Re: QDX power drop

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.


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