KN-Q7A no transmit.


G7HEU
 

Hello all,

Some years ago I bought a KN-Q7A kit for 40 meters but didn't get around to even opening the parcel until a few weeks ago. 

I have finished the kit today. The receiver works well with strong signals to the UK from Spain and Germany on a fairly quiet band. However there is no transmit power at all. 

I can set the bias for the IRF530 as per instructions but on trying to adjust the TX BPF for maximum output I find there isn't any at all. I have checked the voltages for the three power transistors in the troubleshooting table and they are similar to what should be there.

I do have an oscilloscope somewhere at the back of a cupboard so while I find it, (and perhaps a variac), I wonder if anybody has any pointers?

Thank you,

Steve.


Larry Lovell
 

**** Remember, that this is a Single Side Band radio and there will be little if any power out until it is modulated.  
Check the resistance between the tab of the IRF530 and GND.  It should be open.  If it is shorted, it will blow a fuse so you would have no voltage to the receiver, so this is probably not the issue.
Check the resistance between each of the toroids that you wound.  It is sometimes difficult to strip the wires properly.  They should measure a short.
Check the current on the receive and the transmit side.  You should see a difference.  About 70 mA on receive and up to 2A on transmit.
With an oscilloscope you can check the signal on the 680 ohm resistor which is the output from the bandpass filter and input to the transmit stage. Transmit and talk into the microphone.  Should see a signal.
  No signal? There may be an issue with one or both of the NE602 circuits.  
Check the output on the Collector of the C3357 (the surface mount transistor).  Transmit and talk into the microphone.  If that works move to the next stage,
Move the scope probe to the D882 or BD139 (I have replaced this with the BD139 which has better power transfer and bandwidth).  There should be a modulated signal when you talk into the microphone.
And lastly, check the output of the IRF530 to see if you see a signal there. (I am now using an IRF510 device).
Any more questions you can contact me directly at larry@....  I sold the radios and still work with them in the US.


On Fri, Jan 29, 2021 at 2:30 PM G7HEU <stephenphawes@...> wrote:
Hello all,

Some years ago I bought a KN-Q7A kit for 40 meters but didn't get around to even opening the parcel until a few weeks ago. 

I have finished the kit today. The receiver works well with strong signals to the UK from Spain and Germany on a fairly quiet band. However there is no transmit power at all. 

I can set the bias for the IRF530 as per instructions but on trying to adjust the TX BPF for maximum output I find there isn't any at all. I have checked the voltages for the three power transistors in the troubleshooting table and they are similar to what should be there.

I do have an oscilloscope somewhere at the back of a cupboard so while I find it, (and perhaps a variac), I wonder if anybody has any pointers?

Thank you,

Steve.



--
Larry Lovell
73's N7RGW
http://QRVTronics.com
Cell: 214-697-1729


Adam Rong
 

Hi Steve,

It's been years. Thank you for bringing it back.

Have you seen 60mA increase while adjusting TX bias? If yes, the final stage works. If you checked all the voltages ok in TX mode, you have all the transistors in the TX chain working fine.

If you have RF probe, you can measure from C3357 stage, then D882, and finally IRF530 at the secondary winding of the FT37-43 transformer to find out where the power is missing. If you still cannot get any output reading from the RF probe, you can monitor the TX signal from a nearby receiver to make sure you at least have something output. 

Some pitfalls:
1. Adjust trimmer too fast to burn the final or the nearby components.
2. FT37-43 tranformer winding incorrect.
3. TX BPF is not properly adjusted.

Thanks,
Adam