Yeah, these are beautiful! They've inspired me to start learning macro photography. I have one of those Canon EF-M macro lenses with the ring light, I've started to grok the exposure triangle, and it feels really great to have understanding and control vs just trusting phone magic. I still need to figure out how to get decent images out of my microscope, but that's another story.
I didn't think I had a "before" pic, but it turns out I do! See attached. The bond wire fused in the middle, and this was the case for all three dead amplifiers. My guess is that fusing in the middle means a thermal mechanism, whereas mechanical excitement would tend to effect the ends, but that's all just guesswork.
HP didn't do active biasing in any of these designs, but they did do sequencing. Perhaps they should have done active current control. The gate bias is fixed voltage, adjusted manually with those little blue trimpots, one per amp. Sequencing is accomplished by watching the -10 gate bias supply with an op amp. The +8V drain supply, which provides power and cooks the amp if applied without the gate bias, is switched on with a FET and only gets switched on when the op-amp sees the negative rail. This is the case in all three modules. On the 8360 I have access to, I've scoped the drains and the mechanism appears to work. The +8V is never up when -10 isn't.
Err, is it a Doherty pair if the splitter is a Wilkinson and not a hybrid? Eh, I'll just (ab)use the term.
Dead Amp A:
* Narrowband 5.4GHz VCO buffer from low band module
* Found with fused bond wire
* Re-bonded
* Came back to life, hasn't keeled over yet.
Dead Amp B:
* TC702 traveling wave amplifier (see attached doc) from 50GHz doubler
* Found with fused drain wire
* One half of a Doherty pair (other amp wasn't fused)
* Re-bonded
* Zombie: degraded frequnecy response, gate bias needed big adjustment, outside of typical voltage range
Dead Amp C:
* TC724 traveling wave amplifier
* Found with fused drain wire
* One half of a Doherty pair (other amp wasn't fused)
* Re-bonded
* Came back to life, bias needed small adjustment, inside typical range
Notes here:
https://www.dropbox.com/s/2682992emftu6rm/8360%20Doubler.pdf?dl=0TC702 Amp:
https://www.dropbox.com/s/f6ix9seghzt1k9l/TC700%20TC702%20TC900%20TWA%20Amplifier.pdf?dl=0TC724 Amp:
https://www.dropbox.com/s/9kt70tm2vlaw27f/TC724%20TWA%20Amplifier.pdf?dl=0