CRT size please

[6GH8cowboy]

I have recaped a Admiral 20Y1 chassis for another VK member and have been trying to resolve a "width" problem. (very slightly lacking). All caps and a few resistors later I just noticed that 10 and 12 inch CRT is on the schematic. The yoke assy is also at the top of the elevation travel. I'm begining to suspect that some one at one time put a chassis from a 10 or 12 inch set into the 16 cabinet with a 16" tube.
I have been all over this set and cant get any more width. All voltages look good and horizontal wave forms (PP) are up to spec also. I'm about a 1/4 inch short on each side. The focus coil was a bear to get right when centering the raster. The picture looks very good other than the width thing.
At this moment I feel like I'm trying to get water from a rock. (No disrespect to Moses).

Is my perception correct or was this a common, flexible chassis used in 16' crt applications also?

Dave

[Eric H]

Hi Dave, these are probably too obvious but have you tried a different 6BG6 Horiz output tube?
I've also found that the 5U4 can make quite a difference in width if it's not quite up to snuff.

[bandersen]

The Sams and Riders for this chassis only mention the 10BP4 and 12LP4 so I don't think it was originally designed for a 16" CRT.

In addition to Eric's suggestion, maybe you could tweak the value of the .035 uF cap across the width coil ?

[6GH8cowboy]

It is a 12 " crt . I guess the 16 was a late night , tired thing. I sub'd the cap across the width coil and it helped a lot so .047 is it for now.
Now a new concern.
While playing it this morning the screen went dark. This was 20 min after aligning the horizontal freq and lock coil with the scope. It is a horiz osc problem for sure as the freq is waaaay up acording to the scope. All resistors have been rechecked and new caps sub'd out. the 75k resistor feeding the 6SN7 plate via the osc coil is very hot (250 v drop across it)
I suspect the coil is the problem as that is the last thing "touched" before the fail.
I took the tour and removed the coil from the can and noticed a tiny black buble on the side of the horiz lock coil and if I gently squeez it the scope shows an instant frequency swing.
Anybody have one or a suggestion?
BTW I've removed the H output tube to prevent cooking it while testing.
Admiral part# 69B110
Dave

[kx250rider]

Actually the CRT size doesn't affect the width. The deflection angle is the same (70 degrees) among most all CRTs from the 10" up to the 27" rectangular tubes, so basically you can drive any of them with any chassis, assuming you mount the tube physically somehow. The 10 and 12 are often interchangeable by moving the yoke bracket fore and aft and up and down on the chassis, with elongated holes (seen on many models).

You have something else causing the insufficient size, and it sounds like you are on the right track with many of the suggestions above.

Charles

[jr_tech]

No, The deflection angle of the 10BP4 was 50 degrees, 12LP4 was 54 degrees, 16AP4 was 54 degrees, the 16GP4 does indeed have 70 degree deflection.
jr

[teevee]

You can also add about a 20-50 pF High voltage cap across the damper... (A suggestion from one of the ancient druids (Jack Darr) ) Never had to use this suggestion yet..

[Eric H]

Not likely here but I've made the mistake of searching for a problem like this or similar only to find it was caused by running the set off my Isolation transformer which drops the voltage 10% or more.