It sounds that the fault is with excess blue on display. There have tw reasons for this. As you see diagonal lines (thin white lines, all over the screen, displays horizontally, at about an inch or so apart each other); the screen voltage should be suspected. If the screen voltage is more than normal, this kind of fault can occur. The second one, is related to its video output card. If there is any loose solder terminals, especially to its blue color output transistor, or related part/s. If this happens, the voltage at the blue cathode will go less than normal related to the other two [Red & Green] cathodes of the CRT. if this happens or the output transistor itself is leaky, this kind of fault can occur. You have to check by adjusting the screen voltage to a little less first. The screen voltage pot-meter can be located at the EHT transformer itself; marked as 'Screen'.
Try this first. If it do not help you, replace the blue output transistor at the video output card [the card that can be located at the neck of the picture tube itself, will directly plugged on to its back]. Check all the soldering there first. Re-solder all the suspected solder terminals, by applying a little more solder. I hereby type in this details, assuming that, you're somewhat handy to repair this set, as per your descriptions.
If you wish to get some details, check the site linked here. http://electronicshelponline.blogspot.com/
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