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B&W TV EHT voltmeters for black and white TV sets.

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Till Eulenspiegel
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From the 1963 Quarington Radio and Television circuits and data book a circuit of a simple EHT voltmeter.
No indication of the polarity of the terminals but we can assume the base drive for the OC71 will be negative.

EHT Voltmeter

From the June 1957 Practical Television magazine an article about making a kilovolt-voltmeter.
https://www.worldradiohistory.com/UK...on-1957-06.pdf

EHT Voltmeter PT June 1957

I suggest these crude instruments must not be used to test mains derived EHT systems.

Till Eulenspiegel.

 

 

 
Posted : 27/02/2024 3:06 pm
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irob2345
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Funny thing, I have been working on TVs on and off for 60 years and I have never had occasion to use an EHT voltmeter or probe.

 
Posted : 28/02/2024 6:55 am
Nuvistor
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@irob2345 I had a EHT probe made for the AVO 8 meter, didn’t use it for mono sets but required for early colour sets that needed the GY501/PD500 rectifier/shunt stabiliser set up. Turn brightness down to cutoff CRT set EHT 25Kv and the current through the PD500 to 1.2ma, check EHT still 25Kv. Probably not quite the correct sequence, it’s a long time ago.

 

Radar made a commercial Kilovolter in the 1950’s, I didn’t have one.

Frank

 
Posted : 28/02/2024 8:11 am
Cathovisor
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Posted by: @irob2345

Funny thing, I have been working on TVs on and off for 60 years and I have never had occasion to use an EHT voltmeter or probe.

Strangely, I had lots of occasion to use them when working on professional monitors. Often part of the post-repair procedure.

 
Posted : 28/02/2024 8:32 am
Nuvistor
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@cathovisor I can understand that monitors for studio work would have a much higher quality picture needed than domestic sets. The feed was perhaps plain video with none of the PAL encoding.

Frank

 
Posted : 28/02/2024 8:37 am
Cathovisor
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@nuvistor Distribution was normally PAL, but some monitors had outboard decoders - they could switch between RGB feeds from cameras, caption scanners and the likes and then encoded sources would be routed via the decoder. Spot size was critical in flying spot devices like telecine and caption scanners.

The Barco CTVM3 monitors had separate EHT generators so we had to check them after repair, especially if a tripler had been replaced.

 
Posted : 28/02/2024 1:10 pm
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freya
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I have a new in box Kilovolt probe made by Radar i think, tried it a few times on B&W sets and it was accurate compared with my bench meter. My intention had been to donate it to Lucien.

 
Posted : 28/02/2024 6:43 pm
Lloyd
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My old RBM EHT meter does a great job, no batteries required! 

 
Posted : 28/02/2024 9:39 pm
turretslug
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I found a NOS 30kV AVO multiplier for not a lot long ago and acquired it as a curio, I get the impression that it was aimed at the likes of radar techs and others who would have a TS No.1 to hand. It'd be a bit clumsy to try and shove the tip under a TV tube cap. Basically a GRP tube with creepage/anti-slip guards and containing 2x 295 Megohm HV resistors in series. Also long ago, I made an electronic EHT meter for use at work with plus/minus 3, 10 and 30kV ranges, the multiplier consisting of 4x 500 Megohm 12.5kV resistors potted into a piece of 32mm plastic waste-pipe inside the case (I wanted to use 6 in series for a nice round 3 Gigohms, 10uA at 30kV but didn't have space). So that it could be reliably connected to CRT anodes, I made a through-connector from an ERO EHT cap, its head drilled out to accomodate an Araldited-in M10 Nyloc nut with the Nylon melted out, this making a secure and reliable socket for the existing CRT connector to plug into. I wish I'd spirited it out of the door when the place was closed down, it had served us all well for many years for both 'scope and monitor maintenance. I actually have all the parts to build another including HV resistors, nice big RS panel meter and Eddystone box, but no call for it nowadays really.

Posted by: @cathovisor

The Barco CTVM3 monitors had separate EHT generators so we had to check them after repair, especially if a tripler had been replaced.

I recall those triplers with the potted 350 Megohm feedback resistor on flying leads, I seem to recall that the original ERO triplers became unobtainable for our fleet of ageing CTVM3s, but some UK-based HV component manufacturer ran us off a batch quite cheaply.

 

 
Posted : 29/02/2024 5:28 pm
dtvmcdonald
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I have several EHT probes for various meters, as acessories.  They range from 90 megohms to 1 gigohm.

All are rated for 30 to 50 kV. I've used all of them with different meters, including 50 microamp analog panel meters, VTVMs, and digital multimeters. They all work and give correct readings just calculating the current or voltage expected from the source and considering them and the meter as a resistive divider.

However, digital meters don't like AC from these so one needs a cap across the connection to the meter.

Any resistor under about 200 megohms will give a nasty shock from a color TV.

Most of my probes are very vintage. One should measure the actual resistance. One need either a special ohmmeter, or you can make up 110 meg resistor with 5 22 meg ones in series. You can the resistance of series resistors by measuring separately and adding. You simply test something like 3 kV with the series string and the high resistor and ratio the readings.

 
Posted : 03/03/2024 4:17 pm
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