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Old 2nd Mar 2015, 12:58 pm   #128
Heatercathodeshort
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Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
Default Re: Marconiphone VRC74DA

The effects of a low emission tube are demonstated here, the receiver being a Pye V4 that I have recently restored and was soak testing on the bench.
The heater volts were cut by simply placing a 25 ohm resistor across the tube's heater to reduce the available voltage to around 3v in the series heater chain.
As can be seen the picture lacks all 'punch', is grey in appearance but is in good focus generally.
The room lights were turned off for these pictures that are produced from a digital camera, many times brighter than they are on the actual screen.
Any attempt to increase brightness or contrast results in overdriving the weak emission turning the picture the classic negative appearance and giving it an overall 'satin' look. Compare with the picture of the CRT working normally.
The CRT in this case is a Mullard MW36-24 tetrode but the symptoms are identical in all receivers from 1936 to date including of course the EMI 3/16 employed in the 1807 series. Hope this clarifies the diagnosis.
PS. As Simon has suggested, have you repaired/rewound the video peaking chokes? John.
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