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Old 29th Aug 2017, 10:15 am   #97
FERNSEH
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Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Re: Pilot PT650 "Spacemaker" Television

1100 man wrote: "I need to modify the temporary HT part of the mains dropper. At the moment, it's using one 100 ohm section which is only 1/2 inch long. This runs so hot that after a while the solder melts and the stench is unbearable According to my infra red thermometer, it runs at 250 degrees!! 100 ohms gives the correct HT though."

The total value of the tapped HT section of the mains resistor is 93 ohms. The reason why the resistor has a much lower value per section compared with the heater part of the dropper is because the ripple current of the reservoir capacitor has to be taken into consideration. The HT current demand of the receiver might only be in the region of 220mA but we have to bear in mind that the reservoir capacitor has to accept it's charge during the positive half cycles of the mains supply, hence the reason why the HT section dissipates so much heat. To sum up: receiver HT current + reservoir capacitor current.
I'll look through my stock to see if there is a suitable mains dropper available for the PT650.

DFWB.
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