SE/30 Focus Control?

[smrieck511]

Hi, Although I've got everything (logic, analog and psu boards) recapped and working properly, I noticed that the Focus potentiometer on the analog board does nothing. The screen is reasonably in focus but I'm still wondering if this is something I can fix? Possibly as simple as a resistor on the analog board or something? I also think I remember that it relates in some way to the brightness pot (working fine) on the analog board? Maybe that's not correct? Thanks for any help.

 

[LaPorta]

Pics would be 100% helpful with this.

 

[smrieck511]

Pics would be 100% helpful with this.

Currently in the back of a bus rding to a music performance so I can't send pics.... I just mean the focus control is unresponsive. Is there any common reason for this?

 

[Callan]

I would make sure you haven't bent any pins on your tube / check you neck board. Also check for cracked joints on the connectors header pins.

 

[zigzagjoe]

R26 is bad. Replace it with a ~1 megaohm resistor, 1/4W.

You can also use two ~470-530k ohm 1/8w resistors in series, if those are more handy.

 

[smrieck511]

R26 is bad. Replace it with a ~1 megaohm resistor, 1/4W.

You can also use two ~470-530k ohm 1/8w resistors in series, if those are more handy.

sweet that is exactly the answer I'm looking for. Thank you very much.

 

[smrieck511]

R26 is bad. Replace it with a ~1 megaohm resistor, 1/4W.

You can also use two ~470-530k ohm 1/8w resistors in series, if those are more handy.

Also, at a glance, it looks like this resistor would affect the brightness as well as the focus circuit? The brightness potentiometer is working fine. Could R26 still be the culprit? Thanks.

 

[ymk]

Could R26 still be the culprit?

Yes. I had the same exact problem.

 

[smrieck511]

Yes. I had the same exact problem.

ok thanks again.

 

[smrieck511]

R26 is bad. Replace it with a ~1 megaohm resistor, 1/4W.

You can also use two ~470-530k ohm 1/8w resistors in series, if those are more handy.

just being sure… It says 1/2 watt on the schematic?

 

[zigzagjoe]

just being sure… It says 1/2 watt on the schematic?

Right you are, I misremembered. If it says 1/2 watt, then use a 1/2 watt or two 1/4 watt.

I personally had a machine with this issue and used two 1/4W 470K in series to address it.

 

[smrieck511]

R26 is bad. Replace it with a ~1 megaohm resistor, 1/4W.

You can also use two ~470-530k ohm 1/8w resistors in series, if those are more handy.

R26 is showing 1.02 megaohms of resistance. What else might it be?

 

[zigzagjoe]

R26 is showing 1.02 megaohms of resistance. What else might it be?

Hmmm... That's the common failure.

Next would be to check the pot itself while you've got R26 out - should be 2 mega ohms, IIRC, and then if that seems fine, check the connection of the wiper to the neckboard as @Callan mentioned.

 

[smrieck511]

I re-did all the solder joints around that resistor and the potentiometers... I think I understand better now. Unless I'm mistaken, the focus is somewhat subtle? I can see that if I turn it one way, it gets a out of focus in the corners, but is in focus in the center and the opposite is true when I turn the other direction. Sorry for my confusion, but if this is sort of a subtle control, I think it is working correctly.

 

[zigzagjoe]

I re-did all the solder joints around that resistor and the potentiometers... I think I understand better now. Unless I'm mistaken, the focus is somewhat subtle? I can see that if I turn it one way, it gets a out of focus in the corners, but is in focus in the center and the opposite is true when I turn the other direction. Sorry for my confusion, but if this is sort of a subtle control, I think it is working correctly.

Yes that sounds like correct operation. As CRTs wear, the corners go out of focus first in my experience... best you can do is somewhere in the middle to get an acceptable level of focus.

 

[ymk]

I re-did all the solder joints around that resistor and the potentiometers... I think I understand better now. Unless I'm mistaken, the focus is somewhat subtle? I can see that if I turn it one way, it gets a out of focus in the corners, but is in focus in the center and the opposite is true when I turn the other direction. Sorry for my confusion, but if this is sort of a subtle control, I think it is working correctly.

That's expected. The center of the screen and the corners are different distances from the gun.

To tune the focus, I'll put a grey checkerboard pattern up, look at a spot halfway between a corner and center of the tube and dial it in so it's as sharp as possible. The center and corners become slightly less sharp.

 

[smrieck511]

I took my SE FDHD out of the closet and the focus control is definitely more responsive than the one on my SE/30. So the mystery remains.

 

[smrieck511]

I may swap the analog boards. That would at least tell me if it's something on the neck board or CRT vs the analog board.