I have been asked to repair an HF transceiver, a Yaesu FT-990, that does not transmit. The owner had done some research and got an opinion from some repair expert that it is a failed PIN switching diode feeding the power stage. Unfortunately an exact replacement diode is not to be found, at east so far. I did remove the diode and checked it wit a DMM and it seems to pass the normal diode test of conducting in ony one direction. I am hoping that there is additional testing that will verify that it is either failed, and must be replaced, or OK, and I need to keep searching for the problem.
So what other tests can I do for the PIN diode? Besides reverse leakage and forward voltage?

 

Your testing seems to show the diode is working correctly and so the fault is probably elsewhere.

 

Your testing seems to show the diode is working correctly and so the fault is probably elsewhere.

That is why I am wondering if tghere is some additional checking to do. The diode, when the radio is working, passes a fair amount of RF to a higher powered amplifier stage.
I am not at a familiar with pin diode switching at any serious power levels, and so I am wondering what other test might apply.

 

That is why I am wondering if tghere is some additional checking to do. The diode, when the radio is working, passes a fair amount of RF to a higher powered amplifier stage.
I am not at a familiar with pin diode switching at any serious power levels, and so I am wondering what other test might apply.

Yes, if you have an RF signal generator for the band in question you can bias the diode as it would be in-circuit and check the attenuation at the output. If you can't get it to attenuate, it's not working. I don't know if a normal diode test is sufficient to determine this.

 

See page 4 of this note: https://cdn.macom.com/applicationnotes/AG312.pdf