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QMX thump with CW keying

 

I did some digging in the archives and I found some information on the is problem, but I didn't find which cap is the culprit. There was a reference to replace on of the surface mount capacitors with either an electrolytic or tantalum capacitor with a value of 33 uF but I need to know where it goes if I am going to replace it.? I am using a battery pack of rechargeable AA lithium batteries.? They last for quite some time but there is some voltage sag on transmit, and if I understand correctly this will ameliorate the problem.

May I please have information as to which cap (part number) to replace, and the size of the surface mount part?? Any details would be much appreciated.

The QMX+ kicks a$$ but the thump is rather fatiguing.

72

Charles - NK8O
 

The undervalued cap is C430, 2.2 uF. Some have reported success replacing it with 22 uF or 33 uF. The existing 2.2 uF cap can’t filter out battery droops during transmit keying.

Tony

On Sat, Jan 18, 2025 at 8:16?PM Charles W. Powell via <doctorcwp=[email protected]> wrote:
I did some digging in the archives and I found some information on the
is problem, but I didn't find which cap is the culprit. There was a
reference to replace on of the surface mount capacitors with either an
electrolytic or tantalum capacitor with a value of 33 uF but I need to
know where it goes if I am going to replace it.? I am using a battery
pack of rechargeable AA lithium batteries.? They last for quite some
time but there is some voltage sag on transmit, and if I understand
correctly this will ameliorate the problem.

May I please have information as to which cap (part number) to replace,
and the size of the surface mount part?? Any details would be much
appreciated.

The QMX+ kicks a$$ but the thump is rather fatiguing.

72

Charles - NK8O
 

See attached for the way I corrected it.
?
 

Thanks for posting that Stan. Very helpful. Has anyone implemented this work around on a revision 1 board, where C430 lies underneath one of the SMPS boards and there’s nowhere obvious to site the additional cap?

Is the only solution then to source a 33uF surface mount cap of appropriate size and fit that under the board? That would be a very fiddly task indeed given it lies in a string over other, very closely spaced, components.
?
73,
John VK7JB?
 

Me not yet. But I have today ordered some 33uf smd caps. I intend to glue one to the top of the board as close as possible, then run a couple of fine wires to pick up the connections. Plan to do this on my QMX+ and two QMX mini.
 

This is how I did it on my QMX - there is just enough space for the cap on top of the audio jack.
There is space on the bottom side of the board for a SMD cap, just need to find the right nodes to solder it to, using something like 30-gauge blue wire-wrap wire.
 

Thanks Stan - just what I needed to know. ?It can be done! ? I can see that one lead is attached to the top of C430, but can’t quite see which spot you’ve selected for the other leg of the 33uF cap. ?What convenient ?connection did you use for the grounded-leg??
?
?
 

Stan - I’ve just reviewed the revision 1 schematic and I can see that the junction between C411 and C412 is indeed a ground point, so that’s where you place the other lead. ?Now off to the junk box to find a small 33uF cap. ?I’m very hopeful this will be the fix. ?Thanks again.

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