QDX Highband - works normally on 20m, low xmit and receive on other bands #qdx

Frank Cox

I built a QDX Hiband Rev 5 board on latest firmware and it works as expected on 20m. On all other bands output is low milliwatts on xmit is in the low red zone on the WSJT-X display on receive.  It will still decode strong signals on receive and seems to transmit correctly when I listen on another rig. I even got one spot in pskreporter on 10m xmit when the  band was open the other day. (On 20m output is about 5W on xmit and green 40 on receive with many spots and decodes).

I've been trying to debug this problem but I don't have a good theory of what could cause these symptoms. Maybe it's more than one thing. I haven't found any build errors yet or anything that isn't working -- except for the levels.

Any ideas about what would cause this and what I can do to fix it?


-- Frank KK6WUU

Brad McDowell

Do a search for QDX low power on this forum. There are a lot of suggestions and threads on this topic.  There is a troubleshooting guide on the QRPLabs website but it has not yet been updated for the QDX HB.  Still has some good suggestions.  Its on the left hand column, under the QDX tab.

Brad, AB4BA

Frank Cox

Thanks Brad, I'll check it out.

Chuck ke9uw

You need to update the firmware to 1.09?

Frank Cox

Chuck, my kit came with v1_09 installed so I should be up to date

Frank Cox

I've  spent several sessions debugging this issue. I've concentrated first on the transmit problem on 10m. (As said above: output power is normal on 20m and milliwatts on10m)

 I found:

- Band switching for transmit works. DC voltages between the PIN diodes on band switching in each LP output filter is correct for all bands.
- And no significant RF voltage is seen on transmit in any LP filter except the one switched on.

Looking at the LP filter for 10m which includes, in series from input to output D6 - L10 - L4 - D3

- On 10m transmit, the RF voltage on both sides of D6 is 6.1V RMS at 28.1mHZ, 3.4V between L10 and L4, and 300mV on both sides of D3

This last seems to show that most of the 10m RF energy is absorbed inside the LP output filter for that band. (For reference, the 20m LPF is 10V on input and 9.5V output)

Checking the component values again, L10 and L4 have the correct number of turns (11 and 10) on the black toroids. I was able to push the caps over enough to see they have the correct markings. I can't see any shorts, opens of other problems with soldering or the PC board.

The only next step I can think of is to try replacing all the parts in the 10m LPF on the theory that it could a bad component. 

Am I missing anything?

Al Holt

Do you think you would be able to 'scope RF levels at the midpoint of the 10m LPF? This would be at the junction of C14, L4, L10 and C11. It might help seeing if the signal drops there.

Would you say filter response is good using the RF Sweep function in the Terminal Application? Maybe it looks good, but how many dB down is the high point of the curve?

Have you got continuity through L9, the 47uH choke? Just double checking. 

Hans did mention he had success removing two turns from L10 and fully compressing L4.

If you do decide to rework the LPF consider using a lighter gauge of wire. It makes compressing/expanding turns a lot easier.

Thanks and Good luck!


Frank Cox

Thanks for the reply Al,

To answer you questions:

- I got 3.4V between L10 and L4. 

- I'm not sure I know how to interpret the RF sweep but I think it looks OK. I'll attach a screen shot.  The curve at 28mHZ is down from the peak but the same filter is used on 12m which is closer to the peak.

- I get about 1 ohm across L9 which is what I get for the other 47uH chokes.

I'd be interested to know what you think about my RF sweep

Thanks for the help,

--Frank KK6WUU 

Al Holt

Thanks! I should have mentioned it's best doing the sweep with a 50 Ohm load and not an antenna. I'd say it looks pretty good versus what I get on my QDX-HB. Did you have a dummy load attached for the test?
When I run it on mine 10m is like -4 dB. The best I can get on 10m is about 1.5W with 9V. But, I've deviated a lot from the standard build. L10 has 9 turns, L4 has 9 as well. T1 is a bifilar wind 1:1 at 3 turns. I've been experimenting :-)

I think the bifilar wind for T1 helped get me higher output on 20m which is now about 4W.

Since you are reading 3.4V at mid-LPF it sounds like you are dropping a lot across L4, but with that RF Sweep result of yours I'm not sure how that can be.

I'm more confused than ever. I will probably rewire my L10 & L4 to factory specs, but with 24g wire and see where I'm at.

I wish I could be of more help!


Frank Cox


Yes, I'm using a 50 Ohm load. Your scan might look worse but you're getting 1.5W. I'm only getting 300mW

Don't feel bad about not knowing what's wrong with my build. I'm also totally mystified and about to start replacing parts for lack of a better idea ;-) At least I think I have it narrowed down to the LPF but ...

It will be a while before I have parts in hand so I'll turn to my receive problems in the meantime.

Thanks for the help,

--Frank KK6WUU