Update :

https://dl.dropboxusercontent.com/u/29948706/33901.jpg

Now have calibrated distortion in via a matched set of resistors and an external generator. It measured 0.055 % on its own oscillator. I had a 10K rom that nd a 10K from the HP generator with 600 ohm output impedance.

I calibrated, which is to set the level so the needle goes to 100 % and then you switch the switch.

We found that even moving around the room affected the reading when we were reading that 0.0055 %. Also, because of my slight (only one order of magnitude) error, we were measuring 10 KHz, not 1 KHz. Of course all the filters were off, but I do want to try those filters.

I started to get close to the null frequency with the external generator. I saw how the response dropped.

The scope on the left was connected to the monitor, where you can see the TRMS meter input. I am about to chew through these ropes and get a spectrum analyzer to hook up there. The scope to the right has the original HP 339A output going to one channel and the output of the other HP generator sitting on top to the other channel. Thus it is easy to compare the amplitude of their waveforms.

When the amplitude was 25 %, the distortion meter read 25 %. I mean when the internal HP was 3 volts, and the external was 1.5 volts it read 50 % distortion. Since the TRMS convertor is after the input attenuator, is it not reasonable to assume that it will be accurate at different ranges ? that is determined by resistors just like every, and I mean every other thing on the planet.

Well there are exceptions...