I've been following this thread and others regarding what takes out the Finals. I've built 2 QDXs and one QMX. Both the QDX rigs blew the finals within a short time of building. I don't know for sure why in either case. After that I put some serious heat sinking in the QDXs and haven't lost them in the 9+ months since. The other day I really, unintentionally for sure, tested the theory of an open antenna blowing the finals with high SWR. I was in a hurry and hit TX and saw no power out. Switched the meter to SWR and saw a SWR of 10. By that time it had been in TX at least 10 seconds. Quickly killed the TX and connected the antenna. Tried again and all was good. I've had several QSOs in Olivia chat mode with key down times of up to 10 minutes since. All is good. No damage evident yet.?
Why the finals endured my attempt at their destruction I can't say, but I'm sure heat sinking helped.
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On Aug 22, 2023, at 11:45, Chris KB1NLW via <
chrisrey1@...> wrote:
The flat surface of the BS170 contacts the surface of the PC board.? Being a six layer board this should dissipate heat better than the QDX.? (I do put a little thermal paste on mine to help conduct.)? Hans has explained that the washer on top of the BS170 is to press the BS170s down to make good contact with the board and not primarily to dissipate heat.
I have lost one BS170 on my QMX most likely due to high SWR possibly during? auto antenna tuning or even transmitting without any load - though I don't recall? doing this.? Even with better heat sinking it may not be possible to protect the BS170s from the power reflected into the radio from high SWR.? I believe Hans may build in firmware protection based on the SWR hardware sensing in the QMX.
