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QCX-mini with no RF power #qcxmini #20m

corbin.haugen@...
 

I need help with a broken QCX-mini.  I built a QCX-mini 20m back in December and everything was going great until I lost all RF power output.

The radio functions normally, it receives well, tunes well, sidetone works (in both practice mode and transmit mode). Radio was putting out 6W at 12V before all this went down.

Since this problem has come up I have replaced:
  • Q6
  • Q1, Q2, Q3
  • IC3
Most recently I found that on IC3, pin 6 and pins 1,2 were no longer connected.  I jumped that connection and now I actually get some signal on pin 3 of IC3.  That said, RF power is usually 0 (or somewhere close as I am using a QRP Guys Pwr/swr meter, not an RF analyzer) but some times I see a little spike on the output and I see something like 300mW pop up briefly.  I have been comparing the signal at each point with what I get on my QCX+ (10m)

I am thinking that next I should replace C25-C30 or something or maybe Q5 or C33?

I would love any feedback on my troubleshooting or thoughts on what it all could be.  I have a 100MHz o-scope and access to an RF analyzer and Freq Gen if need be.

Thank you!
Corbin
Ronald Taylor
 

Hi Corbin. I think you should do some probing with the scope and your DVM before replacing any more parts. 

Some suggestions:

Connect a dummy load.

Look at the Gates of Q1,2&3 with the scope. You should see a 5 volt "sort of" square wave there. Probably more like a very distorted sine wave. Be sure to do this with the probe and scope input both set to X10. 

Next look at the drains of Q1,2&3, where you will see 10s of volts of something similar if the transistors are amplifying when you key the rig. Only key it for a short time..

If you see a strong signal there, you should do a DC continuity check with your DVM from the center pin of the BNC connector to the C29 end of L3 just to make sure the solder joints are all still good. While you are at it, check from the center pin to ground and you should see the 50 ohms of your dummy load. If it is shorted (0 ohms) you should find and fix that short before proceeding.

If there is little or no signal on the drains of those transistors, then you need to use the DVM and check for 12 volts on the emitter of Q6 (all the time) and 12 volts on the collector when the transmitter is keyed. If that looks good then check for 12 volts on the drains of Q1,2&3 when keyed. If it isn't there, then check the continuity through L4. 

Please do those checks and report back ... 73 ... Ron

On Thu, Mar 23, 2023 at 8:08 AM <corbin.haugen@...> wrote:
I need help with a broken QCX-mini.  I built a QCX-mini 20m back in December and everything was going great until I lost all RF power output.

The radio functions normally, it receives well, tunes well, sidetone works (in both practice mode and transmit mode). Radio was putting out 6W at 12V before all this went down.

Since this problem has come up I have replaced:
  • Q6
  • Q1, Q2, Q3
  • IC3
Most recently I found that on IC3, pin 6 and pins 1,2 were no longer connected.  I jumped that connection and now I actually get some signal on pin 3 of IC3.  That said, RF power is usually 0 (or somewhere close as I am using a QRP Guys Pwr/swr meter, not an RF analyzer) but some times I see a little spike on the output and I see something like 300mW pop up briefly.  I have been comparing the signal at each point with what I get on my QCX+ (10m)

I am thinking that next I should replace C25-C30 or something or maybe Q5 or C33?

I would love any feedback on my troubleshooting or thoughts on what it all could be.  I have a 100MHz o-scope and access to an RF analyzer and Freq Gen if need be.

Thank you!
Corbin

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