Another power meter joins the collection. 432A, but most importantly, this one came with a 478A thermistor mount. I also got the special old style line cord for the 431B, which enjoyed a successful power up with no dramatics.
Nice catch! How much did that set you back?
$300, which is substantially less than good thermistor mounts seem to sell for by themselves. It does work! Probably needs cal though, as the (unbroken) HP cal sticker on the thermistor is dated due in 1998.
That's not bad. The lowest I saw for a working 478A was close to $400.
Yeah, they are expensive because they are still used to calibrate more modern open loop style RF power meters. Keysight actually still works on the 478A thermistor mounts, and lists it for sale from third party distributors. My plan is to do just that; get a more modern open loop meter and use this to calibrate it after sending it to get freshly calibrated (whether I pop for NIST traceable or not...probably not for hobby use).
I did some testing tonight after reading the manuals for the 432A and the 478A. The 478A still has all the cal seals intact, and according to the label, had a NIST traceable cal done on 11/18/97, which came due 11/18/98. The good news? The power meter and my 8566A appear to agree almost exactly. The bad news is my 8664A probably needs calibration, as it is showing about 0.4 dBm off. This particular 478A is a -H88 option which means it's a 1 GHz rather than a 10 GHz mount. But that's ok with me as my intended use case is calibrating the 50 MHz cal signal on an open loop meter.