My 40m version seems to be having fading output power, literally going down with time on my 40m version.? I thought the finals were overstressed with my issues learning to use the GPS, as this dropped power from 3 watts to 1.8 watts on my 40m version.? Replacing the three MPS170 transistors gave the same 1.8 watts.? Now power is fading down to 1.2 watts, and the current seems to be dropping as well.? Is there anything in the circuit that could behave this way?? I checked all the connections in the transmit section.?

Curt

开云体育

Dropping battery voltage.

thanks for replies.? running this on a 30 amp power supply, so no voltage droop.? power output is steady, but lower than it used to be.? only Q6 is warm.? tweaking the turns on the toroids is having no effect.? I questioned my WM2 until running my HW8 which is outputting more power than the QCX right now.? Yes same output power result with 2 different sets of BS170 finals.? I don't see anyone else reporting this behavior.

Curt

Have you checked Vrf or power at the output side of C29, and then each stage of the LPF? I know of one case where there was 5W at input to LPF with each stage showing progressively less until only 1.8 at ant jack. Still no solution to that one.
--
Wes

AE6ZM

Hereford, AZ

?

I have seen this sort of thing (not on one of the QRP Labs kits, though) and found either a bad capacitor (heating and changing capacitance) or a bad inductor (heating and changing inductance). One was a coupling capacitor and another was in the LPF. The inductors that I have seen change were in the LPF.

If you are seeing a larger power output when cool or after a cooling period, then the part heating (bad part or resistive solder joint) will be my guess. Maybe trying some "cooling spray" or air directed at the area to see if the power comes up again. Use a microscope, if possible, and look for cracked capacitors.

Measuring the power before the LPF will produce unreliable results due to the square(ish) nature of the signal before the filter (I have likened class E to a "kick and coast" type of amplifier - one of the reasons that they have high efficiency and low heatsink requirements). You can get a relative idea that there is power there, but the comparison from stage to stage will only be relative and not a power referenced to the 50 ohm output. Possibly using an oscilloscope to trace the signal might show where distortion (it is going to be pretty ugly before the LPF anyway) is introduced. The QCX uses a class E final. Look up some class E waveforms or the expected waveform - it is far from a sine wave until after the LPF. shows the theory - practical circuits have more going on and will be messier.

---------

Paul KE7HR

Like anybody else I can only guess about the reason of your power fade, but my advice is to replace Q6. All the PA current is going through there and even the stronger MPS751 seems to develop problems when you operate the unit with 15+ V and especially when you are doirng that in beacon mode or if you operate into a high SWR, even when the mismatach is not too severe. The TO92 transistor sometimes dies a slow death: It must not directly melt to a lump, smell? and open between Emitter and Collector, often it will only increase the resistance between Emitter and Collector slowly. My MPS751 did just that until the plastic housing cracked horizontally and there was no current flow any more. One defective BS170 kills the other two, because one after the other "PA" transistors takes over the full load and then comes Q6. Fortunately the transistors are all relatively cheap, but my other advice is to order at least 10 BS170s and 10 MPS751s to keep your QCX in business without having to wait for mail delivery. In Germanyy they cost only 20 Euro-Cent (25 US C) for the BS170 and 65 Eurocents (74 US c) for the MPS751. I'd like to use transistor sockets for easier replacement, but they seen to be no longer on the market for TO92 devices.

Good luck

Klaus
DL2QB / WN2Z

Appreciation to all who responded !?

Klaus - indeed - replacing that PNP restored the output power to 3 watts!? so when all that plastic smoke was coming from the BS170 -- the bipolar PNP was delivering the current and took more damage.? so now I know that running high current into a bipolar can change its properties - presumably adding more resistance.? I hope to not to have too many electronics lessons with damaged parts - albeit inexpensive ones.?

Not likely I had a high current PNP in the stash, except now I remember having some high dissipation ones - no worry - I suspect this one will keep me QRV.? If I ever am tempted to run WSPR with the QCX - I will create a lower voltage power supply!? I think I will let the U3S do WSPR.?

I hope everyone still struggling with their QCX finds their issues - meanwhile enjoy the journey with the electronics reaching the ions.? HNY.?

Curt