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Replacing an IRF520 with a more durable Mosfet

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DaveF715

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I have a 10meter Ham rig that is running 8 IRF520's in the amplifier section. The 520's don't take abuse to well especially if running at sustained intervals for CW operation. I was wondering if something like the IRF540 fet could be used as a direct replacement and if it would be a bit more tolerable for long sustained use?
 
The amp is built with 2 sections, each with 4 in parallel and then each goes through the multiplier section or combiner if I read the circuit correctly. This particular radio has yet to release the schematic to the unit. I sent the unit out to the manufacturer the first time and they fixed it but used the same inferior 520's that they use at the factory. This time I bought better quality 520's and installed them myself. The first set failed inside of six months and the warranty chips lasted less than that so I'll wait and see how these new ones hold up. If the data sheet is accurate the new 520's should be at least a bit more durable but I don't have real high expectations. I'm trying to plan ahead for future repairs. Heat is probably the biggest issue and I'm also looking at designing a better cooling system. I should add I'm a newbie when it comes to using mosfets. I've ran strictly vacuum tube equipment up until I bought this new radio so I'm learning RF circuitry all over again. Thanks for your input.
 
Sometimes the capacitors they use in these circuits will change value and will have no indication by just looking at them. I have changed many of these capacitors. The worst ones are the coupling caps between the finals and the output network, generally 500pf to 1000pf. Sometimes the ones on top of the cores are the culprit. Remove them and check them, If they are bad, I replace them with a good quality silver mica. Matching Mosfets are a good idea but not as critical as with bipolar transistors.
 
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