[Electronic M]

This is an interesting set and I look forward to reading more about it in the future.

A few notes. You can measure the 875V boost rail (that's what the Sam's for my 12B13C52 claims it is) with a 600V meter. The highest regular B+ rail is 405V (again working off my Sam's)...If that 405V rail is a constant voltage with respect to ground write that voltage down then measure the difference between the 405V rail and boost (meter - on 405, meter + on boost), then add the measurement from the 405 rail to the boost difference measurement to get actual boost voltage...It's kooky l, but it works in a pinch...Ask me how I know.

If the yoke has made in Holland and a phenolic wafer facing the screen under the rear plastic cover it could be a shorted yoke...I just dealt with one of those yokes on a 16" portable (it used electrically the same yoke and connector as my console) it would warm up and short and after 15min slowly kill the HV...the 15 minute warmup time (as well as H out Grid drive dropping with boost as a red herring) made it a PITA to troubleshoot but I fixed it after 2 days...I was lucky to have one older Zenith yoke that was all plastic (no phenolic) to swap in as a replacement.

IIRC the pulse regulator samples the boost voltage ( I think it does so at a specific point in sweep gated by the damper) and adjusts the H output tube bias to reduce or increase HV. Basically the Boost voltage is a VERY good indicator of the load on the flyback. Assuming the HV system is working properly the only thing loading the flyback that will change the amount of loading is the CRT beam current load on the HV so if you adjust horizontal to vary energy supplied to the flyback with a feedback loop that holds boost constant, then you can effectively hold HV constant too. I may be oversimplifying it a bit as it's been ages since I read about it, and it's rare that I need to troubleshoot one of those circuits.

Pretty much all TVs even unregulated monochrome sets get H output grid bias voltage from the oscillator. Basically the AC signal gets rectified by the grid/cathode and that creates the bias...It works similarly to grid leak detector triodes in 20s radios if you are familiar with that.

If this were my set I would put a meeter on the boost use the plate drive of my B&K 1077 to drive the flyback then unplug the yoke and see if boost improves...That would sniff out a bad yoke fast.