As far as i can tell, q1-3 are outputting , I dont have an oscilloscope to see what they're outputting, but that they ARE outputting something.

This has been sold. Tnx fer looking.
73 de Arnie W8DU

On Wed, Aug 19, 2020 at 10:46 AM, Ben Bangerter, K0IKR wrote:

I set the bias for 113 mA wit

Why?   The amp is class C so the bias should be what ever the board
draws and no more.

The adjustment procedure has the bias at zero and no signal in (CW practice mode)
and the bias is turn up until the board current inreases then backed off to the point
before that.

the waveforms are seriously distorted.  Maybe input transformer miswired or 
soldered.  you could have an  inter-turn short.

Something is wrong and its likely in the input side.

Allison

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Bigger wire is less resistive, both in DC terms and also effective RF resistance.

Related to this topic, is it correct to assume that using a larger diameter wire will not change anything as long as the same number of turns are used? I need to rewind one on a U3S for the 630m band and have slightly heavier wire on hand.

John

There does appear to be a path to ground through the LPF, however, It's not a dead short,

If you have a dummy load connected, as you should, then you will see 50 ohms.
The 104 capacitor check makes Q1-3 failure likely.

73 Alan G4ZFQ

There should be no DC path to ground in the low pass filter, as the RF caps should be many megohms to DC.

I constructed my 50 W amplifier For 20 M at the end of June, and the assembly process went well.  I set the bias for 113 mA with A 20.1 V power supply, no RF power input, and both the input and output ports terminated in 50 ohms.  I tested the amplifier briefly driven with my original QCX, which puts out 2.0 W with a 12 V supply.  My Bird power meter with a 50 W slug Indicated ~15 W to a dummy load.  I then set the amplifier aside while I awaited my QCX+.  That was assembled and aligned, and it delivers ~2.5 W (32 V p-p on the scope) to a 50 ohm dummy load.  When the amplifier, supplied with 20.1 V, is driven by the QCX+, it only delivers ~12.5 W (71 V p-p) To a dummy load.  This is not highly dependent on the amplifier bias setting (3.1 V in this case for the standing current threshold), nor on the power from the QCX+, which can be adjusted by adjusting its supply voltage over a 10 - 14 V range.  I looked at rf waveforms at various places in the amplifier circuit.  Here are waveforms at the gates of the IRF510s, Q1 in yellow and Q2 in violet:

 The drive signals differ in phase by ~180 degrees as expected for a push-pull circuit, but the Q1 waveform in particular appears highly distorted.  The waveforms do not change much at higher bias settings.  This is the waveform at the drain of Q1; that for Q2 is nearly identical.

The frequency of this waveform is ~42 MHz, indicating high third harmonic content.  The amplitude between the cursors is 103 V p-p.  The next waveform is at the secondary of the output transformer at the input to the low-pass filter:

The shape of the waveform is the same as that at the Q1and Q2 drains, the p-p voltage here is 246 V.  The output of the amplifier, after the low-pass filter, is a nice sinusoid at 14 MHz, as expected, but just 92 V p-p, and which dropped to 71 V p-p over time as testing continued.  At no point during my testing did the heat sink become the slightest bit warm to the touch.  The final photo shows the voltage at the input to the low-pass filter with markers to indicate the time interval between the indicated peaks, here 71.6 nS, corresponding to 14 MHz.



i checked the amplifier over carefully, and all the components appeared to be the right values in the right places.  I thought the output transformer might have been reversed.  I removed it and unwound it, and found it was installed correctly.  Then I rebuilt it with new wire and reinstalled it.  While it was out, I examined the low-pass filter with my spectrum analyzer/tracking generator, and saw it had a 3 dB cut-off frequency of ~ 16 MHz, and a loss at 14 MHz of 0.25 dB.  I thought the problem might lie in the T/R switching circuitry, as an incorrectly installed or bad diode could clip the rf waveform and create harmonics.  Checking in both T and R modes, the four 1N4007 diodes all had the correct forward bias currents and correct reverse bias voltages, depending on the on/off condition.  These were examined with amp bias set to zero and no rf input or output.  The rf voltage doubler circuit produced 186 volts when the amplifier was delivering its (measley) 14 watts or so to the dummy load.

At this point, I have run out of ideas, other than replacing the two power MOSFETs.  I have been reluctant to do that except in desperation, being concerned about damaging the pcb.  If anyone has any suggestions for resolving this problem, I will be overjoyed to hear them,

73, Ben K0IKR

There does appear to be a path to ground through the LPF, however, It's not a dead short, and appears to be through a capacitor, which is to be expected?
C29 appears to be working fine as far as i can tell.

I dont know if there is RF , but I'm reading 11.97v (external meter) at the non-lpf side of c29, then 7v on the other side. Interestingly, there always seems to be ~4.7v at the rf output, unless its switched off? Replicated the same behavior of c29 with the other 104 cap, "5.2 watts" in, and practically nothing out

Hi, QCZ+ 700 hz tone in BPF peak is very weak, also IO screen is jumping? Checked toroid connections and resoldered.

First limiting factor is caps with 50V rating.  That can tolerate 20W but only if
the SWR is under 2:1.  Bump up to 100V parts and you in the comfortable
25-30W level. More than that 200V or higher.  As you go higher in power
the voltage is only part of the issue as then currents (though caps) increase.

The cores and wires are less a limit and they can get to 30W or more though
I'd stay under 50W with T37 sizes and 70W with T50 size.  For 100W use
T68 size for reasonable margins.  Wire,  #28 is generally enough but 26 and
24 as power goes up.  You need larger cores to fit larger wire.

Experience is that to 10W life is easy and simple, getting to 50W justifies
the effort to go 100W.  Why?  every thing has to be bigger, heavier, and 
higher voltages and currents.

Allison
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Mates:
I have the following for sale. 99 percent assembled but never powered up or aligned. Just don't have time for it. SO here is what I have:
U3s - Rev 3 F/W 3.12
GPS QLG1
Relay LPF board
LPF for 40, 30 20 meters (40 meter board not assembled)

Price is $50 plus actual shipping (USA sales only, no shipping outside USA)
Please send me an EMAIL to mycall at arrl d ot ne t if you want it.
Tnx de Arnie W8DU

Jim, good information. Thanks...

You should read again John...copied and pasted from the assembly instructions.

Cheers

Vernon

Hi John:

No that is wrong.

Highest band goes in position 1. Read the manual carefully.

Regards

Roger

G3YTN

From what I read in my build instructions, position 0 must have the highest band module.  In my case 10M (28Mhz).

I agree with Curt that the built-in bar graph works very well, but the auto-scaling can be a bit distracting.  as an alternative, I've found that simply listening with a decent pair of headphones allows me to null the adjustments pretty well.
-- 
73,
Brent DeWitt, AB1LF
Milford, MA

Hi Ken
Thanks for your suggestion..... I monitored the 5 v line and I don't think it is dipping.
I think what I am going to do is order another relay board kit..... I am starting to think I have some faulty relays.
I do not see any other answer.... I could be wrong, but I figure if I build another relay board kit, there is nothing
else that could cause these problems. As I look at the wiring schematic, the five relays are attached to the five pins off the
MCU (ic-1)...It seems  straight forward....You can see my board... All the jumpers seem to be right. You can not see the diodes
in the picture, but I checked and they are on the board correctly.... The only other thing is the MCU, but I don't think that is the case. In fact, because
I wired those stranded wires directly to the transmit board and they tend to break when flexed much at all,, I am going to rebuild that kit and I will have a new MCU.... It transmits on 40 meters just fine and that is
the band I am most interested in, so I am fine for now.... But, I do want to try an figure out what went wrong here......
I want to thank everyone for your suggestions and time....I appreciate it very much.... Maybe I can return the favor in the future...
73's to all
Doug WA3ZHG