Restoring a RCA 21CT55

[miniman82]

What is your HV reading with the HV control at max? It’s not uncommon to have to play with the value of the resistors around the HV pot, since any drift there will cause the grid of the shunt tube to not be where it’s supposed to be which drags down the anode.

[etype2]

Originally Posted by miniman82

What is your HV reading with the HV control at max? It’s not uncommon to have to play with the value of the resistors around the HV pot, since any drift there will cause the grid of the shunt tube to not be where it’s supposed to be which drags down the anode.

We replaced a resistor in the HV regulation circuit which was found open. This caused the HV to drop to 10KV and messed up the picture badly. The resistor was an original (made by International Rectifier) special film type, 3.4 Megohm 3 watt. We replaced it with QTY 2 (in series) OHMITE 2.2 Megohm 2 Watt film type which now makes the new resistor a 4 watt unit which is a 25% upgrade for power dissipation.

[miniman82]

That could be your problem, original calls for 3.4 meg and you installed 4.4 meg. Compare the grid voltages on the 6BK4 with what the schematic shows, my guess is it’s a tad high. (Measurement should be taken with brightness at minimum)

Here’s how the regulator works, in case you were unaware:

Plate receives anode voltage, cathode is returned to B+, grid is tied to a boost source usually. If boost rises anode voltage must also have risen, since they are in the same circuit. This higher boost voltage causes the regulator to draw more current, since the grid got pushed closer to the cathode (the tube is normally biased off, with the cathode over the grid). With the regulator drawing current the anode voltage will get dragged down taking boost along with it, in turn taking the grid of the regulator lower and turning it more off.

In this way the regulator tube places a constant load on the anode, but if it’s biased a little too hot to start off with the result will be consistently low HV because the regulator is dragging it down.

The correct HV setting for this chassis is 25kv, I suggest you take Ben’s advice and figure out why it’s low before the flyback protests too much. You also need to know what the horizontal output tube is drawing for current, if it goes much higher than about 215ma, you’re in the danger zone.

We already witnessed another member destroy an irreplaceable CTC-2B flyback by not heeding sound advice, please don’t be the second.

[etype2]

Originally Posted by miniman82

That could be your problem, original calls for 3.4 meg and you installed 4.4 meg. Compare the grid voltages on the 6BK4 with what the schematic shows, my guess is it’s a tad high. (Measurement should be taken with brightness at minimum)

Here’s how the regulator works, in case you were unaware:

Plate receives anode voltage, cathode is returned to B+, grid is tied to a boost source usually. If boost rises anode voltage must also have risen, since they are in the same circuit. This higher boost voltage causes the regulator to draw more current, since the grid got pushed closer to the cathode (the tube is normally biased off, with the cathode over the grid). With the regulator drawing current the anode voltage will get dragged down taking boost along with it, in turn taking the grid of the regulator lower and turning it more off.

In this way the regulator tube places a constant load on the anode, but if it’s biased a little too hot to start off with the result will be consistently low HV because the regulator is dragging it down.

The correct HV setting for this chassis is 25kv, I suggest you take Ben’s advice and figure out why it’s low before the flyback protests too much. You also need to know what the horizontal output tube is drawing for current, if it goes much higher than about 215ma, you’re in the danger zone.

We already witnessed another member destroy an irreplaceable CTC-2B flyback by not heeding sound advice, please don’t be the second.

Thank you very much for this information.