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Hi all,

Yesterday, I finally found the time to finish building my second QMX (Rev.2, 10m - 20m). Initial setup was easy, and after loading the latest firmware, all tests ran smoothly.

Today was its first use, which was also successful with a few FT8 QSOs. But after about 3 hours, the QMX said goodbye with a loud pop. C107 looked like the picture below:

I can't identify a cause for this defect. The QMX is built for 9V and was powered by the stabilized 9V output of a TelentCell LF4100. The antenna was a precisely tuned vertical.

Will replacing C107 be enough? I'm worried about Q107. Could the burn that can be seen in the picture have been caused by C107, or is Q107 also defective?

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73, Joerg DG0JDE

It looks like both of the SMS boards have suffered a catastrophic failure.? Not just the failure you mentioned if the first board, but also on the second board.?

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I would remove those and check the main board with a dmm to ensure they're isn't anything else wrong.?

Your C107 is a 6.3 V spec. (I can see by the one on the 3.3 V board - not an issue there.) I had one blow early on and it took out a few other things on the 5 V SMPS. I repaired it, but testing showed continued problems. I bought a new set of SMPSs from Hans and I've had no issues since. Hans went to a 10 V spec C107 later and I haven't heard of more problems like yours. I believe some of the 6.3 V spec ones were more like 5 V and that's why it blew.?

Anyway, there is a good chance that something else is damaged, hopefully only on the PS boards. As recommended by others, check the Vdd and Vss and +12 V resistances after you remove the SMPSs. If those resistances are good, then I would recommend you simply buy a new pair of SMPSs from Hans rather than try to repair the ones that are damaged.?

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73, Dan? NM3A

Joerg, wich rev. is your QMX? First batches were made with 6.3V rated C107 and later batches with 10V rated. I have a rev. 2 HB QMX and C107 burned similar to yours. As a precaution I switched both SMPS boards and the QMX was running again. Later I found the 5V SMPS was ok, but not a burned spot like on yours.

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I would check both SMPS running the off line test (see QMX QMX_Plus Power supply test Process).

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And like Jason wrote, remove all subboards from the mainboard and check resistance from VDD to GND, VCC to GND, +12V to GND and each voltage rail with respect to the others. All values should be at least a few Kiloohm.

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73 Ludwig

Joerg,

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Yes, both your 3.3V and 5V SMPS are failed.? You may want to order another set from qrp-labs (they are inexpensive) if you don't want to replace components.

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And what Jason said is important, since other things may have failed: measure the resistance between of the connections on the 8-pin and 6-pin connectors to the main board, with both SMPS removed (measure each pin to all of the others in the same connector).? All of the resistances should be high, a few k-ohms or more.? If any have low resistance or are shorted, that might indicate a failed component on the main board.

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Also, please use a good DMM to measure the output of your Talentcell LF4100 "9V" output.? I suspect it is 10V or perhaps more.? Note that according to the LF4100 user manual, the "12V" output will be almost 13V (during transmit) and over 13V in receive - so I highly suspect the same type of regulation for the "9V" output.? (It doesn't say anything about the specific voltage on the 9V output).? When vendors make these power supplies, they often use the "9V" and "12V" specs in a very "nominal" way.???

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I hope instead that you just suffered some sort of power-surge that caused the failure.

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But if your Talentcell LF4100 is giving substantially over 9V during transmit, and you operated FT8, that is likely the cause of your problem, and you will likely have also burned out one or more of your BS170 final transistors.? [A 9V QMX build will receive fine at 12V input, with no damage, but long transmit at over 9V will cause over-voltages on the BS170s along with heat, which can 'fry' the BS170, which in turn can pull excessive current from the power supply and cause various other failures.]

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I'm hoping there was a different cause, but until I see a measurement of the 9V talentcell output, that seems like the most likely cause.? Please share what you find; it will be helpful to the rest of us.

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One additional note - while Talentcells are very popular and very well-made power cells, a USB-C PD powerbank gives much better regulation of its 9V and 12V outputs - both are normally within about 0.1V.? So I always use a quality USB-C PD powerbank for my 9V QMX build, along with a 9V PD-trigger cable (I get Anker powerbanks- a small 10,000mAh battery gives several hours of operation).

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Stan KC7XE

Thanks a lot to everyone for your helpful tips.

I've now removed the two SMPS boards and measured on the mainboard. The resistance between PWR_HOLD and GND is only 250 ohms. Between VDD and GND, it's only 450 ohms. I'm afraid that's not a good sign. It looks like it's a total loss.

Regarding the Talentcell LF4100: As I understand it, the 12V output is directly connected to the LiFePO4 cells. That is why the voltage there can be between 9V and 14.6V, depending on the state of charge. However, the 9V output used for the QMX is regulated. I have now measured the voltage: when idle, it is 9.09V. When I connect my first QMX, it shows 9.1V in the diagnostics menu during receiving and 9.0V during transmitting. This rules out the TalentCell as the cause.

I cannot therefore identify any external cause for this disaster. The power supply was stable, the antenna was adjusted, and the QMX worked without any problems until it suddenly failed, both during reception and transmission. It therefore broke down on its own. Very annoying.

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73, Joerg DG0JDE

Joerg,

No, that doesn't sound good, but as long as the main processor is good, it's worth fixing. If you can identify the failures. It looks as if the failure on the 5V PS took out the 3.3 V supply and who knows what else.?

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73, Dan? NM3A

Daniel,

Without the SMPS boards, I measure a resistance of only 250 ohms on the mainboard between PWR_HOLD and GND. PWR_HOLD is connected only to pin 53 of the processor. I therefore conclude that the processor is defective. I cannot replace it with the equipment I have available, and other components are probably also defective.

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73, Joerg DG0JDE

Joerg, you have mail.

73 Ludwig