开云体育

Dear Folks:

?

I have finished building the 50 watt amplifier kit for the QCX.? I am trying to run the test on page 66 of the manual which simply requires you to do the following:

?

1)? Make sure that the R5 potentiometer is fully counterclockwise.?

?

2)? Put the QCX in "Practice" mode so no RF is generated, but the radio will generate a voltage to key the amplifier.

?

3)? After keying the amp via the QCX, measure the current draw between the amp and its power supply.

?

4)? Once you have keyed the amplifier via the QCX and measured the amp's current draw, R5 is gradually adjusted clockwise until the current draw begins to rise.? Once you see the current draw rise, you turn R5 back to the level just below the point at which the current draw rises.? That is the correct setting for R5.

?

I have done all of these procedures.? I should mention that the green "on" light comes on when power is supplied to the amp and that the red TX light comes on when I key the QCX.? The current draw measurement is 74 MA.? Unfortunately, the current draw never changes even when I turn R5 fully clockwise.

?

I should mention that I did try the same measurement with the QCX practice mode turned off, i.e., the QCX would now be generating RF.? At that point I found that turning R5 gradually changed the current draw from about 97 MA to about 103 MA.? However, no RF is generated (though the QCX generates about 4 watts when it runs barefoot).? This makes me think that R5 is functional.

?

Does anyone have any suggestions for locating the problem?

?

Thanks and 73 de Bernie, KF0QS

Thanks, Adam.

?

I just saw your reply.? I will measure that voltage and let you know tomorrow.

?

Thanks so much for your help.

?

73, Bernie

Adam:

?

I am assuming that the output of 78L05 is the side that connects to R5?? In any event, I measured the voltage at that point and there was no voltage whatsoever.? I even measured the voltage at the terminal of R5 to which that lead of 78L05 goes, and there was no voltage.? I did measure the voltage on the other side of 78L05 and the measurement was 13.3 volts (appropriate since I was running the amp off of a supply voltage of 13.4).? I did make sure that I didn't confuse the terminals of the IC by making sure which one of the three connections was the ground.?

?

I resoldered the pads for all three terminals of 78L05 just to make sure there wasn't a cold solder joint but afterwards there was no change in the reading.

?

Am I correct in assuming that my 78L05 in the amp is defective?? Is there anything else I should check?

?

Again, thanks for your help.

?

73 de Bernie, KF0QS

Adam:

?

I had already checked continuity with the ground lead of the 78L05 when I checked its other two terminals.? There was continuity.

?

Fortunately, I happen to have two spare 78L05 chips available (in the same TO-92 configuration as the amp requires), so I am going to head down to the shack right now and replace it.? I will make sure that R5 is fully counterclockwise and will let you know what happens.

?

Thanks again for the help.

?

73, de Bernie, KF0QS

Adam:

?

I replaced the 78L05 chip with a new one and now have about 5 volts coming out of the output of the chip (actually 4.9+ but who's counting?).? Unfortunately, now that I have that fixed, I still have the same problem, ie, the current draw really doesn't change as I adjust R5.?

?

Admittedly, it may have changed about a tenth of a MA while I was adjusting R5 but that was about it.? Since the assembly instructions say that the rise will be a little bit, I thought that this might be all I needed to do.? So I left R5 at that point, switched the QCX out of practice mode and didn't see any RF generated on the wattmeter I inserted between the dummy load and the amp.? To make sure there wasn't some other problem, I took the amplifier completely out of the circuit, and the QCX alone generated just under 4 watts on the wattmeter.? So something is happening to that four watts but it's not getting to the wattmeter and dummy load when I transmit through the amp.

?

I thought there might be a problem with R5 itself.? So I made sure that its ground connection had continuity to the chassis ground and it did.? Then, with no power applied to the amp, I adjusted R5 while measuring the resistance at various settings (between its input and output, and there was significant change in the resistance from about 2.2 Kohm at the counterclockwise setting and almost no resistance at the fully clockwise setting.

?

Do you have any suggestions as to what I should try next?

?

Thanks and 73 de Bernie, KF0QS

Adjust it as in the instructions?

On Thu, Jan 16, 2025 at 10:08 PM, Bernie Poskus, KF0QS wrote:

I replaced the 78L05 chip with a new one and now have about 5 volts coming out of the output of the chip (actually 4.9+ but who's counting?).? Unfortunately, now that I have that fixed, I still have the same problem, ie, the current draw really doesn't change as I adjust R5.?

?

?

I've not built one of these but looking at the circuit when you adjust R5 the current should increase suddenly and massively.? Setting it fully clockwise will probably destroy the PA FETs.? If you are only seeing a small current increase then something is stopping this.

?

I would start by checking that there is continuity between the wiper of R5 and the gates of Q1 and Q2.? If there isn't then you won't get any bias voltage to them.? This would probably be a problem with soldering the secondary of T1 the trifilar transformenr.

?

Next check for continuity between +V and the drains of Q1 and Q2.? If this isn't low resistance the power supply current won't change.? If it isn't then look at T2 and T3 for wires that aren't making contact.

?

And check that the Sources of Q1 and Q2 are connected to ground.

?

If this seems OK check voltages around the PA FETs with power on.? First check V+, this should be your power supply voltage.? Then check that this voltage finds it's way through both windings of T2 and T3 to the drains of the PA FETs Q1 and Q2.

?

It might be useful checking the voltages on the gates of Q1 and Q2 when transmitting in practice mode, this should rise as you turn R5 clockwise, by half way it should be 2.5V.? Q1 and Q2 should be conducting by the time the gate voltage gets to 4V.

?

If you have +12V on the drains of Q1 and Q2 and volts on the gates of them then there may be a problem with Q1 and Q2.

?

One thing to consider is that the current will increase suddenly and massively when you adjust R5, the instructions really say to avoid this by adjusting very carefully so you only see a small increase, going too far might destroy Q1 and Q2.? A power supply limited to 500 mA could save them.

?

Hope this helps,

?

Chris, G5CTH

?

?

Chris and Adam:

?

I have tried every test suggested by Chris.? There is continuity between the wiper of R5 and the gates of Q1 and Q2.? There is continuity between V+ and the drains of Q1 and Q2.? The sources of both Q1 and Q2 are connected to ground.? The voltages at V+ and the drains of Q1 and Q2 are the same as the power supply voltage (should I actually check the leads from T2 and T3 to see if the same voltages are there, or can I assume that they are present since I see the same voltages at V+, Q1 and Q2?).? When transmitting in practice mode, the voltages on the gates of Q1 and Q2 vary when R5 is rotated (indeed, at counterclockwise, there is no voltage until R5 is rotated at least a little bit) and are around 2.5 volts at about midway through turning R5 (I didn't take it to the max rotation for fear of having the current jump, even though the current hasn't changed much at all).

?

I would almost rather have a problem with soldering the leads on one of the three toroids then have a problem with Q1 and Q2.? But that doesn't seem to be the case.? Wouldn't it be unusual to have a problem with both Q1 and Q2?? I would think one of them would develop a fault rather than both.

?

What do you guys think?? Should I replace both?? Or check something else?

?

Thanks and 73 de Bernie, KF0QS