GaAs FET Troubleshooting: Assembly Not Working

[Ravaner]

Hello. I use 2 GaAs fet in a HF system. This assembly worked properly for several weeks and now doesn't work anymore. Before replacing these 2 transistors I would like to know how it is possible that all voltages ( drains & sources) are still correct, while the complete assembly stopped to work ?

 

[berkeman]

Can you post a schematic? Are they in the output that drives a 50 Ohm load? How much power were you pushing out to the load? Was the load ever disconnected?

 

[Ravaner]

Last transistor drives 680R and power "extracted" from this stage is probably les than µW. My question is why voltages are still the same as when this assembly was Ok.

 

[berkeman]

I think we really need at least a partial schematic and part numbers to help you. Otherwise we would just be guessing.

 

[Tom.G]

Well, a shorted inductor or open capacitor come to mind; and of course a faulty solder joint, cable or cable connector.
Just the usual suspects.