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QDX - Very low RF output after replacing burned finals

[email protected] All Messages By This Member #131956 Well, these spikes did overload my QDX in cw, and the diode on L14 seams interesting and easy modification, and if I ever try cw again it will be with this mode done and possible 1w power. On the other hand, why would one risk the trouble of replacing finals and IC5 AGAIN for doing qrp cw the way it is not designed for? Seams unworthy of troubling about it.. However, as I stated before, QDX seamed to worked just fine in making couple of contacts in Hellschreaber (Feld Hell) after I modified default rasing and falling envelope reaction time and also sample timing too. This was surprising to me as it is a mode based on CW only the signal is varying super fast in metter of mili seconds and this is decoded as fax picture on the screen. I suppose these spikes in Feld Hell are also present but must be much smaller and shorter tharefore not hurting the finals in dangerouse way? But after all, I think I will put this diode at the end at least to feel a little more safer. And not push my luck with cw on QDX anymore, no. ? Srdjan, E78CB
John Z All Messages By This Member #131960 Srdjan, Any time the current through L14 changes value a voltage will develop across L14 in accordance with physics:? V= Ldi/dt. Large and rapid changes in current will produce large voltages. This will take place any time a "key up" condition occurs, requiring the BS170s to turn off completely. For example: - at the end of an FT8 transmission, - at the end of CW dots or dashes. - very likely in other modes as well? This can happen even under rapid load changes, as when an ATU is searching for a match. This last vulnerability is present even in QMX, as its envelope modulator is helpless to protect against rapid load fluctuation. The voltage spikes can be very large. Spikes of over 100V have been observed. The BS170 spec for both avalanche breakdown and for D-G oxide breakdown is 60V.? There is variation in the transistor manufacturing process. Some BS170 will survive this condition, possibly for a long time. Many will not. IMO placing a 1N4148 across L14 as a commutator, as you would across a relay coil,? is simple and low cost insurance against the above.? JZ KJ4A? toggle quoted message Show quoted text On Mon, Oct 21, 2024, 5:16?PM e78cb@... via <e78cb=[email protected]> wrote: Well, these spikes did overload my QDX in cw, and the diode on L14 seams interesting and easy modification, and if I ever try cw again it will be with this mode done and possible 1w power. On the other hand, why would one risk the trouble of replacing finals and IC5 AGAIN for doing qrp cw the way it is not designed for? Seams unworthy of troubling about it.. However, as I stated before, QDX seamed to worked just fine in making couple of contacts in Hellschreaber (Feld Hell) after I modified default rasing and falling envelope reaction time and also sample timing too. This was surprising to me as it is a mode based on CW only the signal is varying super fast in metter of mili seconds and this is decoded as fax picture on the screen. I suppose these spikes in Feld Hell are also present but must be much smaller and shorter tharefore not hurting the finals in dangerouse way? But after all, I think I will put this diode at the end at least to feel a little more safer. And not push my luck with cw on QDX anymore, no. ? Srdjan, E78CB
[email protected] All Messages By This Member #132031 Thanks JZ, the comments in this group are really nice way to learn things from first hand.. and I've learn a lot, that is a nice side of the hobby. ? 73 Srdjan, E78CB ?

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