Rf amplifier not amplifing but rf transistor is ok

Thread Starter

Maz2858

Joined Jan 27, 2021
25
Hi, i build a while ago a 15w rf amp with rd15hvf1 rf mosfet which worked flawlessly but this morning i wanted to adjust the coils by compressing the coils with my finger for max power but suddendly the power went from high to nearly nothing in a second. I tested the mosfet with a transistor tester and it shows ok and i even tried to put a rd06hvf1 in my amp and it still dont amplify, with rd15 it normally outputs 15w and current is about 1A but now it outputs 0.3w max and current is 0.5A
The tester shows with rd15 "Vt 3.52v | C 76pF | Rds 1.1 ohm" + 682mV drop from S to D
I checked also the gate voltage and supply voltage which are ok
Any ideas on whats happening ? Maybe the coils are in such way that no power is made ?? I would be disapointed if i need to buy another mosfet because they are very expensive
 

bertus

Joined Apr 5, 2008
21,524
Hello,

How is the transistor cooled?
When the cooling is not enough, you will experience a thermal run away.

Bertus
 

Thread Starter

Maz2858

Joined Jan 27, 2021
25
it's attached to a big heatsink which is bigger than the rf amplifier pcb itself and it never heated over 45°C
 

Thread Starter

Maz2858

Joined Jan 27, 2021
25
i think the transistor is semi-dead because when it worked i recorded with my smartphone and i see that the current was 1.5A :( so maybe the transistor died but is still detected by my tester
i dont understand why it was using so much current
do you think it died?
 

Thread Starter

Maz2858

Joined Jan 27, 2021
25
hey, when i assembled the circuit in the first place, i burned a ceramic capacitor but it still worked so i leaved it in there but now with rd06 in the circuit when i put a 20pF cap across it, it generates 1W so maybe the capacitor went open circuit? (and created a big mismatch that caused no power output)
i'm gonna try replacing the cap and putting the old rd15 in it
edit: after adjusting, i'm getting 3.5W max, what could be the problem now?
 
Last edited:

Thread Starter

Maz2858

Joined Jan 27, 2021
25
Thanks a lot for your help, i got 18W out of the amp by re adjusting coils + putting 68pF + 56pF in paralel instead of the 68pF (normally 82pF in the circuit)
The amp works just fine now.
 
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