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Old 3rd Feb 2020, 6:40 pm   #1
FERNSEH
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Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Another Bush TV12.

My "Birmingham" Bush TV12 had a no sync fault. Tried another EF50 sync separator valve but no improvement. The 'scope confirmed that the full video vaveform was present at the grid pin of the sync valve but closer observation showed that the sync tips weren't clamped to chassis potential.
As a quick test a silicon diode was connected between the grid of the EF50 and chassis, sure enough the sync tips are now at chassis potential. Turned out the control grid contact on the valveholder is broken. A replacement has been fitted and now the set displays a steady picture, but there are still a number of other faults to sort out.
The first picture shows the high quality ceramic B9G valveholder. Seems a pity to use it for such a mundane task as in the sync separator.

DFWB.
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