HP 180A Oscilloscope problem

Hello, my 180A is having an issue with losing the trace. with no probes or signal, you can briefly see the horizontal trace (as it should be) for a split second after power on.

The beam-finder will sometimes bring it back for a split second also. All of the low voltage power board test points checked out OK on my multimeter.

If I provide a input signal (using the built in 10 VAC calibration to input A) and power-cycle, or press beam-finder, it will show the expected trace, again for a split second.

Turning up SCALE makes the CRT glow brightly as expected. I don't think it is a high voltage power issue, perhaps it is an issue with the sweep?
What could be causing this problem?

A visual inspection revealed no burnt out components, could it be a bad cap?

If you do not already have it the service manual is at
https://www.electro-tech-online.com/custompdfs/2010/10/00183-90911.pdf

I think it was nice of them to put the manuals online.

Thanks, I've been poring over it for the last 24 hours :>

I don't have equipment suitable to measure the high voltage test points, and I am still trying to narrow down what circuit or component is causing the problem.

I also thought it was nice of them to put it online.