Quote:
Originally Posted by mhennessy
Sounds like you have a nice one 
As is usually the case, the dag is not connected directly to chassis - it goes via a resistor, and this is used to sense the EHT current and control the beam limiting circuitry. So my earlier advice to discharge to the dag, not the chassis, was good advice. OK, it's standard practice, but occasionally you meet sets where the dag is connected directly to ground.
Oh, and 25kV into 10Mohm is 62.5 watts. |
Since the glass bulb of the tube with its internal and external dag forms a capacitor, there is only current flowing in the external dag circuit when the capacitance is charging and discharging, so I cannot see a way that the current in the external dag circuit can be used to measure the average EHT current, the EHT current is the beam current and that can be measured at the cathode of the CRT. Most sets I have seen the external dag is grounded.Though there might be some value of measuring dynamic changes.
Also here are my thoughts on discharging the CRT:
There is really no need to discharge it most of the time. The charge is built up in the glass bulb between the internal and external aquadag, the internal dag is connected to the final anode clip. The internal aquadag is also connected to the final anode structure in the CRT's gun assembly, but that is insulated (by a vacuum) from the pins that connect to the tube socket and the other electron gun metallic elements that are connected to the tube base. I have never received a discharge of any kind unplugging the CRT base and touching its pins, even when the bulb remains charged. After all, if there was any leakage across the gun's final anode to focus electrode or other gun structures, the bulb charge would already have dissipated.
So with the set switched off, you can take off the CRT socket and pcb assembly off the CRT without being exposed to the charge stored in the CRT's bulb. You only require to discharge the bulb if you plan to take off the anode connection (EHT clip) which for many fault finding exercises is not required.
There is only one common reason to discharge the CRT, that is if you are removing the final anode cap and might touch that lifting out the CRT or carrying the CRT across a room and get a zap and drop the CRT on the floor as a result.
The total charge stored in the CRT could give an an unpleasant zap, but not too risky, but it might make a person drop the CRT.
Some people short the anode cap out to the external aquadag or ground, it is not really a great idea actually, without current limiting, as it is possible the dag on the internal connection to the anode cap could be damaged by very high peak currents. It would be better to do it with a 100M Ohm high voltage rated resistor, if you really want to do it.
Therefore, the best tool to do it with, is a standard EHT voltage probe that has this resistor already built into its body and you can slip its tip under the plastic EHT cap and its flying wire is grounded to the sets chassis or external CRT aquadag. So with the set switched on you measure the EHT voltage, with the set off, the probe gently discharges the CRT.
Using wires, screwdrivers etc etc, you might accidentally discharge it to a pcb component and create a new fault.