Restoring a RCA 21CT55

[Electronic M]

Your last two posts took the words out of my mouth before I could write them, Ben.


To add to your running at design center max being desirable. Most manufactures tried to do that for a couple of reasons: Brighter picture, and CRT life. The cathode material of a CRT has a somewhat finite number of electrons it can emit before the emissive material is depleted. The higher the HV the less gun current is needed for a given brightness setting, the less current the fewer electrons needed from the cathode for the same result, the fewer electrons used for desired result the longer the CRT life.

Also I'm not trying to knock your tech, but I remember you quoted your tech as saying that the HV regulator "shunts the excessive voltage" which conceptually is wrong. The regulator shunts current not voltage from the HV line to ground. You see every power supply has to obey the laws of Thévenin...That is there is no ideal voltage source. Every voltage source has an internal resistance, and that internal resistance forms a voltage divider with the load (CRT+HV reg tube). In the case of varying loads (such as a CRT) the output voltage varies as a function of load current drawn through internal supply Thévenin resistance. The HV reg is configured to attempt to increase it's relative current draw instep with decreases in CRT current draw (caused by video content), and decrease when the CRT increases. If the current through a Thévenin resistance is held constant (which is what the HV reg tube is meant to do) then the output voltage of the supply is also held constant.

There is also an average current to the HV reg that is adjusted by the HV level pot....The lower you set the HV level the higher the constant/average current (sum of CRT and HV reg tube) is on the HV winding of the flyback. The Thévenin resistance lives in the fly windings in the case of a TV like this, and thus the higher the HV current you put through it the higher the heat and strain on the flyback.

Many RCA flybacks were being pushed hard by design, adding to it does not strike me as desirable.