So yesterday my hp 437B power meter started to show errors in the readings. The CAL output read -6.7 dBm instead of 0.0 dBm. All the readings from my signal generator were also off by that much. In addition, it would not calibrate using the built-in CAL function. I got "CAL ERROR" every time I tried it.
I did notice that some fluctuations in the reading would occur when I tapped the meter case. I didn't find any adjustment that I thought would correct it, so I started inspecting the PCBs. Oh my what did I find?
Pin 10 of J4 has a pin untouched by solder! Not a weak or cracked joint - no solder at all. This J4 connector is where the sensor signals go through - both stimulation and response. Pin 10 and pin 9 supply the 220 Hz drive to the sensor. When I pushed on pin 10 where it entered the PCB while powered on, the error went away. Somebody was in a hurry inspecting this one! So for all these years it was working by merely touching the hole plating. I probably disturbed it just enough to disrupt the conductivity when I disconnected the plug to remove the front panel.
I soldered the pin and also touched-up the other ones. Now it's working correctly again. So inspect those boards when you get new TEA, never know what you'll find.