UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - My first foray into Old model TV repairs. Ferguson 991T.

Heatercathodeshort · 6th Oct 2020, 8:10 am

This symptom has nothing to do with dampness in the scan coils, leakage and other suggestions.
It is purely caused by a low emission tube aggravated by electrostatic charges mainly on the screen faceplate.
It used to be a very common display with older receivers and looking back can only remember it occurring with Mullard tetrode tubes such as the 12" MW31-16/18/74 types, 14" MW36-24/44 and the 17" MW43-64.
It is interesting to note that all of these tubes are non aluminised and at a guess I would suggest that the aluminium backing probably prevents static build up. As Lloyd has mentioned it's just a matter of giving the tube full beam current for a couple of seconds to clear it. Very low emission tubes take a bit longer.

I don't know the type of glass employed in the early 50's tubes but it may or may not have contained a percentage of lead. This may have had a bearing on the fault. Regards, John.

6th Oct 2020, 8:10 am	#113
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: My first foray into Old model TV repairs. This symptom has nothing to do with dampness in the scan coils, leakage and other suggestions. It is purely caused by a low emission tube aggravated by electrostatic charges mainly on the screen faceplate. It used to be a very common display with older receivers and looking back can only remember it occurring with Mullard tetrode tubes such as the 12" MW31-16/18/74 types, 14" MW36-24/44 and the 17" MW43-64. It is interesting to note that all of these tubes are non aluminised and at a guess I would suggest that the aluminium backing probably prevents static build up. As Lloyd has mentioned it's just a matter of giving the tube full beam current for a couple of seconds to clear it. Very low emission tubes take a bit longer. I don't know the type of glass employed in the early 50's tubes but it may or may not have contained a percentage of lead. This may have had a bearing on the fault. Regards, John.