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....? ?I also wonder what the output level would be with 7 volts running the radio.

?

I did a few tests as I to set up my 20 metre QCX+5W.? ?On 9.1V? in? (so less than 8.7V via the protection diode and into? the PA modulator Q6 ) and still got a little over 3 watts out for a the long term beacon.? (it?lasted around 5,000 hours (the PA was still fine) so that should cover quite a few QRP activations)

In testing also noted 8V in (7.6V after the diode) still produced 2.4Watts

?

To achieve under a watt is going to to require a pretty low PA voltage (nearer 5 than 7 ? ) but I would not go there without a spectrum analyser to see what happens with the BS170 gates 'over driven'.? At some point it will likely get hot or nasty or both?

?

I don't understand the 'add an attenuator' suggestions unless you don't care about receive losses ??

Hi Don
Sounds like a much simpler solution?
When you do an RF sweep of 20m what is the level of the sweep in the 14 mhz range... I think the best I got was -12 db with an external toroid and coil
Dave VE3KCL

Another nice one: QDX Assembly Manual for PCB Rev 5, Page 4: Beware that a 12V nominal

Upon completion of my QMX (#331) build I found that the sensitivity on 20 meters to be quite poor, about -92 dBm (5.6 microvolts) for a Minimum Discernible Signal. A topic soon appeared on the forum regarding this as a design problem so I entertained myself otherwise and waited for someone to find a solution.

Since there has yet to be a solution proffered I decided to try the obvious solution of replacing C403, a 22 pF capacitor in the band pass filter, with a 16 pF value. That seemed to be what a ratiometric guess would suggest. Nope! The band pass response did indeed get closer but the 14 MHz frequencies were still highly attenuated. A 10 pF capacitor did little better but confirmed that the tapped inductor ( L401) and the associated parasitic capacitances formed an effective trap for the entire 20m band.

?

It occurred to me that the simplest solution would be to just bypass the 20m band pass filter entirely. I live in a rural area with no broadcast stations or other strong HF signals. If I needed a band pass filter then I could add one externally or try to wedge in one separate from L401.

?

I removed the tap at the 19^th turn of L401 from the solder connection, soldering the two conductors together so to effectively remove the tap from the coil. I then removed C403. I installed a 1 nF (.001 uF) 0805 sized capacitor from the input signal side of C402 (the end shared with C403, C401, and C404) to the previously vacated solder pad of the removed tap. I positioned it a bit diagonally, reflowed the solder at the C402 pad so that it fastened the new capacitor. The other end was then soldered to the pad where the L401 19^th turn tap used to reside.

?

Progressing to the Step 8 tests, I confirmed continuity from IC3 pin 9 to pins 3, 4, and 5 but not 6. Visual inspection confirmed that no potential short circuits had been created.

?

The results have been satifactory. MDS is now -121 dBm (0.2 uV) which is comparable with the other four bands. No new birdies or images. No overloads. Just a 29 dB improvement in receive level. Terrestrial, solar, and galactic noise are now all stronger than the weakest signal that I can detect. Operation on all other bands were unaffected.

?

If new builders want to duplicate this fix might avoid making the 19^th turn tap on L401 and not install C403. The value of the new capacitor is not critical, it just needs to block the DC bias but pass the 14 MHz receive signal when that band is selected. It could be a leaded part, placed from the hole at the input end of the C403 position to the hole where the 19^th tap was supposed to go.

72, Don

The 20 meter rig has been sold; awaiting payment on the 30 meter QCX+.

The 17 meter radio is still available.
--
Jim / K7TXA
Eagle, ID

SKCC 10447
BUG 301

John,
Another safe and very flexible way to go is a step-attenuator.? This site has some good info on QRPp as well as construction of simple 43dB attenuator in steps of 1dB.


--
Cheers,
Chuck

On Tue, Jun 20, 2023 at 09:46 PM, MattW wrote:

how would you recommend repairing the solder pads and replacing this component L202?

I modify and repair boards routinely.? The pads don't need repair, just bridge them with wire.??
I would obtain some fine gauge wire, I use wire-wrap wire or single strands from stranded wire and tin it, carefully scrape the solder mask (#15 scalpel blades work best) from the ends of the remaining traces and tin them.? Any undamaged part of the run is OK.? Use low-melting point solder if you have it and go easy on the heat to minimize heating of the damaged traces to avoid them lifting.
Lap solder the wire, first to the replacement L202 ends then, after forming/trimming the leads, to the tinned traces.? Measure to ensure continuity and stick the wires and component down with cyanoacrylic (Krazy-type) adhesive.? Done.??
We used this exact technique to repair PCB traces for space flight hardware except using fancy $pace grade low-outga$$ing adhe$ive$.
73, Don N2VGU

What an interesting article (and new to me)!? I've made progress, but smoked my QRP dummy load before I could take measurements --- further results await a more capable dummy load.
--
JimC (KQ7B)

Don,

Congratulations on moving forward with your test gear.

There are 2 points on either side of the push-pull PA that you want to measure:? The gate inputs and the drain output of the BS170s.? Since each side is wired in parallel, you only need to measure one of each half.

The gate should look like a 5-volt peak square wave.? This can be distorted; thou should still be recognizable as a square wave.? The drain will be a distorted sine wave.? Here is a plot of a simulation:



Later today, I can capture actual scope traces if you need them.

73
Evan
AC9TU

Let me be clear - I'm not upset at all about how long it may take to get my QMX.? I'm just curious is all and I know Hans is working hard to meet the huge demand.? During the 8 month wait for my K4 I was in an io group where K4 shipping and arrivals was tracked and it was fun to see the progress.

73, Bill NZ0T

I would run at 7 volts and cut the 12v trace to Q6. Then wire from the 12v line at JP27 to Q6 and insert a string of diodes to drop the voltage. This lowers the voltage to the finals. By adding more diodes you can drop the finals below the 7V requirement for the CPU voltage regulator. I marked the changes in yellow. (I haven’t tried this….)
--
Colin - K6JTH?

I have not looked at my QCX+ but it seems easy enough to cut a trace and supply a lower voltage to the amplifier - a 78xx voltage rectifier or zener diode inserted in the amplifier supply line maybe?? A twisted pair to a separate supply would work, but probably be cumbersome.? I also wonder what the output level would be with 7 volts running the radio.

I haven't yet done anything with WSPR, but do have the GPS receiver - lots to learn here!

Harry, KM3D

Thanks to all that have replied,

I have to throw my hands in the air and do a Sergeant Schultz on it and claim to 'know nothing'. That is to say that, at best, I have an only basic hold on the cl. I have juggled through varying OS's through the years but I really love everything that I have managed to do with Linux. There is a massive wish that it was a bit easier for me. I am 'ok' at following instructions but even then it can be an arduous process. I bought this Chromebook that I am posting on with the hope that I would be able to work 'in the field' - 'shack-in-a-bag' style. It is set up in 'developer mode'. I thought that having Linux as the main OS on the machine with the good battery life. AS is the Chromebook is useful. It is unclear what I am doing or to what I am doing it. I know that flavours differ a bit but I don't really 'dream in code'. Perhaps I should submit the forward for the next edition of 'Linux for Dummies'.

The Pi and SBC's are a big thing for me - the fact that I can get a tiny box to talk to another tiny box. QRP and low voltages do it for me. I still am confined to having to use a mac for certain stuff and Windows too. It would be brilliant to have great coding chops but I don't really know what I am doing in this area and possibly I may be looking for advice in the wrong place -? I really think that it is the case that I can do this.

Linux is really good fun - especially in the SBC department. All computers drive me a bit bonkers but it is all part of radio now in a very big way. I love the fact that I once was using my QDX at it's full potential a fantastic little box, ready to hit the road and do some radio yet at the moment I am a bit lost. For radio I want to keep everything I can Linux based. I have the Pi3B+, a Pi4+ 8GB, two Inovato Quadras as well as a couple of other boxes that I wish to dedicate exclusively to radio. Only my two QDX's (a LO and Hi model), two tru(SDX)'s and a Yaesu FT818 are CAT controllable (just an fyi there).

I am glad that I didn't attempt anything last night as I was really wrecked tired. It may be that I can sort things out but I will definitely need a bit of help rather than reporting failures and wasting the time of anyone. The first thing is to determine how to find 'Users and Groups' in the system menus which I think I have to find anyway. I think that I am going to be doing this until I get a positive result. i.e. until I have to sleep.

Cheers,

Tryg de EI7CLB, GQRP #13050

--

It looks like there is enough connection left to make a pad. Put some liquid solder there and heat it without a component. Then check continuity of the traces. You may have to scrape a little resist off and do it a second time but I don’t think so. Once the pad is rebuilt add the component back.?
--
Colin - K6JTH?

I too would love to see how this mod would be done in order to use qcx+ in wspr at less than 1 watt. But the engineering is not in my wheel house.
I have seen this idea requested in several threads before but Ron is the first ive seen to suggest this theory.

--
regards,
Bryan, N0LUF

Datapoint: When I initially powered the qmx I got on the order of
200mA when pressing the left encoder.

Good news, sorta, L202 is in line to power the cpu, so, once you
replace it, fingers crossed, everything will work out. Just for good
measure use a loupe or camera to check the rest of the board just in
case something else fell off.

-mike/w1mt

Does running a 12Vdc QDX at 9 Volts provide a sufficient safety margin (for finals) while tuning up with an auto tuner if the SWR does NOT exceed 2:1 DURING TUNING? I use a 3db attenuator pad so my SWR can never exceed 2:1.?
Thanks,
Howard, W6ZZ