开云体育

OK - I have now replaced IC3 (14-pin NAND gate) and that has brought back the receiver to full normal volume, but I still have no TX output.?

I had to use a Dremel with a disc cutter to cut through the legs and then remove each pin one by one. A new socket has been soldered in and a new 7SN74ACT00N is in place.

On key down I can detect a constant carrier on my HF receiver at the frequency of interest, but it is solid (ie not keyed with the paddle) if that helps.?

Voltages are;
IC3
Pin 1 - key up 5V, key down 2.5V
Pin 2 - key up 5V, key down 2.5V
Pin 11 - key up 0V, key down 5V
Pin 12 - key up 5V, key down 0V

so I think that is working correctly.

The voltages on the centre pin of Q1, Q2 and Q3 are 0V on key up and 3.65V on key down.

I had previously replaced Q5 and Q6 (and Q1, Q2 and Q3) after the fault appeared, but I wonder if the faulty IC3 had in fact wrecked the replacements too. What would readers suggest I check/replace now? I have spare BS170s and MPS751s, but am loath to replace Q5 and Q6 yet again as the board can't take much more desoldering.

Steve G0KYA
?

On key down I can detect a constant carrier on my HF receiver at the frequency of interest, but it is solid (ie not keyed with the paddle) if that helps.
Voltages are;
IC3
Pin 1 - key up 5V, key down 2.5V
Pin 2 - key up 5V, key down 2.5V
Pin 11 - key up 0V, key down 5V
Pin 12 - key up 5V, key down 0V
so I think that is working correctly.
The voltages on the centre pin of Q1, Q2 and Q3 are 0V on key up and 3.65V on key down.

Steve,

I think that looks good so far.
Is there 12V on the drains of Q1-3?
It seems the keying is not getting through from the processor via Q4/Q6, check voltages on these devices.
Probably Q6 is first to suspect - again.. but get the voltages first.

73 Alan G4ZFQ

I had previously replaced Q5 and Q6 (and Q1, Q2 and Q3) after the fault appeared, but I wonder if the faulty IC3 had in fact wrecked the replacements too. What would readers suggest I check/replace now? I have spare BS170s and MPS751s, but am loath to replace Q5 and Q6 yet again as the board can't take much more desoldering.
Steve G0KYA

Just checked. Yes, there is 12V at the drains of Q1, Q2 and Q3 on key down.

I checked with the built-in power meter - 0W at the BNC output, but 2.28W at the drain of Q3. Not sure whether that means there is RF, but a fault in the LPF now. I have continuity from the BNC inner to one side of C29 which suggests the toroids are OK.?

Over to you Alan!

Steve G0KYA

I checked with the built-in power meter - 0W at the BNC output, but 2.28W at the drain of Q3. Not sure whether that means there is RF, but a fault in the LPF now.

Steve,

You may see I've asked about this.
Your problem might be C29 or LPF but I'm wondering if the internal power meter might give an erroneous result on the PA drains. Many have reported 5W or more indicated (not necessarily accurately) on the drains.

73 Alan G4ZFQ

On 06/01/2018 07:43, Alan G4ZFQ wrote:

I checked with the built-in power meter - 0W at the BNC output, but 2.28W at the drain of Q3. Not sure whether that means there is RF, but a fault in the LPF now.

Steve,

I should add that your other readings do indicate that there should be RF on the drains.
My meter question may just be a technical point, not really relevant.
So check connections, C29, and for a short across the output socket.

73 Alan G4ZFQ

You may see I've asked about this.
Your problem might be C29 or LPF but I'm wondering if the internal power meter might give an erroneous result on the PA drains. Many have reported 5W or more indicated (not necessarily accurately) on the drains.

but a fault in the LPF now. I have continuity from the BNC inner to one side of C29 which suggests the toroids are OK.

gm Steve, and you say your receiver is fine but be aware that the pin sizes on 50/75 ohm BNC connectors are different. 50 ohm is larger.

Geoff G4CAO

FIXED IT!? After way too many hours today I finally fixed the problem. It turned out that Q1 had failed. I only found this after replacing Q4, Q5 and Q6 again. After that failed to bring back the TX I suddenly realised that Q1 was getting hot even on receive. I has previously replaced Q1, Q2 and Q3 before I replaced IC3, so either the? replacement was faulty or the failed IC3 cooked it.

Either way, it is now working again and I got a quick 599 from an Italian in a contest this afternoon just as the band was closing.

Anyway, thanks all of you for your help.? So it looks like using the radio on WSPR cooked the 5V regulator. This in turn took out IC3. This may have then caused Q1 to die, not sure about that though. I do think that an IC socket on IC3 would have helped replacement a lot.

Now Jim G3YLA and I have to give a talk about the QCX to Norfolk Amateur Radio Club on Wednesday. So glad it is working!

Steve G0KYA

I suddenly realised that Q1 was getting hot even on receive.
Either way, it is now working again and I got a quick 599 from an Italian in a contest this afternoon just as the band was closing.

Steve,
That's great!
But are you really sure Q1 got hot on receive?
Not just cooling down from an attempt to TX?
It should not be possible for it to get hot on receive as the PA should be off. Maybe it was faulty and shorting all the current to ground.

73 Alan G4ZFQ

Steve,

Good show. ?

Take a look in my photos folder for a picture of my home-made heat sink for the 5 volt regulator chip. ?Standard heatsinks would not fit that particular area. ?They would either block the DC power input block or interfere with the contrast control. ?I used a small piece of aluminum and shaped it to fit around those obstacles.
Take a look for some ideas.

Bill N7EU?

Nope. Q1 was definitely getting hot on receive. Perhaps it was shorted.

Anyway, fingers crossed as it is working now - just wish 20m was a bit better today!

Steve G0KYA

Nope. Q1 was definitely getting hot on receive. Perhaps it was shorted.

If any of the PA BS170s gets hot in RX mode then Q6 must be turned on or shorted.
But if I understand correctly your final action was merely to replace Q1, nothing else?

Have fun

73 Alan G4ZFQ

Steve,

If you get any other problems I have an unbuilt kit for 20 meters you can borrow for spares. I’m just south of Norwich. Hans and I are old pals so anything I can do to help just ask.

Regards,

Steve G0XAR?

Thanks Steve - fingers crossed it will be OK now. Norfolk Amateur Radio Club are likely to use the QCX as a club project, and Jim G3YLA and I (he has a 20m on? too) are giving a talk at CNS school on Wednesday 10th about QRP and the QCX.

Alan - I'm wondering if there was a short on the PCB, which I fixed when I replaced Q1. That was definitely the change that fixed it as it was the last thing I did. I had also gone around the board with an 8X lupe and removed some excess dried flux between the tracks near Q1 - might have been a tiny solder splash there. I have built about six or seven QRP rigs and this has been the most difficult for me to debug so far, but Jim has had no problems whatsoever. That's life I guess!

I'm glad I beat it. I was running out of ideas - and patience!

Steve G0KYA

I'm wondering if there was a short on the PCB, which I fixed when I replaced Q1.

Steve,

Yes that could be it. A faulty Q1 cannot get hot on RX without there being another fault.

I'm glad I beat it. I was running out of ideas - and patience!

Yes, glad you remained sane:-) Three failures at the same time is not what you wanted.

73 Alan G4ZFQ