Dumont RA-110 from ETF

[N2IXK]

Grounding the screwdriver when testing for HV at a rectifier tube plate is definitely a BAD idea.

In most sets, the voltage at the plate of the HV rectifier has a DC component of a few hundred volts, in addition to the high voltage AC pulses. Providing a DC path to ground will take the B+ (from the HOT tube plate), and short it to ground through the HV winding on the flyback (200-300 ohms?) and the (low) resistance of the arc itself. This can easily burn the flyback winding open, as it is not designed to have any appreciable DC current running through it, and it is wound with very fine wire.

A grounded screwdriver should only be used to (briefly) check for DC high voltage at the CRT anode lead, and then only if you don't have access to a real HV meter or probe.

[Username1]

Originally Posted by N2IXK

Grounding the screwdriver when testing for HV at a rectifier tube plate is definitely a BAD idea.

In most sets, the voltage at the plate of the HV rectifier has a DC component of a few hundred volts, in addition to the high voltage AC pulses. Providing a DC path to ground will take the B+ (from the HOT tube plate), and short it to ground through the HV winding on the flyback (200-300 ohms?) and the (low) resistance of the arc itself. This can easily burn the flyback winding open, as it is not designed to have any appreciable DC current running through it, and it is wound with very fine wire.

A grounded screwdriver should only be used to (briefly) check for DC high voltage at the CRT anode lead, and then only if you don't have access to a real HV meter or probe.

The original statement was letting the hv arc to a screwdriver to ground....
No direct contact made.... Just a quick arc, no 1/2 hour welding job, popping
pop corn, roasting a turkey stuff like that..... One quick Zap! Yup there is HV, move on....

.

[N2IXK]

Originally Posted by Username1

The original statement was letting the hv arc to a screwdriver to ground....
No direct contact made.... Just a quick arc, no 1/2 hour welding job, popping
pop corn, roasting a turkey stuff like that..... One quick Zap! Yup there is HV, move on....

.

Understood.

But why take the risk, even briefly, when there is no need for the screwdriver to be grounded to make that test?

High voltage RF will arc to an ungrounded metal object just fine, without the danger of the DC following the arc path to ground and overloading the transformer winding.

[old_coot88]

Originally Posted by N2IXK

Understood.

But why take the risk, even briefly, when there is no need for the screwdriver to be grounded to make that test?

High voltage RF will arc to an ungrounded metal object just fine, without the danger of the DC following the arc path to ground and overloading the transformer winding.

This. Thank you Sir.

[Zenith26kc20]

Never had a problem with RF. The anode lead, on the other hand will usually whack you pretty solid. For that, I use a high voltage probe.

[kramden66]

Tim
not sure where to get a doorknob , i did buy some 15kv 470pf caps from mouser that will sub , you could always pull one from another set that you know is good just to test , or if you disconnect the one in the dumont and bring the set up on the variac and try to feed the set with 100 to 105 volts ac from the variacit should be safe and you may get an answer if the doorknob is at fault , had one take out hv in a rca , a dumont that uses three doorknobs had one that was bad and it caused all kinds of issues with not enough hv and a dim image............ i just noticed you still have the horz output tube in the socket when testing the fly , your supposed to remove it and the hv retifier and the damper , but the damper probably isn't in that hv box , i have the same set and have yet to restore it
mike