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I suspect that my factory built QMX has a blown BS170.

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I was running FT8 on 15m into a resonant antenna tuned with an external SWR analyzer. The rig measured the SWR at 1.26:1. I don't believe that the SWR changed at all during the operation. I checked it again when I noticed the power drop, both on the rig and with the external meter. It was still about 1.26:1. I think I can rule out high SWR.

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Here is a list of output powers that I measured when new with the battery, as recorded at the factory, and after the failure:

20m has 3.7 watts on Talentcell battery (Factory 3.8 watts) (After the failure 2.2 watts)
17m has 4.5 watts on Talentcell battery (Factory 4.5 watts) (After the failure 2.2 watts)
15m has 4.7 watts on Talentcell battery (Factory 4.6 watts) (After the failure 1.9 watts)
12m has 3.5 watts on Talentcell battery (Factory 3.4 watts) (After the failure 1.9 watts)
10m has 3.8 watts on Talentcell battery (Factory 3.6 watts) (After the failure 2.0 watts)

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I use a Talentcell YB1203000 (12v, 3Ah) battery which my DVM is measuring 12.3v fully charged. Could it be that the extra 0.3v is too much for the BS170's especially on higher frequencies? If so, would a "DROK DC-DC Adjustable Buck Voltage Converter Stabilizer 6-32V to 1.25-32V LCD Display Constant Volt Amp 6V 12V 24V Step Down Voltage Regulator Power Module" prevent future failures if I reduce the voltage to 11v?

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I've read about the addition of a couple of zener diodes (1N4756A) to the BS170s of QDXs. Would this also provide protection on the QMX?

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I haven't inspected the rig yet but will follow these instructions found here to repair it:

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I know that FT8 is 100% duty cycle and that can be hard on rigs but I'm wondering if I should just use my QMX for CW. I'd rather be operating than changing blown FETs.

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Thanks.

Paul VA3ZC

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I just found a Mini 360 Buck Converter in my junk box. Can I use that in place of the "DROK DC-DC Adjustable Buck Voltage Converter" to lower the voltage to say 11v.?

It is this one:

Thanks.

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Paul VA3ZC

Paul,

That Drok buck converter requires the input voltage to be 1.5V above the output voltage, so it would work, but would only give about 10.5V to your QMX.? The mini360 would also work, but probably requires an even higher voltage difference, so again no more than 10.5V.? This is true of all simple buck converter designs.? If you changed the QMX to 9V, then either of these would work with a 12V battery to give a full 9V.

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If you want to get 11V with your 12V battery, instead you can use a drok (or other) buck/boost converter - you can get inexpensive small ones.? It will actually run in 'boost' mode to keep the voltage at 11V with 12V in, and you can turn it to whatever voltage you like.

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- I would be surprised if the extra 0.3V was the entire cause of your problem.? Perhaps combined with the 1.24 SWR, long FT8 sessions, maybe a weaker-than-normal BS170, and reasonably high temperatures it would fail in time.? Or maybe your antenna SWR is not completely constant - many antenna setups vary in SWR when the wind blows hard, and possibly to momentary high SWRs due to mechanical flexing of electrical contacts.

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BUT - running your QMX at a reduced voltage will certainly reduce the possibility of failure regardless of the complete reason.? Hans has not endorsed the diodes across the finals, as they may add capacitance and may not be compatible with the SSB update (but they would probably work fine for digi mode).

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I have run FT8 quite a lot on my QMX devices at full voltage, into antennas with <1.5 SWR and never had a failure yet.

Stan KC7XE

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Thanks for the information, Stan.

I'll certainly keep the volts down on FT8 after the repair.

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Is it correct to assume that the 12.3v battery without a buck converter is OK on CW?

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Thanks again,

Paul

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Yes, into a good SWR.?

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Now I have to give a longer answer since someone else is going to say you are adding incredible risk of damage with anything over 12.00V.? But Hans officially states (in multiple places, red ink, in his documents): 12V max and 5W max for the QMX/QMX+, which he believes is safe for all modes.? (Except WSPR beacons, where you may also want to reduce your voltage.)? Note that his spec does not say 12.0V or 12.00V; he specifically did not want to be that precise.? He also says do not go over 13V, and sets that as the default protection threshold, and to not use 12V sources that are not really 12V - all of which I interpret as staying within a few percent of 12V. ? And he has also said (in individual message responses here) that for CW (and other low duty cycle modes) into a good SWR that he wasn't so much concerned about a few tenths over 12V.

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I personally have a power source that I charge to 12.4V.? I use it on SSB and CW until it is down to 12.0V (less than 1/2 hour of use), then I use it for FT8 or whatever mode for the next several hours of its capacity.? And that has never given me a problem.

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Stan KC7XE

FWIW a plain old LM2940 CT 12V/1A low drop voltage regulator will certainly also work? if you're concerned. Only one additional component (output cap) required. With say 2V*.8A = 1.6 W max. to dissipate a 1" square tab of 1/16 sheet aluminum should do the job. At 1-2$ per unit they are price competitive. However,? you will lose .1-.5V when your source voltage drops below 12.5V, such as when using a 3S LiIon battery. Your QSO partner certainly won't notice this :-)

I'm thinking I may rewind T501 to make it a 9V rig. Then use the Drok converter and set the voltage to 8.5V from my 12V battery.

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Since the Drok needs 1.5V, the most I could supply the 12V version of the rig from a 12V battery would be about 10.5V, which should yield about 3 watts (Han's chart on Pg.4 of the assembly manual) whereas the 9V rig supplied with 8.5 V should yield about 4.5 watts. That seems like a win and the BS170s should be less stressed.

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Paul VA3ZC?

I wired my first QMX for 9V, and have been very happy that I did.

I also use a PD power bank and 9V trigger cable for it, and it has worked very well.? The powerbank measures right at 9.0V, but at transmit there is about a .2V drop in the cable, so I am always transmitting at about 8.8V, which is a fine safety margin, even for FT8.? Note that because of the steeper voltage curve for a 9V build, you don't need to reduce the voltage as much for the same margin.

And the power bank I chose is an Anker 10000mAh unit that is smaller than the QMX - a perfect match, which gives me several hours of operation.

Stan KC7XE

Anker banks are good; and it's worth noting for any future folks reading this thread that you can?get 12V PD USB-C cables, but they work with fewer banks and adapters. Notably, no recent Anker AC plug or powerbank supports 12V output.?

Also, many larger powerbanks/"solar generators" now have 12V DC output plugs. Always test them! I've measured many that will happily drift all the way up to 13.7v. I use a Drok buck/boost converter on the 12v output of a Anker Solix bank and have gotten consistent 12V into my QMX+ that way.

73!