Hello All,

I have a problem with my QCX-40 rev. 4 (2017). After meticulous assembly, following the manual step by step and checking and re-checking all parts, I hold in hand a nice board that receives well but transmits nil.

Here are my findings, following the test sequence in the troubleshoting guide (sec. "Transmit signal"). Supply is 12V DC (verified). Tests done with a good DMV and a scope.

1. On power up I get erratic current readings on my power supply: anything from 135 up to 216 mA. Consumption stays more or less constant during one power-up/down cycle. During the last tests tendency was towards about 170 mA. On TXing slight drop (!) in current (from 170 to 150mA).

2. [OK] Reading at power terminal = 11.98.

3. [OK] ClK0 = 1.67 V = ClK1; C2 (left) = 3.36 V, D2 (right) = 4.98.

4. IC3
Pin 3: RX = 5.83 [should be 0], TX = DC (no square) 6.08 [should be DC 2.5 and square].
Pin 4 & 10: TX = 1.89 [should be 1.65].
Pin 9: TX = RX = 0.78 [should be 0].
Pin 8: [OK?] TX = DC 2.58. (That's a square wave, not stated in the troubleshooter though.)
All other values are spot on.

5. Q1-3
Get hot quickly after power up even without TXing.
On TX no "big" signal at drain. In fact, there is no difference between TX and RX: its always the supply voltage (minus negligable loss). I can see no sq wave on the scope.

6. [OK] L4 has continuity (also the other Ls).

7. [OK] Q6 is the right type (MPS751). At collector RX = 11.58, TX = 11.61.

8. [OK] Q4 gate TX = 5.19.

I wonder whether someone has seen this before and/or has a suggestion as to how I can bring the thing to live. Any help much appreciated!

73 de andre dm1aa

Andre,

The finals (Q1 - Q3) and the final DC switch (Q6) think the board is in transmit all the time. There should be 0 (zero) volts on Q1 thru Q3 and the collector of Q6 when receiving. So we have Q6 being switched on when it should be off. Try pulling one end of R42 out of the board and see if the 12 volts on Q1 thru Q3 drops to zero. If it does, we can now trouble-shoot without "magic smoke". If it doesn't, Q6 is installed backward, hosed, or both. If it is installed backward, replace it. The erratic current reading was the insides of the transistor melting.

If we now have no voltage on Q1 thru Q3, check the voltage on the gate of Q4 in receive. It should be 0. Next, set the keyer for ultimatic, set it for code practice, and check that it makes both dots and dashes. If the gate of Q4 stays at 5 volts through all of this, the micro has a bad flash. Either replace the micro or download the firmware from QRPLabs, get the eeprom file from Hans, and reflash the chip.

Jim

Hi Jim,

thank you very much for your clear instructions. I have taken out one leg of R42 and looked at Q1-3 and Q6 again. Result: Still

Q1-3 drain RX = 12V
Q6 collector RX = 12 V

Q6 is not installed backwards. So it must be "hosed" (I had to look that up ;-) ). I was preparing for this and had ordered a batch of replacements which should arrrive this week. But there is this nagging question: Why? Just a bad part or is there something else wrong which will also hose (!) the replacement.

73 de Andre

Q1-3 drain RX = 12V
Q6 collector RX = 12 V

Andre,

Proceed cautiously.
A few constructors have reported failure of the PA stage which has damaged Q6, Q1, Q2, Q3, and U3.
This seems to occur if the CQX goes into TX mode without a dummy load connected. But the cause is not always certain.
I suggest you regard Q6, Q1, Q2, Q3 with suspicion. Remove them all.
You have seen 5.83 volts on pin 3 of U3 which is more than it's supply voltage. This too I would regard with suspicion. The only way it can be high is if voltage is fed through a faulty BS170. At least check the voltages on U3 with the other devices removed.

73 Alan G4ZFQ

Q6 is not installed backwards. So it must be "hosed" (I had to look that up ;-) ). I was preparing for this and had ordered a batch of replacements which should arrrive this week. But there is this nagging question: Why? Just a bad part or is there something else wrong which will also hose (!) the replacement.
73 de

OK, Alan, many thanks for vy plausible sounding advice. So I shall remove Q1-3 along with Q6 and test pin 3 of IC3 with these Qs off the board. What should I do with R42? Leave it off for the test or reinsert it?

73 de andre

So I shall remove Q1-3 along with Q6 and test pin 3 of IC3 with these Qs off the board.

Andre,

Yes.

What should I do with R42? Leave it off for the test or reinsert it?

It does not matter. It may be best to leave it out just in case Jim has any more ideas. It is a pity we all sleep at different times:-)

73 Alan G4ZFQ

Hi Alan and Jim,

I have taken out Q1-3 and Q6 (R42 still loose).? Power up: Draws 175 mA in RX (as before).
Measurements at IC3 in RX: All pins ok, except

pin 3 = 4.89 V (should be 0), pin 9 = 0,32 (should be 0).

So the voltages at these pins have reduced a tiny bit but, alas, not enough yet. So where to go from here?

73 de andre

开云体育

Hi, Andre

OK, we still have 12 volts on Q1 through Q3. Q6 is definitely shorted. So we first "defuse the bomb" by removing Q6. Then check, in receive, the voltage on the gate of Q4. This should be 0 (zero) in receive. If it is not, you have a microcontroller problem - probably bad firmware (my QCX had this exact problem when first constructed, which I corrected by reflashing the microcontroller. Luckily I yanked the power before anything was damaged).

If you are not an experienced electronic repair technician, I would recommend Q6 removal by cutting the wires off the device close to the device body, then using a solder pencil and tweezers to remove the wires one by one. You can then re-open the holes in the board with a thin toothpick or a steel sewing needle while heating the solder with the solder pencil. Solder wick doesn't always work to re-open the holes, and you may end up damaging the board with the wick (ask me how I know).

If you find that the voltage on the gate of Q4 is high on receive, DO NOT INSTALL A NEW Q6 until the microcontroller problem is corrected, either by flashing the chip or installing a new one. Also, just to be on the safe side, check the resistance from Q4 drain to ground. With R42 disconnected it should be infinite. If it isn't infinite, replace Q4. If it is infinite, reinstall R42.

Jim W4JED

Hi Jim,

tnx, once again. I removed Q6, and also Q1-3. Voltage at Q4 gate is 0, as it should.
Next I checked resistance from G4 drain to GND (R42 disconnected): 140 ohms. So I replaced Q4.
(I also checked Q5 to GND: 140 ohms. Is that OK?)
So the microcontroller looks ok. Can I now put back Q6, Q1-3, and R42 without first calling the fire brigade?

As Alan said, pity that we don't sleep at the same time.

73 de andre

Forgot to say: After replacing Q4 resistance from drain to GND is infinite.

Measurements at IC3 in RX: All pins ok, except
pin 3 = 4.89 V (should be 0), pin 9 = 0,32 (should be 0).

Andre

That makes IC3 faulty.
Looks like Jim and I are both right.
It's the first time I remember Q4 causing the PA breakdown.

73 Alan G4ZFQ

Hi Alan,

so you are suggesting that I should replace IC3, put in the new Qs and reconnect R42, right? Funny, when I soldered the IC3 directly on the board, it didn't feel right: I thought that there really ought to be a socket in between -- just in case ;-)

Anything that I could try before embarking on this major piece of surgery? (I don't mind the time spent but I am afraid of ripping off pads or traces.)

@Jim was suggesting that the microcontroller (IC2) is off. That would be much easier to replace, as it is mounted on a socket.

73 de andre

so you are suggesting that I should replace IC3, put in the new Qs and reconnect R42, right?

Andre,

It is a pity Jim has not commented about this.
But you have said that in RX mode IC3 pin 3 is high, 5V. This is wrong.
You say all other voltages on IC3 are correct. (Pin 9 is 0.32V, near zero, may be normal.)
That shows the IC is faulty.

I am unsure of the sequence of events. Jim has shown Q4 was faulty. (Not the micro as he had experienced.) But it seems to me that merely switching Q6 on would have no effect if the BS170 gates were at zero volts.
But in the fault situation you noticed current variations. When testing you saw 5.83V on IC3 pin 3. I think this must have come from the 12V on the drain of a BS170 that had shorted drain to gate.
It would be good if someone else commented.

Anything that I could try before embarking on this major piece of surgery? (I don't mind the time spent but I am afraid of ripping off pads or traces.)

Remove it in the way Jim suggested. Some recommend a Dremel-type tool to cut each pin before removal. A small, sharp sidecutter may be an alternative.

73 Alan G4ZFQ

--

Hi Alan,

I understand Jim as saying: IF Q4@gate > 0, THEN suspect IC2; ELSE suspect IC3. So we are now in the ELSE-clause, as you also suggested. I have the sidecutter on the bench so as to remove IC3 while waiting for the replacement chip to arrive. This time I'll use a precision socket as a go-between -- unless someone dissuades me otherwise.

@Clint: Thanks for this vy good thought! I was tempted to use a Dremel, at least on the middle pins. Now I'll definitely try to do it just with the sidecutter.

73 de andre

开云体育

Hi, Andre

I would expect Q5 source to be just about 0 (zero) ohms to ground, and the drain should be just about infinite. 140 ohms would send whatever is coming out of the transmitter into the receiver. Not a good thing. So Q5 appears to be bad. The purpose of Q5 is? to connect the receiver to the antenna when receiving, and to disconnect the receiver front end on transmit.

Install the transistors, leave R42 dangling, connect a voltmeter to one of the output transistor drains, connect a dummy load, and apply power. If the drain voltage remains at 0, reconnect R42 and try again. Still no volts? You've got a radio. 12 volts? You're still transmitting and receiving at the same time. The dummy load is to keep the voltage on the drains from spiking and blowing the new transistors.

73

Jim

Hi Jim,

those 140 ohms at Q5 looked fishy to me too. Now I know why -- thank you. The present state of the board is this:
a. Q4 has been replaced.
b. Q1-3 have been taken out.
c. IC3 has been taken out (was a surprisingly easy job with a sidecutters and a combination of desoldering wick and pump).
To do:
- Replace Q5.
B. Put in new Q1-3.
C. Put in new IC3 (this time on a socket).
I? should get the new NAND-gate by the end of the week. With it installed I'll test again.

73 de andre

Hi Alan and Jim,

SUCCESS (well, more or less, see below). I have replaced Q1-3, Q4-5, Q6, and IC3. Now RX draws 125 mA (display installed) and shows a whopping 3,4 W TX-output with 12V input. The drains of Q1-3 show on TX the desired big signal on the scope. So many thanks, once again, for your super help. I have a working TX.

Sadly, however, now the RX side is fouling up. During calibration maximum I-Q balance was only obtained with C1 completely open. So I took off 4 windings from the third secondary of T1. Now C1 maximises in a reasonable position. But I have a lot of noise and next to no signal. The noise reacts strongly to tapping on the board. Moreover, when I tune to a frequency (RX only!), then my nearby other receiver responds with a strong continous tone (around 650 Hz) on that very frequency. This is only a preliminary report. I'll have to look at this more closely and will start by measuring the IC3 pins again.

73 de andre

I have a working TX.

Great! That's a good start.

Now C1 maximises in a reasonable position. But I have a lot of noise and next to no signal. The noise reacts strongly to tapping on the board.

Andre,

The calibration signal is applied to the antenna input from IC3 by way of R43. If it calibrates you should receive something. Maybe a bad joint on T1?

when I tune to a frequency (RX only!), then my nearby other receiver responds with a strong continous tone (around 650 Hz) on that very frequency.

This is to be expected, it is a direct conversion receiver, the local oscillator will appear to be strong on a local receiver.

73 Alan G4ZFQ

Happily I can annouce: complete success! I had overlooked a foam pad that was clinging to the soldering side of the board and completely messed up the RX. After taking it away I had a whow-experience!? The RX seems to pick up all signals that I can hear with my big rig and has a very nice filtering, very quiet. Just nice to listen to.

I now have a fully functional QCX before me. The building experience was, well, tough, but thanks to you, Alan and Jim, very educational and eventually succesful. After that I love the little thing even more ;-)

So, thank you so much and alle the best to you!
Vy 73 de andre dm1aa