Royal Star a bit of picture bother HELP please

[lloydwells]

Don't ask why but ever since I first saw one I'd wanted a KB royal Star I finally managed to get one two months ago I have the set up and running but can't seem to cure an issue with the picture on it. The bottom half of it is fine the top half I have wider lines causing an egg shape on test card ( see photo ) the very top of the test card is squished when I bring it down on to the screen. All out of spec resistors have been done all the control pots are fine the valves are good so I assume it's scan coil position related ??

[simpsons]

Hi Lloyd

Do you have the schematic?

If not Google: "kb pvp20 royal star service manual," scroll down to radio-tv.co and the field (frame) circuit is shown on that site.

Which controls are fine? Is the frame linearity preset R101 working as it should? If not the capacitors C84,85 & C86 should be checked.

Chris

[Heatercathodeshort]

What have you adjusted? Does the frame linearity control have any effect? Have all the wax capacitors been changed in the frame osc/output stage including the ones that look like resistors and the brown mini Hunts and are they the correct values? Is the cathode bias resistor the correct value? What is the effect of reducing the height and recentering the picture/raster?
Horrible to work on. Removing the 4BA screws that mount the mains dropper and moving it out of the way helps. Good circuit but nasty construction. John
PS Sorry Chris we snapped!

[lloydwells]

Everything on the board is either new or within tolerance it's all been checked and checked again against the original KB service manual and that included checking everything was through the correct holes in the board as the KB data comes with a rather handy board layout diagram the picture shows the set with the linearity control doing as much as it will to correct it .

[greg_simons]

Looks like the frame linearity pot just needs a tweak.
Greg.

[Sideband]

The linearity on mine is perfect. It has to be a capacitor or resistor problem. Make sure any capacitors replaced are correct value. Does the linearity control work correctly? The old pre-sets can change in value so it might not be able to cover sufficient range....just a thought.

[Sideband]

Originally Posted by Heatercathodeshort

What is the effect of reducing the height and recentering the picture/raster?

Another good point....

[Sideband]

Here's a quick picture of mine....the tube isn't brilliant and this was taken just after a gentle boost. If anything the line linearity needs adjusting on mine but frame lin is fine

[lloydwells]

Sideband as mentioned the resistors and caps are in the correct place and the correct value checked multiple times against both the factory data and the trader sheet the linearity control measures 32.3K which should be near enough and has an effect centering the picture makes no odds the height control works fine what I have noticed is when it's as near as I can get it the lower half of the picture is slightly compressed. The only caps that haven't been changed are anything measured in PF everything else is new and was tested before fitting just because a component is new doesn't mean I trust it.
One last thing Spike Milligan's grave does and doesn't say " I told you I was ill" the cemetery wouldn't allow it and half the family were against that inscription it is therefore inscribed in small lettering on his stone in Gaelic not English.

[Nuvistor]

The width seems a bit short, see photo in post #8 for comparison, is the HT voltage correct?

[Nuvistor]

Another thought is to check the boost HT, the boost voltage is used through the height control to supply the charging capacitor, hence the sawtooth waveform,
The boost HT is used as the charging voltage to allow a more linear scan, a low voltage will affect the linearity more at the bottom of the scan.

[Sideband]

Originally Posted by Nuvistor

The width seems a bit short, see photo in post #8 for comparison, is the HT voltage correct?

That's a good point....I hadn't noticed the lack of width. Check the HT first and if that is OK check the boost HT. If the boost is low, try a boost diode. If the boost is OK, try normal width adjustment. If still insufficient width then it could be the line output valve (think it's a PL36). Either way, more width is required which will affect the overall picture shape. Has the boost capacitor been changed?