On Sep 19, 2023, at 14:48, Cliff <ae5zaham@...> wrote:

John,

Thanks for the explanation. I fully understand. No controversy here.

73,

Cliff, AE5ZA

On Sep 19, 2023, at 14:40, John Z <jdzbrozek@...> wrote:

Cliff,

Sorry to hear about the busted FET!

The modulator in QMX serves a number of special functions that QDX does not require.

CW wave shaping, to eliminate annoying? transmitted key clicks, is one example.

Someday it will be used to create. synthetic SSB.

Hans also uses it to gracefully wind down the transmit power at the end of each transmission. That should avoid a damaging Ldi/dt spike from the PA's power feed inductor.

QDX does not benefit from that spike protection, as it has no modulator circuit. Hence, you will see discussions about the use of commutating diodes, zener diodes, or other means to protect the QDX finals.

These discussions always seem to drag along some controversy. There should be none. The Ldi/dt spike is real and it puts the PA transistors into a dangerous out-of-spec condition.

JZ KJ4A?

On Tue, Sep 19, 2023, 2:14 PM Cliff <ae5zaham@...> wrote:

John,

Looking at the schematic for the QDX there is no modulator like the QMX and the L14 which is similar to the L507 in function is always at the input voltage, 11V in this case. At least in the QDX it's normal to have the input voltage on that coil all the time, but I guess not in the QMX.

73,

Cliff, AE5ZA

On Sep 19, 2023, at 11:34, Cliff <ae5zaham@...> wrote:

Thanks John,

Well, the gate had 6.5V on it. The Source and Drain had 11V. Wasn't sure whether that was 6.5 was high or low. Low I was expecting to be 0V. I lifted the Gate pin and now the pad for the gate is still 6 V, but the Drain is 5.4 and the Source is still 11V. It appears as if Q507 was being turned on. Interesting with the gate disconnected from the pad the idle current drain still hasn't dropped. Seems as if that wasn't causing an increase in receive current.

What do you think?

73,

Cliff, AE5ZA

On Sep 19, 2023, at 00:07, John Z <jdzbrozek@...> wrote:

Cliff,

You have confirmed that there is a problem at the PMOSFET modulator.

When all the BS170 are good, they are all off in receive mode and you can expect a normal receive current level even if the modulator is stuck "on". When one or more have failed, a stuck modulator will allow elevated receive current.

Your reading of 11 volts during receive is proof that the modulator is turned on, wrongly.

The next step is to measure the gate voltage at Q507 in receive. If it is low, Q507 is being driven on by the four 'difference amplifier' transistors that compare the DAC signal from the processor to the output of the modulator and produce the gate drive signal for Q507. Determining which have failed would be the next challenge.

?If the gate voltage us high, and Q507 is conducting, Q507 is shorted.

...

I see a preceding post that goes on a remarkable run-on over your use of sockets. There is a grain of truth in there, amongst several misconceptions, in that socketing does somewhat impede heat flow out the leads of the BS170. That was neither your initial problem nor your biggest problem now.?

In the future I would advise a thin layer of thermal compound under the transistors, and between them and the washer. too. Do not over tighten the nut and screw as thermally expanding transistors will be squeezed badly.

The molded case of modern TO-92 transistors contains a thermally conductive filler to allow the case to participate in heat rejection. The via holes that pepper the plated pad that the transistors sit upon are not helping that at all. They impart surface irregularities ( and hence air gaps) that get in the way of good heat rejection. A little thermal compound helps overcome that.

Good luck, JZ KJ4A?

On Mon, Sep 18, 2023, 11:14 PM Cliff <ae5zaham@...> wrote:

Good thoughts JZ.

When I see the idle current go up I open things up and find one of the BS170s is causing it. Pull the bad one and it goes back to normal receive (idle) current so I doubt that the modulator is on, but will verify it anyway.

In receive has 11V on it. Does that mean that Q507 is bad? Sorry I'm not familiar with how Mosfets work? Could the very bad current draw when I had left off the antenna caused that? That looks like a pain to remove without a hot air rework system which I don't have.

73,

Cliff, AE5ZA

On Sep 18, 2023, at 21:05, John Z <jdzbrozek@...> wrote:

Cliff,

I think you are correct when you suggest that something more is going on with the rig.?

I suspect that the PMOSFET modulator may be stuck "on". It would not afford protection against destructive Ldi/dt spikes from L502 if it were stuck.

You mention idle current going up when BS170s fail. I presume that you mean current drawn while in receive. That would be another indication of a fault in the modulator section. The modulator should take PA current to zero in receive.

If you see voltage on L502 during receive, that would confirm a stuck modulator.

You might be able to go further in diagnosis using just a multimeter, but at that point it may be better to send your QMX to Jeffrey W. Moore for repair.

JZ KJ4A?

On Mon, Sep 18, 2023, 8:46 PM Cliff <ae5zaham@...> wrote:

Hi Paul,

Thanks for the response.

I've had it fail while in different modes. Olivia usually has the longest key down times since it's quite slow. This last time I was in Feld Hell which is one of the easiest modes there are on the rig. It's not a constant transmission, but many with very short pauses mixed in. The rig had been sitting for several hours so it wasn't even hot.

No scope here, sad to say. Testing the Q503 with digital VM and verified 2.5 volts on all final gates.

You raise an interesting point about spikes. Maybe the first time it was damaged was bad enough that it damaged something so there are spikes that are damaging the finals. What that would be I don't know, but maybe one of the experts on this list may have an idea.

Power and SWR are the same on a dummy load. Could try simulating a QSO with the dummy load, but it might take a long time to get enough time on the rig to act up.

The Power/SWR Meter is built into the dummy load and is in between the tuner and the rig so I take it the SWR is what the rig sees. Putting the SWR meter after the tuner shows 1.3:1 SWR. Tuning with the LED indicator in tune mode ends up with the same settings on the tuner and the current draw is the same so I assume power out is the same also.

I agree, something I/we don't understand is going on.

As it is now I'm getting reticent to use the QMX as it's unreliable these days. I have 2 QDX rigs I could use, but prefer the QMX if it can be made reliable by figuring out what is wrong. Of course future firmware upgrades should make it even better.

73,

Cliff, AE5ZA

On Sep 18, 2023, at 18:04, Paul - AI7JR <paul.hanchett@...> wrote:

You mentioned Olivia-- Is that the predominate mode here? Were there other modes where you had a problem? You mention 8-10 minute key-downs, is that typical of your operations?

Are you able to confirm proper waveforms on the gates and drains of the BS170's? No spikes, proper voltages?

Could you simulate a QSO into a dummy load, do the currents look the same? You mention the SWR is 1.0, is the load impedance presented to the QMX actually 50 ohms, or something else? (Could it be that the output impedance of the QMX isn't really 50 ohms --likely, really-- and when we adjust for 1:1 SWR, maybe the antenna isn't being adjusted to be?50?ohms?)

Your experience suggests to me that we don't understand everything about what's going on!

Paul -- AI7JR

On 9/18/23 14:53, Cliff wrote:

I built this QMX within a a month of the release of the kit. It worked great for 2 1/2 months or so. Solid even with long key down modes of 8 to 10 minutes. Power out about 4 watts.  Then I unknowingly left the antenna disconnected and called CQ 3 or 4 times in Olivia mode. I happened to look at the Buck/Boost Converter and noticed it was in current limiting mode so immediately shut off the power. Damage done. <Sigh .....>

I installed sockets for the finals and replaced all 4 BS170s and the driver IC503. Worked fine for an hour or maybe 2, btw SWR is 1.0 with a manual tuner. Then, with the antenna attached, it again blew a final during a QSO and showed current limiting. Replaced the bad BS170 and all seemed fine. IC503 looked good. This scenario  has repeated a number of times now. Replace the bad BS170, check IC503 and it'll run fine for an hour or two of QSOs. I even replaced IC503 and all the finals again just to be sure all was ok. 

I've also seen twice where the idle current increased from 130 to 200 ma. That was an indicator of a bad BS170 also.

I'm being forced to conclude that there is something else going on besides the finals just dying because of operator error. All parts are from Mouser so quality should be good. Power out is still about 4 - 4.2 watts. Current is about 750 ma, sometimes nearly 800ma (the increase puzzles me) and voltage is 11.0 volts. SWR is 1.0.

No physical shorts that I can see. It's also not always the same BS170 position that goes bad. The time before last it was Q505 and just now it was Q504.

I sure hope someone has an idea of what can be going on. I've been running out of the case to be sure it wasn't heat build up.

Any and all suggestions welcome.

Thanks.

73,
Cliff, AE5ZA