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 dummy load and a mini QRP Power Meter like this one to measure power:
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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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