Featured
Latest
Colour on B&W s...
 
Share:
Notifications
Clear all

Forum 141

Colour on B&W sets

10 Posts
6 Users
0 Likes
891 Views
Focus Diode
(@focus)
Posts: 729
Honorable Member Registered
Topic starter
 

Having worked on a few BRC 950 and related sets I've discovered this phenomenon not present on other B&W models.

Yes the Chroma dot effect is present on mistuning yet the colour remains B&W if you know what I mean!

Here's a couple of examples, the first from the 13" GEC 2015 portable which is pure B&W then compare with the 16" 960 portable where the colour info actually glows! Sets running on 625 of course.

Edited_ImageEasyImageEditor_20170103_303.jpgEdited_ImageEasyImageEditor_20170103_307.jpgAnyone else encountered this?Freeview-test-card.jpg

 
Posted : 07/01/2017 5:52 pm
Nuvistor
(@nuvistor)
Posts: 4594
Famed Member Registered
 

 I am using my phone to see the photos, presume it's the second and third photos that are showing the problem.

It looks like poor video frequency fault to me, at one time it was named 'plastic picture'.

If you have a sweep generator check the IF response and components in the video amp. If the picture is turning silvery towards negative when you turn up the brightness it may also be a low emission CRT  that is making it look worse.

Frank

Frank

 
Posted : 07/01/2017 6:09 pm
PYE625
(@pye625)
Posts: 5121
Famed Member Registered
 

I see exactly what you mean, it is like the colour is being amplified along with the luminance and fed to the CRT.

The GEC might have a more selective IF response that filters out the colour sub-carrier?

  

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 07/01/2017 6:40 pm
Nuvistor
(@nuvistor)
Posts: 4594
Famed Member Registered
 

Is there a 4.43Mhz rejector in the video amp, could need adjusting.

Frank

Frank

 
Posted : 07/01/2017 9:23 pm
sideband
(@sideband)
Posts: 4216
Famed Member Moderator
 

I think it does depend on the video response. The sub-carrier is at 4.433MHz so well within the 5.25MHz capability of the video amp of a B/W set. I think some later sets had a rejector (basically a notch filter that had a sharp cut-off at 4.43MHz). Early sets may not have been fitted with them. My Pye 11U (1964) will show chroma dot pattern if carefully tuned. Slight off-tuning gets rid of it but of course at slightly reduced video response but it really doesn't notice on a normal picture.

 
Posted : 07/01/2017 10:16 pm
Focus Diode
(@focus)
Posts: 729
Honorable Member Registered
Topic starter
 

I don't see this as a fault condition at all, I rather like the effect! Just noticed sets with the 950 chassis and their variants have this "feature". It only really shows up on a Test Card and not normal viewing.

The 11" portable is also prone to this although not quite so pronounced.

Nuvistor's right about the CRT in the 960, it is passed its best. Perhaps they'll be a CME 1602 out there somewhere.

 
Posted : 08/01/2017 10:55 am
Nuvistor
(@nuvistor)
Posts: 4594
Famed Member Registered
 

The Thorn BW range of sets were made to service its large rental market and they tended, in my opinion, to cut corners to get the costs as low as possible. Sometimes it worked out other times not so good. If the sets were accepted by the customer that would be all that would be required.

They made and rented a lot of TV's though and always easy to repair,  cannot take that away from them.

Frank

Frank

 
Posted : 08/01/2017 11:56 am
turretslug
(@turretslug)
Posts: 567
Honorable Member Registered
 

That looks like a rather striking example of subcarrier rectification- one of the acknowledged shortcomings of NTSC/PAL-type encoding, I get the impression that the attitude during development was, "Oh,doh_gif , another compatibility shortcoming, oh well, not that many domestic mono TVs have a great HF response in their video amps, so there won't be much subcarrier left by time it gets to the tube...." Curious that it should be so marked in these particular sets, perhaps the video amp response is HF peaky. Quite possibly, I'm grabbing entirely the wrong end of the stick here, and it's something else completely, though.

 
Posted : 08/04/2017 9:32 pm
Nuvistor
(@nuvistor)
Posts: 4594
Famed Member Registered
 

If there was colour on the screen however 'wishy washy' it was a colour set.

What this thread is discussing is the dot crawl effect that can be seen on some BW tv's due to the 4.43Mhz colour subcarrier. The PAL system used an offset of the line frequency that was 75/25, the NTSC system used 50/50. The dot crawl is less noticeable on NTSC due to the 50/50 cancellation, the cancellation is incomplete on PAL. On some sets it's worse than others, the dot crawl can look a bit sparkling especially if the crt is poor emission.

You can get similar problems the other way, i.e. BW areas of the picture giving the wrong stripe colour. Fine detail on tweed jackets and shirts. Always avoided on camera if possible. The fine detail is around the subcarriers frequency and the CTV think it's a colour to reproduce. I seem to remember it's called cross colour.

Frank

 
Posted : 08/04/2017 9:41 pm
Jayceebee
(@jayceebee)
Posts: 2014
Prominent Member Registered
 

turretslug said
not that many domestic mono TVs have a great HF response in their video amps, so there won't be much subcarrier left by time it gets to the tube...." Curious that it should be so marked in these particular sets, perhaps the video amp response is HF peaky. Quite possibly, I'm grabbing entirely the wrong end of the stick here, and it's something else completely, though.  

I think that's plausible, looking at the frequency gratings with the 960 I can see that 4.5MHz is very well defined and can even see 5.25MHz whereas on the GEC 4.5MHz is not well defined and there is no 5.25MHz response at all.

John.

John.

 
Posted : 08/04/2017 10:37 pm
Share: