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Old 8th Feb 2019, 1:31 am   #15
mhennessy
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Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Microvitec Cub monitor model identification and repair

Quote:
Originally Posted by Argus25 View Post
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.
That is how beam limiting is done in the vast majority of sets that I've encountered. Only a small number of those had the dag grounded directly. The way it works is really simple: the dag (and negative end of the EHT generator) goes to chassis via a resistor. There is 25kV nominally between dag and EHT. Beam current - via the cathodes - passes from EHT through to chassis. Hence, beam current passes through the resistor that connects dag to chassis.

If you look at the Microvitec schematic, it's trivial to see how the potential of the dag, relative to chassis, is used to effect the beam limiting. In many other sets, the same signal is used to vary width and height to reduce "breathing".

If you'd like more details, there are countless explanations of this in manufacturers' manuals and in articles in Television magazine. Of course, there will be variations on the theme, but this is how it's usually done, I promise

The sets that did have earthed dags (often Sony) took the negative output of the EHT system to ground via a resistor. The effect is the same, give or take the time constant formed by the capacitance of the CRT. The Sony schematics can be a pain to follow, especially on a screen, but usually the width and height control elements can be fitted on the screen together with the line output stage, so it's easy to see how they work, and the signal from this that goes off to the signal processing parts of the set is usually labelled "ABL" (auto beam limiting) - knowing that helps you to follow a Sony schematic.



Quote:
Originally Posted by Argus25 View Post
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.
I agree that you don't always need to discharge the CRT - indeed, I said so in my first post in this thread, albeit with a suitable caution/disclaimer. Yes, you probably* won't get a shock from the tube base pins.

However, what you didn't say: you must keep the lead from the dag connected to the chassis (usually connects to the tube base PCB at least - for flashover protection - but there might be other connections in some cases).

If you don't do this, you have a 25kV battery connected to the chassis via the EHT cable, meaning there is 25kV between the chassis's PCB and the dag. You usually get away with this because the EHT generator usually contains no reverse path, but in this case, there is a chain of resistors joining EHT to chassis, and so you could end up with -25kV at the dag, relative to the chassis. It won't kill you - especially as it comes from a high source impedance - but there's a risk of damaging static-sensitive components, etc. Far easier to just discharge the CRT to eliminate all doubt IMHO.


* Be aware that some Sony CRTs contain internal resistors from EHT to various connections on the tube base pins. See attached...



Quote:
Originally Posted by Argus25 View Post
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.
Earlier I mentioned a homemade CRT discharge probe. The bulk of that actually came from a Philips TV set (a K12 IIRC). It was attached to the same braided wire that was sprung onto the rear of the CRT - the standard way to connect to the dag. An extra length of braid was then crimped onto this probe, provided by the manufacturer for convenience. The set was sadly scrap, but I spotted this and rescued it. Anyway, the point of relating all this is to say that it seems to be fine to short directly to dag - certainly, I believe that we can safely trust Philips in these matters - they probably know a thing or two about CRTs. See also the attached extract from the K30 service manual.

I also mentioned the dielectric absorption point - easily overlooked.



Quote:
Originally Posted by Argus25 View Post
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.
Yes, I agree that an EHT probe is an excellent way to do it. I have a Fluke 80K-40, which is basically 1GOhm. It's a relatively recent acquisition, and I haven't had the chance to try it with a CRT monitor yet, but I've used it to set up oscilloscopes. It's comedically huge. Inspires confidence though

Hope this helps,

Mark
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