CTC 10 HV affected by brightness

[Turle]

Been stuck on this for weeks. Initially I thought it was a focus problem, but The HV on my 10 drops to around 14kv whenever the brightness is up, and it jumps around depending on what’s on screen, I have no idea what could be causing this. I’ve pulled the cap off the regulator tube and it made no difference, the rest of the hv tubes are fine. the power supply seems to be working fine. I even tried a different CRT but it made no difference, does anyone know how I could fix this??

All help is appreciated. Thanks again

[old_tv_nut]

I presume the high voltage is normal when the screen is dark?

Have you replaced the tubes or only tested them? Tubes in the horizontal section can test good but be weak in actual use.

[old_tv_nut]

Another thought: with a black screen, check the 640V boost voltage to see if something is pulling it down.

[old_coot88]

Does the picture 'bloom' (expand), go dim and out of focus with brightness turned up?

[old_coot88]

OOPs double post

[Turle]

Originally Posted by old_coot88

Does the picture 'bloom' (expand), go dim and out of focus with brightness turned up?

Yes that’s pretty much exactly what it does. The HV drops and so does the focus voltage

[Turle]

Originally Posted by old_tv_nut

I presume the high voltage is normal when the screen is dark?

Have you replaced the tubes or only tested them? Tubes in the horizontal section can test good but be weak in actual use.

Yes when the screen is dark the voltage is normal, and I’ve substituted all the tubes for NOS ones with no change

[Turle]

I’m so lost I tried a few more things. None of the voltages or resistances look concerning. I checked the flyback it seems fine. Still no change

[old_tv_nut]

To repeat, is the boost voltage normal with a black screen? Does it go up and down with the high voltage as brightness changes?

[Turle]

Originally Posted by old_tv_nut

To repeat, is the boost voltage normal with a black screen? Does it go up and down with the high voltage as brightness changes?

Oh whoops I missed that part, doesn’t the boost run off the HV? So it should be affected the same?

[old_tv_nut]

Originally Posted by Turle

Oh whoops I missed that part, doesn’t the boost run off the HV? So it should be affected the same?

Yes, it should be affected the same. But my question is whether it is up to normal when the high voltage is normal, or could it be lower than normal all the time and loading the horizontal somehow.

[timmy]

Seems like the hv is not regulating or responding to the load.

[old_tv_nut]

Originally Posted by timmy

Seems like the hv is not regulating or responding to the load.

No, if it was not regulating, high voltage would go too high with a dark screen. The regulator is a shunt regulator, which means it draws more current when the CRT draws less, to keep the high voltage constant by creating a constant total load. In this case, high voltage never tends too high and therefore the regulator never draws current. This gives the symptom that disconnecting the regulator cap makes no difference.

[timmy]

I had a silvertone ctc12 clone with the same problem hv would not go over 19 kv higher the brightness the lower the hv I checked and changed literally everything and was never able to figure it out. I basically gave up on it but it did work nice but was not right.

[Yamamaya42]

As old_tv_nut mentioned, more info would be helpful.

For example.

HV reading at anode with brightness at low/mid/high
640v BOOST source with brightness at low/mid/high
Damper plate (junt of L30 / C90) with brightness at low/mid/high

This might give some clue as into what the problem is.