I worked a little on it last night and I was able to get the heating problem resolved somehow, but the controls are still not working properly. the right button seems fine, but the left button works only sporatically, and same for the rotary encoder. Also, No matter the direction the rotary encoder is turned, the value goes down.

I will post a couple pictures here of my progress. Does anyone out there have a picture of the QCX mini board that I could look at to see what the traces should look like? I’m a bit conserved that the ground pins from controls header are shorting making contact with a trace they shouldn’t, but I having difficulty figuring out what it should look like.

Thanks!
Cal

John,

it may not be obvious, but substituting a 2 Ω resistor for L14 changes the principle of operation of the Class D PA. As originally designed, it is a current switching PA, and requires a parallel-tuned output network. As modified it has become a voltage switching PA, and requires a series-tuned output network. Since the original QDX PA design was for a current switching PA, this may explain why some have observed the output filter getting hot.

These two classes are noted in the RSGB Handbook (12th edition, p10.6) and Device Evaluation for Current-Mode Class-D RF Power Amplifiers to be found at at figures 3.4 and 3.5. [There are also many IEEE papers on class D, but these seem to be behind the usual IEEE paywall.]

L14 is employed to act as a current source. Thus when the output filter sees 50 Ω resistive, a similar load will appear at L14, but when things are reactive, there is the potential for undesirable voltages at the PA MOSFETs. (A simple measure of SWR says nothing as to how reactive the load might be.)

All that is not to say that a re-designed PA stage might not be more resilient against non-50 Ω loads, but it is not a simple change. The original design has a great deal going for it. (And even valve (tube) PAs can flash-over with too much SWR.)

73, Stay Safe,

Robin, G8DQX

On 12/05/2023 12:12, John Z wrote:

One more thing on this. Tony AC9QY and I concluded that using a 100 nF capacitor to bypass the center tap of T1's primary to ground, combined with replacing L14 with a 2 ohm resistance, was a good thing.

The inductor L14 adds no value in this circuit, and even contributes a dangerous LdI/dT spike at the end of transmission. The 2 ohm resistor in its place offers some additional protection against low-Z high SWR, which is the high current case. The PA self-throttles in response to excessive current. The loss of power under a 50 ohm load is insignificant.

The combination of high transistor breakdown voltage and high current protection makes the PA nearly bulletproof regarding SWR. Simulation demonstrates that nicely.

JZ KJ4A?

On Fri, May 12, 2023, 6:51 AM John Zbrozek <jdzbrozek@...> wrote:

Hello Tony,

Another Tony. AC9QY, and I have been running LTSpice simulations of the QDX PA using TN0110 transistors.

They are similar to the Microchip TN0106 transistors that you have been testing,? differing mainly in that they have a 100 volt drain breakdown specification vs. 60 volts.

The simulation results look extremely encouraging, even? on 10m, but we had some questions about the adequacy of the real drive level available to the gates.

Learning of your actual results in real hardware is fabulous!

Tip of the hat for trying that! Bravo!

If you are an LTSpice user and interested in running the simulation I would be delighted to send you the .asc file for it.

73, JZ KJ4A?

On Fri, May 12, 2023, 6:18 AM mux_folder2001 <canthony15@...> wrote:

I have been experimenting with different transistors for the PA in my HB and LB QDXs. We have been having heavy rains the past two days and my antenna is providing some bad mismatches of up to 2.8 to 1. I used this condition to test several transistors for robustness and have good results to report. The high SWR conitions I used were caused by load impedances less than 50 ohms as measured at the transmitter end of the coax with my nanovna.

First off, the BS170s do not tolerate these kinds of mismatches and blow quickly. But I have been unable to blow up either the TN0106 or VN0606 transistors. Note that these two both have reverse pinout with respect to the BS170 so the flat face has to be mounted upwards such that it is pressed against the washer instead of the ground plane. This does not seem to bother the transistors and they run cool with a normal 50 ohm load. My units are wired for 9 volts and that is how I did my testing. I operated the VN0606 for an hour at 2.5 to 1 SWR. While the unit gets warm, it does not fail. The TN0106 behaves similarly but I seem to get more drive into the low Z loads and the transistor heats up a bit more. AT 50 ohms, the two transistors behave similarly.?

Neither QDX has any other modifications from stock. No diodes or capacitors are added. Just the transistors have been changed.

The transistors were ordered from Mouser.

I also tried out the 2N6660. This is a higher power device in a TO-39 can. While it works, it provides much lower output, even into 50 ohms (on the order of 1 watt). I tried using just two devices but that made even lower power. These are expensive and the QDX does not provode enough drive for them.

The 10 meter performance of both the TN0106 and VN0606 seems to be better than the BS170.

I would be interested in hearing of peple trying these on their 12 volt units.

Tony

AD0VC

---

From: Anthony Casorso <canthony15@...>
Sent: Saturday, April 29, 2023 7:11 AM
To: [email protected] <[email protected]>
Subject: Re: [QRPLabs] 3 tips for not blowing the finals of the QDX #qdx

?

I have only had my HB for about a month and I have blown a couple of PAs due to matching issues. That aside, this QDX is FANTASTIC! The design is remarkable and well executed. I love the electronic aspects of the hobby so I always end up fiddling with things but I would be proud of this design if it were mine. Just amazing what was done for the price. This thing makes me happy! I have ordered another for the low bands.

Tony

AD0VC

---

From: [email protected] <[email protected]> on behalf of Afghan Kabulldust via <kabulldust=[email protected]>
Sent: Saturday, April 29, 2023 1:57 AM
To: [email protected] <[email protected]>
Subject: Re: [QRPLabs] 3 tips for not blowing the finals of the QDX #qdx

?

Dave et Al,

I have used the first prototype High Band since I designed the filters last year. I used at the home QTH and then took it to QRP Labs HQ for assessment and ultimate implementation. I have used that and another HB almost continuously. I’ve operated with it portable in Prague, Rome and Istanbul with various wire antennas out hotel windows on fishing rods and wires on lead sinkers slingshotted into local trees. I used to use a small 12V PS with a variable buck down and meters but now, due to noise reduction measures, I run all my QDXes and RaspberryPIs on Lithiums, 2 in series that start at 8.4V and I usually change them and recharge when somewhere at 7.3V. RasPis I use a 30,000mAH power bank.?

The one thing I am very careful about is antenna matching. I used to use LC matches with plastic printed spherical variometers as the variable L. I now use FRI-matches with 3D printed large toroids. I also have a switch that puts a LED swr bridge in line and a switched LED current monitor antenna wire.?

The first and original HB QDX is still going strong! It was built for 9V but usually operates at an average of 7.8V.?

Careful management of the antenna match is the most critical to long QDX life!

73

Ross

6

On 29 Apr 2023, at 07:25, Hans Summers <hans.summers@...> wrote:

?

Hi Dave, all

The washer isn't the heatsink. The heatsink is the PCB. The washer and bolt are just the mechanical means of bolting the flat side of the transistors to the PCB.

As others have said... Thermal paste doesn't harm but doesn't make a lot of difference, either. Be careful too because some pastes are electrically conductive and that won't go well. Make sure you don't use that.

Personally I have found the QDX PA reliable and durable as have many others. A lot of failures have posthumously been attributed to sustained chronic SWR mismatch or acute mismatch during auto tuner use, supply spikes etc., over-voltage, including from fully charged 12V-nominal batteries.

Remember also that on any forum such as this one, you will tend to hear far more frequently from people with a problem, than those without. You tend to get the mis-impression consisting of the proportion of people with problems, out of the collection of people with problems. The actual proportion of people with problems, out of the total of all users, is a lot lot lower (and difficult to quantify).

73 Hans G0UPL

On Apr 29, 2023, at 8:45 AM, Dave Sergeant G3YMC <dave@...> wrote:

Humour aside Razvan does have some valid points. I have not yet put my 
QDX into transmit mode but already have concerns about the design of 
its PA stage. The requirements for not exceeding 12V (in a 12V HB 
version) or daring to transmit into anything other than the religious 
50 ohm load is an indication that the design is maybe just a tad close 
to the limits for comfort. It almost made me laugh when I attached the 
little washer to the rounded side of the BF170s asking myself if I had 
mounted them upside down. Surely you should put the heatsink on their 
flat side I asked.

In the desire to make the thing as small and cheap as possible it seems 
common sense has been overlooked. A couple of TO220 case devices on a 
slighly bigger board and a bit more metal to heatsink them would have 
made a far more robust PA and removed the need for all these 
discussions on this reflector.

No criticism of Hans of course, the QDX is an incredible little rig, 
but as an engineer in my professional life I would have not done it 
that way.

73 Dave G3YMC

On 28 Apr 2023 at 15:15, DL2ARL wrote:

 you could also sprinkle over the BS170s some Water out of the Holy
 Fountain of Lourdes from
 southwestern France. It would help as much as thermal paste, but it
 will have the big advantage of
 not doing no harm and cost less.

On 15/05/2023 09:51, John Z wrote:
> Revs 1,2 and 3 use 330 ohm resistors at R2, R4. The referenced power
> calculation of 338 mW per resistor would be valid and excessive heat
> would occur.

John,

The surprising thing is that this does not seem to be causing many
failures. It was noticed long ago, some did modify theirs but the
subject does not surface very often.
(The rev 4/5 upgrade was a big change, not easy to copy.)

I guess excessively hot components will have a shorter life.

73 Alan G4ZFQ

About the T1 getting hot:

/g/QRPLabs/topic/qdx_t1_heat_thermal_design/91641546