Dumont RA-110 from ETF

[Penthode]

Originally Posted by old_coot88

Wow. Are you serious about connecting it to ground?
The only way i've ever done it or seen it done is to draw an "air arc" with an ungrounded but well insulated screwdriver.
The air arc is just about as long anyway since it's RF. And it doesn't risk overloading the xfmr as a straight-to-ground current path might.

Yes, I'll ground the screwdriver. I just make sure I do not ground the HV Plate lead.

I just want to ensure the current goes to ground and not through me! One should be able to draw an arc when the tip is about 1/2" away. This is a quick test and should harm the circuit if a tube is used

[old_coot88]

Evidently it doesn't hurt anything. Guess it's just the thought of arcing it straight to ground gives me the willyjeebers.

[Kamakiri]

I test HV with a neon bulb. I was getting HV (although not as much as I'd expect) all the way to the cap of the 1B3, then nothing after. The resistor tests good. Maybe I'll replace the doorknob before I pronounce the flyback dead. Where do I source those, anyway? Or what style do I use? Never replaced a doorknob before.

[Username1]

Without a HV meter, the neon bulb is the best way to check HV, They glow with anything
above 50V, and if you do some experimenting with the bulb and a working set, then you
have an idea how close you have to be to a HV source for it to glow.... A working HV
source will set it glowing X inches from the 2nd anode wire, x inches from the transformer,
x inches from the Horiz plate cap.... It was the first device we in tv repair school made,
and learned to use to measure HV at several spots in a tv.

Electricity will only travel from a positive - negative potential to the oppisite potential..
Same for an arc. If you don't ground the screw driver, then the next lowest potential
is through all high resistance paths to ground.... Like you and the handle.....

Like lightning, very high potential will overcome the high resistance of distance and
the poor conducting properties of air...

The neon tester we made in schooll was to test electrical outlets, It was made out
of a piece of house wire, a resistor, and the neon bulb, which was tucked into a piece
of the wire's insulation... Pretty neat little thing. So we held it by the wire to test for
HV, kinda held it backwards... But you have to be aware that the magnetic field, or
electrostatic field will be making the light glow, and inducing a voltage into the wire
as well.... So if you are not careful you will get a taste of it.... Or get too close and
get a spark to jump and get a bigger shock....

.

[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.

[Kamakiri]

I've never had the guts to play the arc-cap-to-screwdriver game. Neon bulbs are cheap.

[BigDavesTV]

Yep, I quit playing the "arc-cap-to-screwdriver game" 35 years ago, when I got zapped good fashion, in a Customer's home, from a 23" Philco tube color set, in a metal cabinet! I'll never forget lessons learned from that one!

[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

[Phil Nelson]

Originally Posted by Kamakiri

Maybe I'll replace the doorknob before I pronounce the flyback dead. Where do I source those, anyway? Or what style do I use? Never replaced a doorknob before.

I can only recall replacing one doorknob. Anyhow, Surplus Sales of Nebraska has some. You might need to adapt the mounting a little -- it has been a while.

http://www.surplussales.com/Capacito...cap_trans.html

(Hey, I didn't say they were cheap )

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Kevin Kuehn]

There's always a bunch on ebay.

http://www.ebay.com/sch/i.html?_from...citor&_sacat=0

[Kamakiri]

Replaced the doorknob, same result. Since my flyback tester told me that the flyback was shorted, and since everything else has been replaced, I'm going to stick with that diagnosis.

And now, the search begins.....