Old colour CRT TV only displaying black and white!

[Finndersen]

I've got my hands on an old colour Toshiba CRT TV, and its only input is RF antenna. So the only way to get video signal to it is with a RCA to RF modulator.

I've got a modulator that works and gets an image on the screen, but it's not colour. I've tested with a modern flatscreen LCD and a newer colour CRT and it works fine.

The image on the old CRT is monochromatic but can be tinted by adjusting gain on independent R/G/B knobs on the electron ray gun board (after taking cover off). Adjusting the "colour" knob on the front does nothing.

Any idea how I can get it working?? It doesn't seen to be decoding the chrominance properly or something. Would there be any any way to interface directly with the RGB signals to the ray gun?

I attached pictures of the TV and what it looks like with Red component turned all the way up

 

[davenn]

Any idea how I can get it working??

make and model would be VERY helpful
have you looked for a service manual or at least a schematic ?

 

[Finndersen]

Is a Toshiba Blackstripe C-1411 colour tv, can't really find anything about it online

 

[kellys_eye]

Does the set auto-detect NTSC/PAL and what system is the modulator using?

 

[debe]

A set that old if you want a circuit for it you will have to find some one who was a Toshiba agent some years ago. I was back in the 80s but have since sent all the service manuals to a paper recycler as they were taking up too mutch room & stoped repairing TVs 15 years ago as it wasn't worth it any more. LCD tv are soo cheap now.

 

[Finndersen]

I'm not sure if the set is auto detecting but it should be PAL cause I'm in Australia. I'm using a raspberry pi for the video source, set to PAL mode ( new lcd and other crt say pal mode when it's connected) I originally had a modulator which didn't handle PAL well, so now have the current one which works fine on the other TVs (colour and sound).
It's hard to come across any other RF video signals to test with it..

 

[73's de Edd]

Sir Finndersen . . .


" independent R/G/B knobs on the electron ray gun board"

Set those controls BACK to where they initially were and having the set producing a proper black and white grey scale.

Assuming that you are using a chan 3 or 4 RF signal coming into the set.
Tune in a broad cast or use a VCR or DVD signal to come in on 3 or 4 and then tune in to that channel amd confirm that you can then be turning the fine tuning control ( The small grooved black knob located concentrically within the CENTER of the VHF tuning knob). . . .usually, rotating to the right . . . to be WITHIN the tuners reception passband, to be able to receive color info.
Usually you will loose the picture and it will white out if turning too far to the right, then you come back just until that cross line interference just clears up and then there should be color drop in.
(If being tuned up with the main COLOR control at max , initially.)
Have that fine tuning knob TOO far to the left . . . and you will never receive color.
ALSO you need to have the contrast knob . . . that is also the AFC control switch . . . pulled out, so that it is not trying to correct . . .why you are trying to tune in. (Akin to auto pilot trying to override a pilot on the controls.)
When through, push in the contrast / AFC control to see if the sets color still holds

Thasssssit . . . . .

73's de Edd

 

[kellys_eye]

I came across this 'lack of colour' problem many years ago and it was similarly based i.e, a colour RF source would work on 'some' monitors but not all.... I was attempting to use colour CCTV cameras on the RF input of a particular LCD TV/monitor.

ISTR that this was caused by issues with the display device having poor colour burst signal processing (or the source having a poor colour burst signal quality). It was an issue I never fully resolved as it was simpler to use a display that actually 'worked' rather than spend too much effort on resolving the actual issue.

At the time, I posted requests for advice across various forums but I don't recall any specific replies that were any more useful than what I eventually 'did' and the matter dropped from my attention after the appropriate (working) display device was used.

If you ever resolve this issue I'd be keen to discover if my suggestion that the colour burst signal was the cause it would settle a distant nagging in my brain....

It 'may' be because the RF modulated colour signal is missing (or poorly presenting) the phase reversal part of the PAL signal....

 

[Finndersen]

So I've got some good news!! Thanks 73's de Edd for the info, it's definitely down the right track of my problem.
I actually "solved" the issue by adjusting a potentiometer on the circuit board of the TV (in the signal processing area). It appears this adjustment works similarly to the frequency fine tuning control you're talking about, cause I turned it to the right until colour showed up! But going too far and it will all turn to white noise again.

However it seems that the actual fine tune adjustment knob on the front of the set (inside VHF tuning knob) doesn't do anything... I turn it multiple times in either direction and nothing changes. It feels quite loose and can wobble around and turns very easily and can turn continuously.. doesn't feel like it's attached to a standard potentiometer, do you think it's broken?

Also the channels don't appear to match up, the RF modulator is set to channel 1 but the set picks it up on channel 5. I haven't been able to get it to work on any other RF modulator channel. It's also very temperamental, I can have it tuned in on channel 5, but then if I change to 5A or 4 and switch back, most of the time it won't pick it up again and will be just noise. I'll have to randomly switch back and forth into it finally works.
After I've adjusted the knob on the board to make colour work, it becomes almost impossible to tune in to the channel again either after turning off the system or changing channel. If I have it tuned in on 5 and adjust knob to get colour, then change to channel 4, channel 4 will have the audio and switching back to 5 is complete white noise. I usually need to adjust the board knob back again, tune in by randomly scrolling channels back and forth across 5 until it latches on again, then readjust knob to get colour..

What were you referring to by channel 3or4? Why those?
It seems like the tuning of this whole set is a bit out of whack

 

[debe]

The central Fine tune knob has to be pressed in to engage the adjustment as its turned.

 

[Finndersen]

Omg it's all working great now haha. I guess the fine tuning was out of whack when I got it! Is now working fine, has colour and can stay tuned after turning it off or switching channels

Although it goes B&W and loud noise after turning AFC off, is that bad? Can I keep it on all the time?

Thanks everyone for the help!! Turns out the main problem was me not knowing how to use these ancient televisions!

 

[debe]

You fine tune with AFC off.

 

[Finndersen]

70s de edd advice appeared to be that AFC is on while fine tuning?

 

[debe]

Not correct you tune first then use AFC if necessary.