Re: Spectrum Analyser TR4172
I'm a bit befuddled here now.
Has anyone ever experienced a transistor failure mode in which it's DC operating characteristics look absolutely normal, but exhibits a loss of gain at RF levels in which it was designed to operate? I modeled the whole Q19/Q20 circuit with the opamp and Q21 section (seems to be operating as an AGC) and my DC measurements are within a few millivolts of what I see on Multisim. I see virtually zero gain, if not perhaps a bit of loss while probing with my RF millivoltmeter's high impedance probe, but in Multisim I see at least twice the voltage going out as going in, so it should be about 6dB of gain at least. This is odd as well, considering the manual states that this amp should provide 14.2dB of gain. Normally, I'd just wholesale replace the transistors, but since this board is such a pain to remove to replace them, I'd rather be a little more conservative in my method.
Thoughts?
Regards,
Matt
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