Thanks, do i need to turn down, contrast, brightness etc to achieve zero beam current?
Sorry, yes I should have made it clear, turn brightness, contrast and colour to minimum.
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Thanks Chris, I’ll give that a go tomorrow, there’s one other thing about this era of TVs that’s got me puzzled, bit of a daft question but when the set is off the screen looks grey, how does it achieve black when it’s on?
when the set is off the screen looks grey, how does it achieve black when it’s on?
I'm not sure I get what you are inferring, the grey appearance of the glass face-plate has nothing to do with black level reproduction. The phosphors, guns, shadow mask and associated circuitry of the CRT TV do. Perhaps you're confusing the "black" look of modern flat panels as somehow being equally responsible for black level, which of course is not true either.
The circuitry of the CTV ensures that each gun reproduces the three pictures accurately upon each other, whilst ensuring the three primary colours are mixed in the intended proportions at all brightness levels. This is critical when displaying a black and white picture on a CTV. Your TV uses RGB drive, the signals are fed to the cathodes with the appropriate levels of DC bias to feed their control grids. The level of the signal representing black is fed to the cathodes and accurately maintained at a fixed value with respect to chassis despite changes in signal amplitude. Simple "grey-scale" tracking sees the DC grid bias fed to all grids, usually at between 100-150V-ve with respect to the value for black level in the cathode signals.
Some CTV's and their circuits produce outstanding blacks/whites such as the Decca Bradford, others such as the RBM A823, very soft and some might say entirely lack lustre pictures.
Not sure if any of that answers your question and there are far more technical folk here from the trade, both service and broadcast who can likely provide a more comprehensive answer than I.
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when the set is off the screen looks grey, how does it achieve black when it’s on?
A good question.
Take a cloudless night sky and stand near to a bright lamp. The sky looks black. Turn off the lamp and you see the sky is not black, but full of stars. In other words, it is the contrast of lighter areas on the screen that make the darker areas look black.
If I'm talking nonsense, I'm sure someone will correct me. ?
To understand the black art of electronics is to understand witchcraft. Andrew.
That makes a lot of sense to me, a bit of an illusion, like motion or seeing colours other than primaries from the three phosphors.
Found this article interesting:-
https://cosmosmagazine.com/biology/we-see-contrast-because-our-eyes-are-always-moving
Think about the really old round CRT's that had a virtually white phosphor...I remember my old Ekco T283 with a CRM141 that was just like that. To improve the contrast ratio of those old tubes, the manufacturer put a tinted toughened glass in front of it that also provided implosion protection. Most were tinted with a purple colour although some where pink if I recall correctly. Viewing a picture without the tinted glass gave a very poor contrast....the whites were whiter than the white phosphor which made everything in between look grey. Turn the light out however and the contrast improved enormously. With the tinted glass in front, the contrast was improved for daylight viewing....most TV manufacturers then recommended viewing in 'subdued' light.
Strictly speaking, there was no black....there was white and shades of grey and black was the colour of the phosphor. Our college lecturer described it as 'an absence of light'....basically during black parts, the beam is cut off.
I remember during the early 70's when I first started in the trade, some Bush sets appeared with a very dark phosphor....like charcoal grey and I always thought they gave excellent contrast pictures. I think they only had a clear glass in front or it was only very slightly tinted.
most TV manufacturers then recommended viewing in 'subdued' light.
And to this day television production galleries are always worked under subdued ('operational') lighting.
I remember during the early 70's when I first started in the trade, some Bush sets appeared with a very dark phosphor....like charcoal grey and I always thought they gave excellent contrast pictures. I think they only had a clear glass in front or it was only very slightly tinted.
The Rimband CRTs introduced towards the end of the 60s made it impossible for the faceplate to shatter, so later sets, such as you remember, had no need for a separate implosion screen. It was the glass of the crt that was tinted or, as you suggest, the phosphor.
When all else fails, read the instructions
Ooh crumbs! This will teach me for not reading the thread properly ? ! I joined this very late on and saw the picture of the set and was thinking it was a late model A823B! Now I see it's not but is in fact a Z718, another set that I have so ignore me - other than the pat on the back bit ?
Tas
I took the opportunity to re-jig the dining room furniture while the better half was in work so have got the TV in its final placement. I needed to install an electrical socket (lucky there was enough slack on the cable feeding the socket in the conservatory), also fed through the aerial cable from said conservatory. I’m using a DVD/VCR combi as modulator for the Freeview box so have siliconed some angle steel lengths under the combi so it can perch under the TV.
Home and work life been a bit manic over the last week or so but still got the mains hum issue on my todo list for this set.
John
Was the rejigging of the room approved and signed off? ?
Looks very nice in that position.
Frank
ha ha - yes-ish, sometimes it's better to seek forgiveness instead of permission.
Back on the set again this week, the old digital-box gave up so picked up a replacement from Argos, not a bad little piece of kit for the price (£35), found nearly 200 channels on a loft aerial. I tried replacing 7C1 & 7C2 with equivalents from Cricklewood but the pesky hum persists.
The DVD box I’m using as a modulator does seem to be impacting the end result though as you can make out a faint outline of it’s screensaver when watching TV so I’ll see if I dig out an alternative device.
