Quote:
Originally Posted by earlyfilm
Re: +495 volts on focus anode on schematic
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.
James.
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James,
Something still doesn't add up. The focus voltage does not change much over the full operating range of the crt as the load is negligible. The RCA manual does indeed say that besides the Junior Voltohmyst you need the optional high voltage probe which allows measurements to 10KV. That must have been used only for the 7500V measurement as the Voltohmyst was capable of 1000V on it's own, so if the 495V is correct, the meter was used without the HV probe.
The loading of the meter is a fixed 11meg, not 11meg/volt, but even with this extra load, and with the focus control turned to the low side you still end up with 966V. With the control to the high side you get 1250V. I'm still guessing this was a misprint in the original RCA manual and just got duplicated in Riders and elsewhere.
I suppose one of us could go measure it on a working set. Since modern meters are 10M input impedance, any digital or VTVM should work.
Darryl