Four years and one pandemic later, I finally managed to get this set.
Actually, not this set, but another one just like it. It's been sitting for a while because color sets still intimidate me. But it's time. Cabinet's in bad shape, CRT tests great. Let's get it going; I have a place to put it where it will get some occasional use.
The 9B23P70 chassis seems to be mostly identical to the 12A/B13/14 hybrid American sets, with the odd empty hole in the chassis where the color tubes should be. The 6JD6 and 6EJ7 are missing, along with the 6GH8. There's a second DuraModule underneath a shield where the 6GH8 would otherwise be that has a couple of DIP ICs on it. I'm using the Sams 1157-2 as a reference, though I am trying to track down someone who may have the actual factory schematic.
I already brought the set up on a variac, it produced a picture for about 1 second along with some loud popping from the HV area, and then the screen went dark. Sweep/HV tubes were removed and tested and seemed to be OK. I put them back in the set, powered it on again, but no brief picture and no high voltage, confirmed with a HV probe.
I did some preliminary checks and determined that the Horizontal section is working correctly up to the flyback, voltages on the HO tube are correct, I have about 330V of drive on the grid and it looks right. My first thought, given the brief flash of picture, was that the HV was too high and a failsafe had shut it off. Now that I have the Sams I understand that's not how it works. I did voltage checks around the damper and HV regulator and everything on this side of the flyback seems to be off: both the grid and cathode of the 6HV5 have less than 100 volts, and I don't think there is any boost voltage. I don't have a meter that can read that high and my high voltage probe doesn't even wiggle when I probe the boost source. I also notice that a few of the resistors in the horiz and HV circuit have been cut and tacked back in, and upon opening the HV cage I found the HV rectifier socket had been unscrewed from its mounting though all the wires are attached, the tube is in place, and plate cap is connected. So I'm not the first person wondering why there is no HV.
And I've kind of hit a wall. I don't understand how this part of the HV section works, so I'm not sure where I need to go from here. I've been doing a lot of searching online for descriptions or explanations and mostly I'm just turning up other threads here talking about VDRs, and a mention that the grid voltage on the HV regulator "comes from the horiz oscillator", which I
really don't understand.
I would like to understand how this part of the set should work so that I can troubleshoot it, can anyone point me in the right direction? Is there a Zenith service manual that gives some detail?
Thanks everyone.