desire to transmit on, say, the 17 m band, and then on the 12 m band, there isn't a direct way to put a 2 minute break between them. There will be a new transmission in the 12 m band immediately following the 17 m transmission, thus losing the timing of the band-hopping sequence. There are however some work-arounds: 
  • RX mode. If the mode is changed from WSPR to RX for a particular frequency, a 2 minute receive cycle is inserted between transmissions. In the example above, this would be the 15 m band. In this way a 2 minute break will be inserted between the 17 m and the 12 m transmissions. This requires firmware 3.11 and up (U3S only, not U3). A receive module is not required.
  • Aux bit. There is an indirect way if you have two antenna outputs as I have on my U3S. Assume a dummy load is connected to the second antenna output. By setting up a 17 m transmission for antenna 1, then a transmission on any band to antenna 2 into the dummy load, and then finally transmission on the 12 m band on antenna 1, a two minute break without transmissions can be achieved. The antenna is selected using the Aux configuration parameter documented in QRPLabs AN003. This feature is present even for the U3.
Some of the ideas here come from a discussion of band-hopping on the Groups.io list with contributions from G4APB. Taking the three considerations listed here into account, the U3S will be able follow the recommended WSPR band hopping sequence.


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