QDX power drop

Brad McDowell

I finally built my Hamvention acquired QDX for 9v. (12v initially, changed to 9v).  It ran great putting out 4-5 w on all bands.  40-50 successful QSO's.  I then decided to try it on 60m and got good signals, output about 3 watts.  However I forgot my EFHW is not resonant on 60 m, SWR around 8.  Subsequently my output power has dropped to 2 watts or less on all bands (still getting some QSO's).
The BS107's all test ok, the T1 and IC5 seems a little hot (T1=46C, IC5=35C) I understand the QDX doesn't like high SWR's and cascading failure of BS107's and IC5 can occur.  My trouble shooting skills are limited.  Where should I begin?


Al Holt

Are you testing the BS170's in or out of circuit? They're all in parallel (I believe) and just 1 might have opened leading to lower power out. 
FETs aren't the easiest to test with an Ohmmeter. A low-cost component tester is a great inventment!

Good luck!


Evan Hand

Hi Brad,

I would suggest that you tell us what test equipment you have.  I have found a digital oscilloscope is probably the most important.  For component testing, I use a transistor tester. 

I start troubleshooting power issues with the QDX at the low pass filters.  In your case, it worked well before, so I suspect the LPFs are OK.

The next area is to verify the gate drive signals on the BS170s.  If there is not a 2.5volt DC signal when transmitting, I pull the BS170s and test again.  If I now have a good 2.5volts on all of the gate pads, then most likely, there are one or two BS170s that have failed.  Test the BS170s.  The critical parameter is the turn-on voltage.  The gate capacitance is next.  All 4 should be close to the same values.  They should test less than 3 volts to turn on.  Replace the ones that deviate more than 10% from the others.  I have enough BS170s in my parts bin that I replace all 4 at once and put the ones that test good into a separate storage location for use on non-critical power circuits.  The 4 come from the same batch.

If you have an oscilloscope, you would do the same tests looking for a close-to square wave on the gates with the BS170s in the circuit and a nice 5volt square wave with them out.  A dual-channel scope can also verify that the gate signals are 180degrees out of phase.

I hope this helps, and let us know what you find.


Brad McDowell

I tested them in circuit but did not trust the results.  You motivated me to remove the BS107's and test externally.  Sure enough, the last one I tested was bad.  Replaced it and am back up to full 4-5 watt output.  Gotta love Quickchip.  Also, I guess I confirmed again the QDX does not like high SWR's.