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Old 14th Feb 2007, 12:29 pm   #6
Ray Cooper
Retired Dormant Member
 
Join Date: Jan 2007
Location: Porthmadog, Gwynedd, UK.
Posts: 199
Default Re: Baird T18 - focus problem.

To state the fairly obvious: the bulk of the focus coil current is provided by the line and field output stage valves' standing feeds. So if there's any problem here (V3 or V6 taking more current than they should be) this will push the focus control up to and possibly beyond the end of its range. It's well worth checking that these feeds are about correct (assuming that you can find out what they should be, that is) because if they're excessive this indicates a further fault which may cause damage.

Another consideration might be that if your replacement CRT has been standing out of use for a while, it may have become slightly soft. Small amounts of gas within the tube will certainly affect the focus. If so, the focus should improve somewhat as the tube gets further usage.
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