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Old 23rd Nov 2020, 2:34 pm   #12
Heatercathodeshort's Avatar
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 7,874
Default Re: CRT Rejuvenation orientation

Another thing to note is that reactivation often changes the cut off point of the gun assembly.
The effect of this which requires a slightly higher grid voltage to maintain a crisp brilliance level. Usually this is well within the range of the brightness control itself but on rare occasions a reduction in the value of the resistor on the top of the brightness control is required to maintain a good range.

I think the microscopic space between the grid and cathode is reduced during the reactivation process. [Tiny amount of cathode material removed].

Another rather odd effect that Mullard tetrode tubes appear to suffer from is a very high resistance internal short probably between the first anode and grid but I am open to suggestions.

The effect is an overall lack of 'blacks'. Plenty of brightness and the contrast control usually appears to overload the video amplifier as it requires a high setting to maintain a reasonable picture. Often increasing the brightness control above an acceptable level also appears to increase the contrast!

My trick here is to reduce the first anode supply by about a third.

This is easily carried out by removing the supply from the A1 pin. Connect a short wire from the A1 pin [pin 10 Mullard Duodecal] to the slider of a 2mohm preset with its top end connected to the original supply wire and its bottom end to chassis. Adjust for the best black level then if you wish the potential divider can be constructed with fixed resistors. It's a good idea to fit a 220k resistor between the supply and the pot.

OK it's a bit of a bodge but in many cases it results in a much better picture with these very old but fully operational Mullard tubes.

Just to add it is very difficult to test for internal leaks in CRTs. Even with the very best of meters the very high impedance involved together with the microscopic current required to display a fault symptom.

Heater cathode shorts are often intermittent but are easily visually diagnosed with experience. It is the only short that can be detected with a test meter and then only if it is practically a dead short.

I worked closely with one of the best tube rebuilders in this country [Central Tubes]. He rebuilt just about everything, EMI Pyrex, Bonded, Rimband and colour.

He rebuilt the coded M5 BBC colour monitor tubes the screens of which were constructed with what was then known as the 'rare earth' red phosphor. The BBC wanted to keep all the monitors with the same colour match.

Believe me the CRT could produce some really strange symptoms. They were exciting times. Very hard work but you never thought of that back then.

I hope these jottings may be of some help. Put those tube blasters in the bin! Regards, John.
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