Quote:
Originally Posted by miniman82
As Wayne pointed out, I think you're misunderstanding how the 6BK4 does what it does.
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Sorry guys. I think I my post was misinterpreted. I guess I wasn't clear.
Since the 6BK4 is a "shunt" regulator, I realize it shunts current to maintain a constant load current, hence supply voltage. Also to regulate, the 6BK4 does not sense current, it senses voltage. The boost B+ voltage is sensed which should be directly proportional to the HV.
What I was suggesting is that if unregulated HV voltage is at or below the point the 6BK4 begins to conduct, the 6BK4 will not shunt any current, hence will not regulate the voltage.
I measured about 23kV with the 6BK4 cap disconnected. (The cap removed effectively removes all HV regulation). When the cap is connected, the voltage remains about the same. This means the 6BK4 is not conducting, probably because the unregulated HV is too low.
The question I have is if the focus voltage remain constant at the point the HV begins to drop (HV out of regulation), this means the problem is the unregulated voltage at the output of the HV rectifier is too low. This in turn implies that the HV pulse arriving at the HV rectifier is of insufficient amplitude. Further, because the focus voltage remains the same (constant CRT load), the HV transformer core is not being over saturated with the HV supply current demand. In other words, the transformer is okay.
On the other hand, if the focus voltage drops with the HV, the transformer core is perhaps becoming over saturated and I would suspect a fault with the transformer.
It is interesting to note that the schematic suggests the HO Tube cathode voltage is 7.5 VDC. This means the cathode current should be 7.5/24 = 312mA. I do not think the current should be appreciably lower than this because I believe the flyback pulse would be reduced which in turn reduces the HV.