Rca ctc-5

[Penthode]

Originally Posted by DaveWM

I will have to check the cathode current again. I did not check the line voltage, but the fly is getting pretty hot, and I do not seem to have any HV blooming (but I do have the video out contrast mod in). My concern is the heat in the fly HV tire. what currents are you seeing? are you checking the AC in and setting at 117v?

I bought several HO tubes to compare with.

I was measuring about 210mA with the efficiency coil adjusted to minimize current. I believe the current should be a bit higher than this. The schematic inicates there should be 7 volts on the cathode with respect to ground. I measure 5.5 volts.

My thought is that adjusting the efficiency coil for slightly higher current will result in and increase to the unregulated HV. However, as Nick points out, the problem may relate to a limitation of the HV transformer.

I found the 6BK4 regulator is not really regulating the HV. This appears because the HV unregulated (with the 6BK4 cap off) is only about 23kV. When increasing the brightness hence the beam current in the CRT, the HV sags below 23kV.

Here is what I'd like to test: if the focus voltage does not track with the HV eg the focus voltage remain constant with the HV dropping, this means it is an unregulated HV supply issue. That is, the HV transformer is okay but the pulse to the HV rectifier is not of sufficient amplitude. If however the focus voltage also sags with the HV, then the HV transformer is likely at fault, as Nick suggests.

As for raising the B supply with a variac on the 117vac side, if the HV regulates better, that would suggest the former and not the latter.

Do you agree?