Topics

0.03w RF output, 1w at transistor drains, Help? #qcx


 

Hello, I built my QCX Plus kit the other day, and have been using it without any luck, so I decided to check if there was any RF output. It appeared that, there was not. I measured .13w at the output of the LPF, but 5.2w at all transistor gates, After re-soldering all the toroids and ensuring continuity, i probed again, with no change. Eventually, while probing, there was a sharp 'snap' sound and the transistor watt measurements dropped to 1w, and .03 at the output of the lpf. I do not know where to go from here, help?


Ben
 

That 5.2W is the max reading if i remember. Are C25 and C26 still OK? Did you use a dummy load?


ajparent1/KB1GMX
 

There is not 5.2W at the transistor gates ever!  Maybe 5.2V.

IF the power out is really low and the power at the drains blown devices
are often the first call.   However before going that far and a new build
soldering is usually the first and mostly likely suspect.

Hif you heard a snap look at the devices (bs170s finals) and
Q6 for visual clues.

Allison
-------------------------------
Please reply on list so we can share.
No private email, it goes to a bit bucket due address harvesting


 

I was not able to get a dummy load, however I used an antenna that i've tuned to be very close to 50ohm

The 5.2w was at the Drains, instead of the gates I measured no power at the gates. all the soldering looks fine, and all the finals have no visual markings on them at all

After removing c25 and c26 from the board to test them, it appeard that c26 has a small crack in it, however it appears to measure correctly for continuity and resistance, i am unsure about its true capacitance as i do not have a capacitance meter
I do not have another 391 capacitor, what can i do now?


Ben
 

For measuring capacitors and inductors I have recently bought the Peak LCR45. Really like it. With this meter I found that I had fried my C25 and C26. You can buy a replacement part to be sure, or buy a meter to verify I guess?

Regarding the 5.2W. I found the source: https://www.qrp-labs.com/qcx/rfpower.html

Quote:
The scaling of the resistors R56 and R58, which form a potential divider, is such that the maximum reading from this RF power meter is 5.2W. If you see 5.2W on the screen, it means "anything from 5.2W and up". You maxed out the processor's Analog to Digital Converter input. 


scot forshaw
 

I’d order another Cap and check Q6 output at L4 also. Q6 can be problematic.

Hans sells the caps or get a complete LPF kit from him they are cheap enough.

2E0WWV
Scot

On 9 Aug 2020, at 20:24, Ben <ben@...> wrote:


 

I'm planning on using this with the 50wpa, and that has its own LPF of the same band, would it be okay if i bypass the LPF  in order to continue troubleshooting? As for q6/l4, one one end of l4 its 1w, the other end is 0.12w


Ronald Taylor
 

You can bypass the lowpass filter if you terminate it with a dummy load and make sure C29 is still in the circuit to block the DC coming from the transistor drains. I don’t think it’s such a great idea to run it to the amplifier without the LPF as it is full of high level harmonics and the amplifier will be amplifying all of those as well as the fundamental. The amplifier’s own LPF May not be able to adequately attenuate the harmonics. 

L4 is a choke (also part of the Class E resonator circuit) so it passes DC from Q6 to the final transistor drains. So it will have measurable RF on the final transistor end and only DC is expected on the Q6 end of L4. 

Ron

On Sun, Aug 9, 2020 at 15:41 spam via groups.io <spam=craftxbox.com@groups.io> wrote:
I'm planning on using this with the 50wpa, and that has its own LPF of the same band, would it be okay if i bypass the LPF  in order to continue troubleshooting? As for q6/l4, one one end of l4 its 1w, the other end is 0.12w


scot forshaw
 

I would ask Hans to confirm, 

he may have qualitative results. RF 101 says the 50W PA would amplify everything coming from the QCX+ of course, and without a pre LPF that will include all the unwanted harmonics. Relying on the final LPF to get rid of these at 50w may not be a good idea IMHO. The fidelity of that LPF would need to be very high. 

As the PA is being used, I don't see any air miles in omitting the internal LPF as the gain would be negligible. Also the impedance matching of the PA to AMP may not be as expected.

The Class E amplifier is intrinsically linked to the highly balanced LPF.

My suggestion is to 1) Ask Hans, 2) don't omit it for the fear of unwanted issues in the Class E final amplifier stage. These class E amps are very susceptible all kinds of issues if the load/LPF is not correct.

Scot
2E0WWV



 

I'm unsure at this point, the LPF appears to be working fine even with the broken cap (if it is broken), passing RF through properly, but the LPF itself just isnt being fed with any power


 

Where can I continue to probe through? I do not know where to continue from here.


Shirley Dulcey KE1L
 

A low pass filter with a broken, absent, or improperly connected capacitor will still pass RF just fine. What it won't do is properly act as a low pass filter. A broken capacitor will make the high frequency rolloff less steep and raise the cutoff frequency a bit; not good if you want to keep your transmissions legal. (It will also change the response curve of the filter, likely to an undesirable shape, because one of the inductors has now effectively become much larger, as two inductors are now directly in series.) A low pass filter has series inductors to impede high frequencies and shunt capacitors to drain high frequencies off to ground.

A broken, absent, or improperly connected inductor, on the other hand, will stop a low pass filter in its tracks. There will no longer be a path for RF through the filter, aside from whatever capacitive coupling may happen in the neighborhood of the broken inductor.

The answers would be opposite for a high pass filter.

On Mon, Aug 10, 2020 at 10:14 PM spam via groups.io <spam=craftxbox.com@groups.io> wrote:
I'm unsure at this point, the LPF appears to be working fine even with the broken cap (if it is broken), passing RF through properly, but the LPF itself just isnt being fed with any power


 

There is continuity through all inductors in the path, and from the rf-output and the input of the first inductor, so i do not believe that is the probelm


 

Update: The output at the drains has returned  to >=5.2w, and i have found that at C30, that wattage is going in, but no wattage is being detected at the other end. Is this correct? I thought that capacitors blocked DC, instead of AC/RF


Shirley Dulcey KE1L
 

C30 is part of the circuit that makes the Class E PA resonant near the frequency of operation. It's connected between the RF signal and ground, so like the capacitors in the low pass filter (C25 through C28) it will carry high frequencies to ground while blocking low frequencies. You should not see RF on the end of C30 that is connected to ground; even if high frequency RF energy has been carried there, the other side of your probe is also connected to ground so there will be no difference in voltage (or nearly none because resistance in the ground plane can cause slight differences). You SHOULD see the signal on the side of C30 that is not connected to ground, as it is connected directly to the PA drains; if you don't, there may be a broken trace on the circuit board. Check for DC continuity between those points with the QCX turned off; if you don't have it, you'll need to run a jumper between those points on the board. The drains of Q1 through Q3, the high side of C30, the low side of C33, and the low side of L4 should all be dead shorts to one another.

The important question is whether wattage is appearing on the other side of C29, which serves to block DC from passing through to the low pass filter and from there to the antenna. That's a series capacitor so it acts as a high pass filter; the cutoff is far below the output frequency of the QCX circuit so the signal passes through unimpeded. If C29 is an open circuit or not soldered properly, no signal will appear on the other side of it.

From there you can also check the signals at the various points in the low pass filter. If it stops dead after any of the inductors (L1 through L3) it means that there is a continuity problem with that inductor; the most common problem is failure to remove the insulation from the enameled wire. If one of the capacitors in the low pass filter has become a dead short (or has a soldering short across it) that could also cause the signal to disappear because it would all go directly to ground at that point.

0.03W is close enough to zero to count as equivalent here. That amount of power could get to the output through stray coupling somewhere in the circuit rather than the signal getting there through the intended path.

On Tue, Aug 18, 2020 at 2:04 AM spam via groups.io <spam=craftxbox.com@groups.io> wrote:
Update: The output at the drains has returned  to >=5.2w, and i have found that at C30, that wattage is going in, but no wattage is being detected at the other end. Is this correct? I thought that capacitors blocked DC, instead of AC/RF


Alan G4ZFQ
 

The output at the drains has returned  to >=5.2w,
If you are using the built-in power meter function this reading does not measure watts . There should be a DC voltage on the drains, the power meter will respond to that. The reading is meaningless.

No reading at the other side of C30 could mean several things. For example:-
1] No RF being generated. Maybe faulty Q1-3.
2] C30 faulty.
3] RF shorted to ground after C30. Check with ohm meter.
4] Bad solder joint.

73 Alan G4ZFQ

and i have
found that at C30, that wattage is going in, but no wattage is being detected at the other end. Is this correct? I thought that capacitors blocked DC, instead of AC/RF


Alan G4ZFQ
 

Sorry NOT C30, I mean C29.

No reading at the other side of C29 could mean several things. For example:-
1] No RF being generated. Maybe faulty Q1-3.
2] C29 faulty.
3] RF shorted to ground after C29. Check with ohm meter.
4] Bad solder joint.
73 Alan G4ZFQ


Wes AE6ZM
 

And remember - trying to measure watts at the FET finals is meaningless as the waveform there is square. Meaningful RF readings can be found only downstream of a properly functioning LC circuit (LPF). 
--
> I finally got it all together...now I can't remember where I put it<

VY 73,
Wes

AE6ZM

Sierra Vista, AZ

 


 

I'm measuring '5.2w' on the input of c29, and only 0.14w on the other side, I'm assuming this is not correct here


 

There was an extra 104 capacitor included with my kit, Should i replace c29 with that?