Re: +495 volts on focus anode on schematic
Quote:
Originally Posted by tubesrule
I'm not sure why they show that. The voltage on the focus anode is going to vary between 1475V and 2250V based on the resistor values shown. The RCA spec sheet for the 7DP4 says it should average around 1800V for 7500V operation. Definitely need to use caution and a high voltage cap.
Darryl
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Darryl,
RCA specified that all voltage readings were to be read with a Voltohmyst. Those meters were very sensitive service meters for the era and only have a 11 meg ohm per volt circuit load on DC. The WWII era Voltohmyst was limited to
1,000 volts DC, without the high voltage attachment. (Confirmed by the manual for my RCA 195-A, although the meter scale goes higher. Duh ! )
Note the little asterisk beside the +495 volts on the schematic.
The asterisk means they specified that pin 6, the focus anode, was to be measured
only with the brightness control turned fully counterclockwise. This would bias the Kinescope to cutoff.
I suspect cutoff would drop the focus voltage to within the range of the Voltohmyst. It also would increase the 2nd anode voltage to 7,500 volts, which most 1946 service shops could not measure.
This has nothing to do with curing the present vertical problem, but once the set is operational, it would be an interesting experiment to perform, since the focusing anode voltage dropping with beam cutoff, goes against conventional logic.
James.