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   James Morris (jmorris@social.kernel.org)'s status on Tuesday, 30-Jan-2024 17:40:12 JST James Morris
   Not sure if this microwave power sensor is repairable. It’s an HP P468A waveguide thermistor mount for 12.4 to 18 GHz. It was bugging out, and I’ve rejuvenated one of these before (using the guidelines in the manual) but it didn’t work this time. I’m puzzled by the mechanics of this, what the adjustment screw actually does, and why is it attached to a small metal block which is maybe held with conductive paste? It’s supposed to finely match the inner and outer thermistors: one is in the waveguide and the other is inside the housing as a reference. The waveguide thermistor is heated by the RF signal, and the power meter detects this, via a bridge circuit which tries to balance the current in each thermistor.
   
   The wonders of 1970s microwave tech. We explored space with this stuff.
   
   #hamradio #amateurradio #microwave #electronics
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     ohiorob (ohiorob@mastodon.world)'s status on Wednesday, 31-Jan-2024 07:32:25 JST ohiorob

     in reply to

     @jmorris

     I was just working on a Narda RF meter and thermistor mount from the 70s myself, wondering what some of the unfamiliar components do as well.

     James Morris likes this.
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     James Morris (jmorris@social.kernel.org)'s status on Wednesday, 31-Jan-2024 11:47:38 JST James Morris

     in reply to
     Ok, I think I understand this a little more now. The horizontal bar in the waveguide is a thermal mass, which heats up in the presence of RF. The detector thermistor is attached to it (too small to see here), and it will closely follow the temperature of the bar. A bias current runs through the thermistor, and also the isolated compensation thermistor. The comp thermistor has its own thermal mass, which is the gold plated square, and a couple of rectangular blocks, which I assume were selected and pasted in by hand during construction, and this can be finely adjusted via the screw. Not sure exactly where the compensation thermistor is yet, and the circuit diagram shows a capacitor here somewhere. Could be both are in the white block, which looks like a modern SMD cap.
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     James Morris (jmorris@social.kernel.org)'s status on Wednesday, 31-Jan-2024 11:50:25 JST James Morris

     in reply to
     Close up of the thermistor, which is a tiny bead in the middle of the photo.
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     James Morris (jmorris@social.kernel.org)'s status on Wednesday, 31-Jan-2024 11:52:00 JST James Morris

     in reply to
     I guess the black paint is for optimum heat transfer to the thermistor? Gold plated everywhere else for RF efficiency.