[Tom Albrecht]

Gentlemen,

I have a pretty nice black and white picture on the set now (with sufficient red gun contribution as well). Color circuitry not working at all, but we'll get to that later.

What I'm having trouble with right now is focus stability.

I modified the HV regulator circuit to regulate well at a lower voltage for the time being, since my HV supply is a bit low. That allows the HV to be stable and well regulated at around 15 kV.

However, the focus is still not stable at all, and varies wildly (from nice and sharp to so badly out of focus you can't see anything). Probing the focus voltage, I see that it is moving around quite a bit (easily a factor of 2 with scene changes). Convergence transformer is out of circuit, so it is not the problem.

That got me wondering about whether the focus control was in fact correct. I had noticed when I got the set, that a previous tech had added extra resistors both above and below the focus control, which I thought peculiar. Measuring the resistance of the focus control I find it is only 1 M instead of the 5 M called for in both the Sams and RCA schematics.

Then I checked the DC convergence control and find it is only 10 M instead of the 15 M called for in the schematics.

Can you take a look at the attached pictures (first three) and let me know whether the controls look original? Or were they both replaced at some point by a tech that took the time to get shafts of the right length with no evidence of cutting, etc.? Maybe he couldn't get the right values.

Much to my surprise I found in my junk box a NOS 15 M "high voltage" control, IRC type HV-15 (last two pictures). Always wondered what that would ever be good for. I may swap that in for the convergence control (although the 10 M in there seems to work fairly well).

What I'm really curious about is why my focus is all over the place. With the 1 M control in there, the load on the focus rectifier is a bit lower resistance than it should be -- 11 M versus 13 M (I have added 2 M above the focus control to get a usable range, so only 2 M missing overall from the chain). That doesn't really seem like enough to really make such a big difference. Obviously the focus range adjustment is much smaller than it is supposed to be, but that wouldn't appear to explain the instability.

Would having a low second anode voltage somehow cause the CRT itself to draw more current on the the focus electrodes and overload my focus supply, in a manner that appears to be scene dependent?

It also seems to like to focus with a fairly low focus voltage -- more like 1-2 kV instead of the 3-4 kV in the schematic.

The 1X2 tests good, and I've also swapped it for an NOS tube, so I don't think that's the problem.

Not quite sure what's happening here.