Front view of oscillator. An MT-197/APA-6X shock mount has since been fabricated to complete the correct appearance.
There were several changes in this oscillator from the standard H-P 200C audio oscilator from which it appears derived. One was the use of a metal 6AC7 in place of the normal 6V6. That makes some sense, given that the load presented by the AN/APA-6 analyser was a known, as opposed to the normal use of the oscillator for general purpose testing.
What is a bit puzzling is the replacement of the directly calibrated frequency dial on the 200C with the expensive National interpolating dial. The puzzling part is that a change like that is normally accompanied by a printed volume of frequencies versus dial readings, so that the interpolated readings could be converted and recorded with a high degree of accuracy. Instead, H-P provided a chart, shown below, that on a good day might be read to two places. The only conclusion that I could come to was that it was useful to have the interpolated reading to identify a specific enemy radar, but that didn't require an exact associated frequency in the after-action analysis. I haven't found any documentation to clarify that issue.
