Service manual for my consumer TV says G2 voltage is 1000v. I actually haven't measured how much I am increasing it because at the time I did the mod, I didn't have a high voltage oscope probe, only a probe that went up to 500v. According to the literature, gun design is the biggest determining factor for voltage to spot size. Without the datasheets for these tubes that show the spot size versus voltage plots (an example was shown in the Display Labs Inc slides I linked in the OP), we won't know how much spot size will change unless we experiment.ElBartoME wrote:900V is coming off the flyback and going to the neckboard. From this voltage the focus and screen voltage is derived.
According to the service manual the voltage range for G2 is between 320 and 440 volts. There is a series resistor to the potentiometer of 680k. I could change that value to squeeze out a bit more voltage like say 550 volts.
I have a JVC BM-H1310SU (also re-branded as the Panasonic BT-H1390Y) arriving in a week. It is a 750 TVL dot mask, and it is an example of a broadcast monitor that does NOT tie the G1 anode to ground. This was a question raised earlier in this thread as to why monitor designers wouldn't use this trick if it would help achieve higher resolutions. Here is the neckboard schematic, where it is clear that G1 has a negative voltage relative to chassis ground.
Looking at the schematic, this monitor appears to make G1 more negative during horizontal blanking. This is one of two methods for ensuring that the cathode to G1 voltage is "blacker than black" during horizontal retrace. This allows for a better spot size because the RGB drive amplifiers can operate at their peak voltage range when scanning and then during retrace they are at a black cathode voltage and G1 is pulled negative for blacker than black voltage. The other technique, which is older, is the design used in my consumer CRT, where the RGB drive amps are used at a lower voltage when drawing each line and during retrace the amps increase cathode voltage to blacker than black.
Here is the schematic for the consumer CRT's neckboard that I modified. You can see that it simply ties G1 to ground and if you oscope the RGB drive amplifiers that control cathode voltage, you can see that when scanning lines, the cathodes do not operate at their peak voltage range, they only go up to about 200 volts for black... but during horizontal retrace (hblank), the amps increase cathode voltage above 200 volts for blacker than black.
The benefit to the fact that the JVC BM-H1310SU's neckboard controls G1 voltage is it should be even easier to decrease the monitor's spot size by changing the bias voltage for the hblank signal going into the amp that drives the G1 anode. I am not sure how much range there will be to play with though as the service manual provides limited information. I will use my oscope to reverse engineer.
Incredibly Useful Technical Articles
Zahid Rahim published numerous technical articles in the 1990s on CRT design. I linked to one of them in the OP, where Zahid talks about controlling spot size... This one is another good one to read, and this article of his describes how to drive a 27-inch consumer grade CRT with a 0.37 mm dot pitch at 1600 pixels horizontal resolution. This comes out to horizontal phosphor triad to spot size ratio of approximately 1.2 triads per cathode ray spot. This is yet another data point that the dot pitch bottleneck that many have brought up in this thread is not a bottleneck for increasing TVL, and that monitor TVL can be increased with more aggressive driving of the CRT anodes. In the early 1990s, Zahid even demo'ed this large consumer grade CRT driving 1600 pixels horizontal resolution. Here is the picture of his demo from the article: