Hello All,
The resistor mentioned before is burning again. When I tried to adjust the odd horizontal position control, that's when the resistor burned. I can find no other reason for it. The horizontal position control is connected to 350 volts on one end and -150 volts on the other end. The wiper connects through some resistors to V240 on the sweep chassis. When measuring this control I find some odd readings. As I adjust it and I'm getting towards one end,and I'm measuring from the wiper to an end, when I get near the end the resistance goes the wrong way. It goes up for a while and then goes down. At the low end it never reaches zero and there are skips while adjusting it. This a multi-turn control. I'm not sure that the control that's supposed to be there can handle the high voltage without the tube to help it. There is no V246 in this scope. For some reason, it was eliminated and replaced with this control. There isn't even a place for a V246 on the chassis.

I'm going to adjust the control while the scope is off measuring the resistance on the 350v line. I'll bet somewhere in the adjusting range it does something odd.

Please help with this.

Thanks,

Dave

What you are describing is what's known as a " Heipot " control. It is a wire wound precise precision pot. I would bet it's burnt up inside.

Error in spelling .. It is Helipot. If you just Google that, it shows pictures and descriptions.

Warren,
I know what it is. How are you so sure that it's "Burnt up inside"? I measure 25K ohms across the entire resistor. When I measure from wiper to the far end, I get 25 k ohms, but if I back it up a little bit it goes up to 30 K ohms. This is what makes no sense. If this is NG, I need to find another almost immediately. It says on the label that this is a 100K ohm pot. It's made by Amphenol. It's connected to the circuit. So that would account for the 25 K across the control, but 30K measured between the wiper and an end seems odd. It shouldn't be more than the total resistance of the resistor.

Thanks,

Dave
:Error in spelling .. It is Helipot. If you just Google that, it shows pictures and descriptions.
:

Warren,
When the scope is off it appears to be good and it turns quite easily. How are you so sure it's burned inside? How can I tell that it's "burnt up inside"? The resistor R756 is burning again. Something is drawing current at an enormous rate through that resistor. This is getting extremely frustrating. When is it going to work and stay working? I can find no loads anywhere on the 225 volt line or anywhere else for that matter, yet the resistor burns.
I see something on the screen but it quickly moves to the left. I try to move it back to the right but before I can, the resistor smokes and burns. The focus and intensity do have some effect but the dot is away from the screen before I can see it.

Thanks,

Dave

:What you are describing is what's known as a " Heipot " control. It is a wire wound precise precision pot. I would bet it's burnt up inside.
:

Being it is wire wound, and would have a wattage rating. it is subject to having a burnt spot in it. With the wiper connection lifted, you should have a very smooth clime, and decrease reading using a ohm meter. If it jumps, opens, erratic readings. It's got a problem inside.

Warren,
I cannot find any problems with the control at all. The reason for the rise in resistance is because of very large electrolytic capacitors connected to the control. I don't think that that's where the problem is. But where is it? WHy does the resistor burn and smoke if there's no load to be found on the 225 volt line?

Thanks,

Dave
:Being it is wire wound, and would have a wattage rating. it is subject to having a burnt spot in it. With the wiper connection lifted, you should have a very smooth clime, and decrease reading using a ohm meter. If it jumps, opens, erratic readings. It's got a problem inside.
:

Dave
I really don't know a direct answer to your question. You did say the resistor did not burn, until you turned the Horz centering control. That leads me to believe you have a clue where to start isolating the problem in a systematic way. One thing does come to mind. The CRT is electrostatic. (No Yoke) There are high voltage Mica capacitors in the Vert. & Horz. Deflection circuit. I have found those shorted before. Like in the 7 inch Motorola 7JP4. Similar problem. The Beam shoots one way and stays there.

Warren,
That may have been a coincidence. By the time I tried to move the beam, it was already too late. The current was already too high. I'll look for mica capacitors. I don't remember seeing any. If they are there I'll try replacing them. Your description of the symptom is correct. The beam moves to the left and never comes back.

Thanks,

Dave

:Dave
: I really don't know a direct answer to your question. You did say the resistor did not burn, until you turned the Horz centering control. That leads me to believe you have a clue where to start isolating the problem in a systematic way. One thing does come to mind. The CRT is electrostatic. (No Yoke) There are high voltage Mica capacitors in the Vert. & Horz. Deflection circuit. I have found those shorted before. Like in the 7 inch Motorola 7JP4. Similar problem. The Beam shoots one way and stays there.
:

Warren,
There is indeed a 4.7 pf capacitor connected to the right deflection plate. The connection to the chassis is though a 2.2K ohm resistor. Also, it is connected through a 47 ohm resistor to a 6CL6 grid. I'll check that capacitor and let you know what I find.

Thanks,

Dave
:
:Warren,
: That may have been a coincidence. By the time I tried to move the beam, it was already too late. The current was already too high. I'll look for mica capacitors. I don't remember seeing any. If they are there I'll try replacing them. Your description of the symptom is correct. The beam moves to the left and never comes back.
:
:Thanks,
:
:Dave
:
:
:
:
:
:
:
::Dave
:: I really don't know a direct answer to your question. You did say the resistor did not burn, until you turned the Horz centering control. That leads me to believe you have a clue where to start isolating the problem in a systematic way. One thing does come to mind. The CRT is electrostatic. (No Yoke) There are high voltage Mica capacitors in the Vert. & Horz. Deflection circuit. I have found those shorted before. Like in the 7 inch Motorola 7JP4. Similar problem. The Beam shoots one way and stays there.
::
:

Dave,

If you find yourself in need of some 6CL6 tubes, let me know as I have a fertilizer truck load of them.

Radiodoc
*****************

:Warren,
: There is indeed a 4.7 pf capacitor connected to the right deflection plate. The connection to the chassis is though a 2.2K ohm resistor. Also, it is connected through a 47 ohm resistor to a 6CL6 grid. I'll check that capacitor and let you know what I find.
:
:Thanks,
:
:Dave
::
::Warren,
:: That may have been a coincidence. By the time I tried to move the beam, it was already too late. The current was already too high. I'll look for mica capacitors. I don't remember seeing any. If they are there I'll try replacing them. Your description of the symptom is correct. The beam moves to the left and never comes back.
::
::Thanks,
::
::Dave
::
::
::
::
::
::
::
:::Dave
::: I really don't know a direct answer to your question. You did say the resistor did not burn, until you turned the Horz centering control. That leads me to believe you have a clue where to start isolating the problem in a systematic way. One thing does come to mind. The CRT is electrostatic. (No Yoke) There are high voltage Mica capacitors in the Vert. & Horz. Deflection circuit. I have found those shorted before. Like in the 7 inch Motorola 7JP4. Similar problem. The Beam shoots one way and stays there.
:::
::
:

Radiodoc,
OK. My 6CL6s are working, I think.

Thanks,

Dave
:Dave,
:
:If you find yourself in need of some 6CL6 tubes, let me know as I have a fertilizer truck load of them.
:
:Radiodoc
:*****************
:
:
::Warren,
:: There is indeed a 4.7 pf capacitor connected to the right deflection plate. The connection to the chassis is though a 2.2K ohm resistor. Also, it is connected through a 47 ohm resistor to a 6CL6 grid. I'll check that capacitor and let you know what I find.
::
::Thanks,
::
::Dave
:::
:::Warren,
::: That may have been a coincidence. By the time I tried to move the beam, it was already too late. The current was already too high. I'll look for mica capacitors. I don't remember seeing any. If they are there I'll try replacing them. Your description of the symptom is correct. The beam moves to the left and never comes back.
:::
:::Thanks,
:::
:::Dave
:::
:::
:::
:::
:::
:::
:::
::::Dave
:::: I really don't know a direct answer to your question. You did say the resistor did not burn, until you turned the Horz centering control. That leads me to believe you have a clue where to start isolating the problem in a systematic way. One thing does come to mind. The CRT is electrostatic. (No Yoke) There are high voltage Mica capacitors in the Vert. & Horz. Deflection circuit. I have found those shorted before. Like in the 7 inch Motorola 7JP4. Similar problem. The Beam shoots one way and stays there.
::::
:::
::
: