Wednesday, November 3, 2021

Video: QDX now operational

My QDX is now repaired and seems to have quite a hot receiver. As I write at 2:30pm local, I'm receiving stations from NA, SA, EU and Africa on 20m FT8.

My problem was a shorted primary-secondary winding in T1. Fortunately, I used a current-limited power supply when I initially attempted to power-up the newly completed kit. Power supply voltage dropped to zero (with 11V dialed in) and there was no indication on the kit's LED.

There was unmistakably a short somewhere.

While building the kit, I was surprised at the ease with which the insulation could be scraped from the 22g wire used for T1. I think I could have removed it with my fingernail so it was the first component I suspected when I had this problem. I believe that, in the process of winding the transformer, the insertion of one wire scraped the insulation from the winding already placed.

I removed T1 from the board, re-wound it with wire I had available and tested resistance readings again, that were faulty before. Success - no shorts between ground and VCC.

So I applied power and got a fast blinking, then a steady, LED. Success.

Connecting to the PC gave the hoped-for "Device Found" and then "QDX Ready" notification by Windows. WSJT configuration went exactly as described in the manual and signals started filling up the screen.

A brief test transmitting on 20m with 11VDC applied gives 3.5 watts out.

I don't want to transmit for a full FT8 cycle with the small wire now making up T1 but am 100% confident that everything now works correctly. I will re-wind T1 with the proper gauge wire once it arrives.


 

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10 comments:

PE4BAS, Bas said...

Nice kit John, I see it's a SSB trx only. CW not possible? A workaround would be MCW which is basically CW made by the soundcard from your computer. Don't know if there is another possebility to work CW with this trx? Anyway, it will be a great trx for portable digital use. 73, Bas

OE1MWW said...

Hi John, congrats you found your fault on the spot.
You saying 11 volts gives 3.5 watts out on 20m.

Did you wind T1 using 3:3 or 3:2 ?

73's de OE1MWW
Wolfgang

VE9KK said...

Good morning John, yes the smoke test is always the moment we hold our breath! Nice, in your case there was no smoke. BUT it's never a good feeling when power is applied and things don't happen as they should. It's good you found the issue fast and were able to rectify it to get the unit up and running, so at least you know it works and what the issue is.
Very nice YouTube video as well.
73,
Mike
VE9KK

John AE5X said...

Thanks Mike. According to Amazon the 22g wire will arrive today so I should be able to make contacts with the rig this evening or tomorrow. I'm very impressed with the receive capability.

73,
John

John AE5X said...

Hello Wolfgang, I wound 3:2 for 12 volts. When I get T1 wound with the proper wire (later today I hope) I will use 12V and make whatever adjustments I might need to the spacing of the turns on the LP filters. Did you get a QDX also?

73,
John

John AE5X said...

GM Bas - it is strictly a digital transceiver. It uses a unique method of modulation and neither SSB or CW are possible. As a result, there is also no unwanted sideband or carrier to be suppressed. I don't believe there is anything like it yet being used to create a digital signal.

Hans explains it in detail beginning at 2:10:
https://www.youtube.com/watch?v=j6Vnh8eiEpg

He also said that 375 more of these kits may be available around mid-December.

73,
John

OE1MWW said...

Hi John, yes, the kit arrived yesterday. I built a QCX+ and a QCXmini, so I got used to those little parts etc. ;-) Found in my junk box several cooler for the Raspberry Pi (14x14x7mm). Broke out two spokes in the middle, drilled a 3.2mm hole, now the M3 nut fits inside on top. Will use 0.2mm Double Side Thermal Conductive Adhesive Tape between cooler and ground plane for the 4 transistors. The decision for the transformer is still pending. Several little stamp-sized boards with a step-down circuit are in my drawer. So I could use my std. 13.6 V power supplies (same for portable use) and set the output to 9V. Did some tests with the step-down in recent days. Stable output up to 2 A, no pumping during peaks and could not hear any QRM at all.
Good luck with the QDX, have fun and DX, 73 Wolfgang

Sverre Holm said...

Very nice, I might be tempted to get one just for the fun of it!


73
Sverre
LA3ZA

John AE5X said...

Or for the uniqueness - there's nothing else quite like it.

73,
John

John AE5X said...

Sounds good, Wolfgang - congrats on getting one of them! Yes, I've been thinking of heat dissipation as well. A 5W RF output, being generated in a small enclosed case, outdoors, in Texas in the summer (eventually) - something additional to the included washer heat-sink seems sensible and a heatsink on top of the transistors would be ineffective due to their curvature. I may just put a little thermal compound between the transistors and the circuit board while performing surgery on T1 later today ;-)

73,
John