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QMX: almost zero power out all bands

 

I recently finished building the QMX kit, 20-10m version, Rev 2. The receiver works great on all bands, and all systems function well except transmit.
I have built 4 other QCX Mini kits successfully and yes, I suspect toroid shorting or lack of continuity in the Tx circuit somewhere but would like some hints as to where to start.
(PS: all Tx testing was done with a 50 ohm dummy load/wattmeter).

These are my symptoms:
  • If ¡°Protection Enabled at SWR 3¡± and transmitter is keyed "S" immediately appears and no output - all bands
  • If "Protection Disabled" quick key down at low voltage (9v on a 12v wiring) and test with nearby receiver shows extremely weak signal on 20-17m and slightly stronger signal 15-10m but QRP wattmeter reads nil

    Given the lack of power out on any band I suspect problem lies before the band filters, and possibly around T501. However, if there is little to no power out why is the SWR circuit even involved?
    Thanks for your attention.
    Larry VE7EA
 

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First place to look from what I read is T507 and the solder connections. The heavy wire and ground plane make soldering difficult. If you didn't scrape the enamel off before soldering you will have difficulty burning it off. Takes lots of heat, but don't wreck the board either.

73,
Cliff, AE5ZA



On Jan 15, 2024, at 18:46, Larry VE7EA <lagagnon@...> wrote:

I recently finished building the QMX kit, 20-10m version, Rev 2. The receiver works great on all bands, and all systems function well except transmit.
I have built 4 other QCX Mini kits successfully and yes, I suspect toroid shorting or lack of continuity in the Tx circuit somewhere but would like some hints as to where to start.
(PS: all Tx testing was done with a 50 ohm dummy load/wattmeter).

These are my symptoms:
  • If ¡°Protection Enabled at SWR 3¡± and transmitter is keyed "S" immediately appears and no output - all bands
  • If "Protection Disabled" quick key down at low voltage (9v on a 12v wiring) and test with nearby receiver shows extremely weak signal on 20-17m and slightly stronger signal 15-10m but QRP wattmeter reads nil

    Given the lack of power out on any band I suspect problem lies before the band filters, and possibly around T501. However, if there is little to no power out why is the SWR circuit even involved?
    Thanks for your attention.
    Larry VE7EA
 

Yeah, what kind of throws me is I was very diligent about scraping off enamel on every toroid beforehand and double checking continuity. Oh well, back the soldering iron I go....
 

Larry,

Post an image of your Diagnostics page while in TX. What is the PA voltage showing? What is the PIN bias current showing?
Also, check C526. It may have been cracked during assembly.

JZ KJ4A


On Mon, Jan 15, 2024 at 8:04?PM Larry VE7EA <lagagnon@...> wrote:
Yeah, what kind of throws me is I was very diligent about scraping off enamel on every toroid beforehand and double checking continuity. Oh well, back the soldering iron I go....

 

Make sure the BNC isn't damaged or shorted.
--
73, Dan - W2DLC
 

Found the problem!

The Source leg to Q507 (an AOD403 MOSFET key shaper circuit) had not been soldered to the board during factory install of the SMD components.
I hope this will not be an issue for other boards out there.?
I enjoyed the troubleshooting process, despite the hassle. I determined the issue must be in the initial Tx stages and started looking there.
Thanks for all the suggestions - it really got me thinking and looking.

73 Larry VE7EA

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