Re: V6.1 Blown output transistors and pcb trace #v6 #ubitxv6-help

Vakarcs Szilard

Here is a very useful single ended IRFP 250 PA project, delivered by YC3LSB:

On 2021. 12. 28. 9:42, Vakarcs Szilard wrote:

I suggest not to use in push-pull any switching mode mosfet for RF purposes. Those transistors are simply not designed to work in AB mode. Mosfets are getting hot in a few milliseconds and there is no possibility to cool down such a small piece. Better is to use a single mosfet with higher voltage, such 24 V and 3 A. Special designed RF Mosfet transistors are working nice at 13.8 V, but they are way more expensive than those cheap fake IRF510.

Try to use IRFZ type mosfets, but all those shall be selected to be near identical in all properties.

On 2021. 11. 16. 13:16, Stan Mason wrote:
Thanks for that info, Raj.  Very helpful!  You’ve given me a great place to start and PA bias needs to be set regardless after replacing the IRF510s.

The primary reason I bought the uBitX was to have an open source platform with which to learn and experiment, so I’m going to read the ARRL Handbook sections on MOSFET characteristics and RF amplifier design before starting with the board.  

A previous commenter, Evan AC9TU, mentioned a file of transistor measurements from a V4 board you posted and speculated that the RF readings should be the same on the V6 board.  I’ll check that out also.

Stan K3PW

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