Hi all,

I have finish building that scope kit.
I recappped all capacitors.

Electrolytics C10, C11 & C12 are rated for 450vdc, but
when i turn "ON" the scope, the B+ at C10 is too high at 500 Vdc, then stabilize to 470vdc.

I suppose electrolytics can be overloaded for a while, but not all the time.

According to the specifications, i should have 420Vdc at pin #8 of rectifier V4.

Unless i use a variac to operate the scope, should i add a damping resistor across C10 ?

thanks

Hi Vianney

Is line voltage high? If line voltage is 10 volts high this DC voltage will be 40 volts over.

Could use a bucking transformer or line dropping resistor on input to the power transformer. 500 volt caps are also available.

:Hi all,
:
:I have finish building that scope kit.
:I recappped all capacitors.
:
:Electrolytics C10, C11 & C12 are rated for 450vdc, but
:when i turn "ON" the scope, the B+ at C10 is too high at 500 Vdc, then stabilize to 470vdc.
:
:I suppose electrolytics can be overloaded for a while, but not all the time.
:
:According to the specifications, i should have 420Vdc at pin #8 of rectifier V4.
:
:Unless i use a variac to operate the scope, should i add a damping resistor across C10 ?
:
:
:
:thanks
:

You might also check the heater voltages. If they are also high your tube life will be diminished.

If the heater voltages seem OK you can just insert a dropping resistor between the rectifier cathode and the first filter cap. You will need to first measure the B+ current draw in order to select the correct resistor value; but for example, if your B+ current is 60ma, you will need an 830 ohm resistor to drop 50 V. A 5 watt resistor (derated from 3.3 watts calculated) would be adequate.