VECTOR: G05_802 Glowing Blob in middle of screen ?

Hi Guys... Just joined this list because I've aquired an Asteroids Cab...

I'm Based in the UK near Liverpool and have a reasonable bit of electronics
/ tech background.

 To start the ball rolling, I'm currently having a problem with the monitor
and hoping someone can help me please.

 I'm getting a glowing blob in the middle of the screen with the controls
set up to give a fairly nice picture.

 I've done a 'mock up' picture of what I'm seeing here...
 http://uk.geocities.com/[email protected]/Astyblob.jpg

 I've just set up the HV to about 11.5 Kv ( it was 12.7), I need to
 measure/check other voltages yet (its a bit hard to work on at the moment).

 With the game board powered down.. I still get this 'blob' , and the spot
 killer is illuminated. I've also tried another deflection board, and have
 the same problem. I have tried adjusting the brightness and contrast...
with
 the brightness right down, the blob is still faintly visable but obviously
I
 would have to play the game blind, If I turn the brightness  past normal
 levels, the blob becomes a sharp dot / spot like the your ships bullet.

 When I play the game, the blob is always in the middle, BUT when I go to
 high score name entry.. the blob dissapears like magic and then reapears on
 the next 'game screen'....

 I'm puzzled about this as this is the first Vector I've worked on.. Its
been
 suggested that the tube may need rejuvenating, but I can get a nice bright
 picture, whats the syptoms of a bad tube ?  I've not got another monitor or
 tube to try and do a swap test.

 There is a dot of screen burn in the middle, but its been suggested that
this is a symptom of my problem (when its been turned up) and not the cause.

If any one can shed any light on this please, I would be very grateful.

 Thanks for reading,
Regards, Colin