1972 Ferguson Colourstar 3713 - Thorn 8500 Chassis

Chris,

You might want to check my archived posts in the sale section in case any of the other bits came from the same set, in which case you would be welcome to any I have still got for the price of postage.

Niall

These are 100% bars which were not actually transmitted!

At the time 100% colour bars were too stiff a test for anything transmitted, but were used internally by broadcasters.

The BBC used 95% bars and the IBA used 75% bars - also known as EBU bars - EBU = European Broadcasting Union.

That latter needs a little qualification, as to many broadcast engineers EBU and 75% bars are very different beasts; the percentages refer to the saturation of the bars, but to a broadcast engineer "75% bars" are where all the bars are of 75% amplitude - so 'white' is in fact grey (see SMPTE bars). In EBU bars, white is the full 700mV.

The original BBC-generated colour bars also had a small portion of white again after the black bar, allowing you to determine where the edges of blanking were.

In 95% bars the chroma envelopes do not go below black level, contrariwise in EBU bars the envelopes do not go above white level.

Catho, you beat me to it on the bar type explanation. Saw the post last night, was too tired and too late to write coherently. The attached page from IBA Tech Review book 2 shows the 4 common types of bars. It doesn't really explain why 95% bars are so called. The chroma signal is the same amplitude as for 75% and EBU bars. I think it's related to gamma correction.

I'm struggling to remember why myself (and neither my Evesham notes or my copy of VTW are readily accessible at the moment) but gamma does come into it. I also remember the phrase "subcarrier rectification" as a phenomenon of the non-linearity of the CRT, but I think that had more to do with the display of colour pictures on monochrome CRTs.

95% "BBC" bars always look sunnier on the screen than the somewhat sombre EBU bars.

Occasionally referred to as "dirty bars" by some! 100% bars as you know are/were only ever used internally, and in the days of composite video they were used for correctly setting the saturation of a picture monitor by switching it to "blue only" and then adjusting the saturation control so that the bars appeared to be of equal amplitude. Quite often this feature is now achieved by the use of SPLUGE, which has a small rectangle of white/100%blue/white above the top white PLUGE rectangle, and again switching to blue only and adjusting the chroma control until the bars are of even amplitude. This was much easier when monitors were run in "Simple PAL" or PAL-S - i.e. the delay line (PAL-D) wasn't involved and you had a manual phase control so the one line delay wasn't showing on the transition between blue and white. Naturally, the phase control was adjusted on the magenta bar for nil Hanover bars.

SC rectification in the CRT helps to correct for the lack of constant luminance when a colour picture is shown on a monochrome CRT. It's purely a side effect, I don't think it was intentional but it helped compatibility.

That was it, thank you.

After a couple of tests recommended by other engineers and a retest on CRT tester, which confirms red at 10% Green and Blue on 85% this CRT looks to be useless. I'm chasing red when there is not enough to be had.

It does rather look like it is game over until I can get a new CRT, unless as I have nothing to loose now I try bopping the Red gun.

Just bopped it on the Leader, I now have 80% emission.Lets see what difference thats made.

We have red back, so all that faffing about of the last few days, this proved that the leader was right, 10% red emission was not enough to get this set back in the game.

Result ?  a testament to your dogged determination. Well done Chris, I really did think it was all over when the LOPT let go. Let's hope the emission holds.

John

Let's hope the emission holds.

John

Nope I watched it fall away as I was trying to set up. ? 

Just put it back on the leader it had dropped from 80% to 40% and is still falling. Looks to have stabilised for the moment on 30%, I suspect it may fall further. Its a 20%-ish improvement on what I had before but I doubt its enough to be usable.

I think I have to accept I've done all I can for this set, unless I can find rocking horse poo in the form of a suitable 19" colour CRT.

If its the original tube I expect that it has already been "reactivated" years ago these tubes were never the best.
Rob T

Emission has continued to drop and is now at 25%, still enough to provide a reasonable B&W picture, colour no. The colour bars have again degraded as a result of the red dropping of the cliff.

I've set this up with a reasonable grey-scale and thats where i'm leaving it here. Perhaps this story may continue if a CRT can be sourced.

Its along time until October when I plan to tackle the G6, so in the meantime, I think it might well be another thorn offering up on the bench next in the form of a multibroadcast 7773 Thorn 9000. Its as rusty as this fergy so I expect more fun and games. Lets hope it has a better outcome.

There are a couple of tubes that could be used as a replacement eg A49-191X or an A49-120X. This company is showing 1 of the latter in stock but I dread to think what they may be asking.

http://www.chelmervalve.com/TBC/A49--120X/20083

Also the Hitachi CNP190 often pops up on ebay, as mentioned earlier in the thread the CRT from this could also be used with some modification to the focus circuitry.

Hi all,

If there is nothing to lose on this CRT, would there not be any advantage in giving the heaters just a little more voltage - nothing silly or OTT, but just enough to lift the red emission to a useable level?

If that proved impossible to achieve, then stay at "Plan A" with a useable B&W picture.

Marion

Final pics to wrap this up

Black And White

Colour, if you can call it that

Ready waiting for a tube

It's a real pity about that poor red emission, and after all that effort.. One can only speculate that was the cause of this set's demise.

If I can get to the local junk auction, I'll keep an eye out for one of those Hitachi sets, or if the gods are smiling, an 8000!

Don't hold your breath though, otherwise there will be an overabundance of blue, and very little red again.

Just wondering, is it worth risking another 'bop'? I can remember some of those old G6s of ours needing two, and in some cases three, bops to get a decent emission, and keep it.

Marion

Just wondering, is it worth risking another 'bop'? I can remember some of those old G6s of ours needing two, and in some cases three, bops to get a decent emission, and keep it.

Marion

Already done that this afternoon Marion.

The leader has three levels of rejuvenation. I tried level one, that achieved nothing, I tried Level 2 that gave a minor increase, so I attempted the final level (Super Rejuvenate). That gave me the good result with 80% emission. The rest you know, shortly after the emission started to fade. When I put it back on the tester it had already dropped to 40%, I watched it fall further finally settling around 25% emission, which is better the the 8% I had at the start of the day.

I dare not give it a wallop again as I think it would probably strip whats left of the cathode. At least this is better than nothing. I did think about fitting a 20% boost TX. I briefly ran the CRT at 8.3 and Red climbed back up to 80% but I think I will leave it as is for now.

Hi Chris,

I think you've done all you can in that case, and wisdom should prevail.. As you say, something is better than nothing, and finishing off the cathode will achieve nothing.

I hadn't realised you had already gone three rounds with it.

There is a CRT out there, somewhere, I just know it.. All we need is a good bloodhound to go and find it.

Marion

One convergence panel is en-route to Chris, so let's see how it performs...

It will need a small amount of work to refurbish, but nothing major.

Marion

Excellent news Marion, as I mentioned to you the Green static on mine seems to be intermittent, despite the fix. All I need now is replacement CRT to replace my tired one. Not much hope there buy I've learned with the RGD LOPT anything is possible if you wait long enough.

Wow! that was quick, it arrived 5 mins ago.

Cheers Marion, When crt turns up for the TV, I will set about sorting this out, until then.....

Chris