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Sharp CP1 (Rediffusion badge) - colour "flaring" to right

 
AidanLunn
(@aidanlunn)
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Posts: 773

Hello all

I understand this is a fairly common problem in colour TVs and I have seen it before on an old family TV (old 14" Sony AE1 that was unfortunately chucked out). Whenever there is enough current (past a given amount) to any of the three guns, the primary colour of that gun flares to the right. E.g. a Hollywood movie, the fake tan of the actors, red flares to the right, excess blue sky next to houses on Homes Under the Hammer, blue flares to the right and on the gold run board on Blockbusters (which has green hexagons), green flares to the right.

I've totally forgotten the cause of this - is it resistors and capacitors on the tube drive board or the matrix or is it in the colour decoder?

(PS, I had an interesting discussion with a heating engineer here who used to work on the benches at Thorn here in Bradford - anyone heard of a fault diagnosis computer that used a series of bulbs, whichever lit up would diagnose the sector of the set with the fault?)

The colour control is on full and the fault does not show up on B&W.

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Topic starter Posted : 02/08/2012 11:34 am
malcscott
(@malcscott)
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Hi, it is the Rediffusion SP1 chassis. These sets date from about 1981/82. The crt,s in these will be a bit past their best. Try increasing the A1 volts a little bit

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Posted : 02/08/2012 11:49 am
Terry
(@terrykc)
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... The colour control is on full ...

Aidan, the saturation (colour) control should never need to be on full so, assuming that this is necessary for a normally saturated picture, it points to overloading somewhere in the early chroma stages as it attempts to compensate for low gain later on, which is why ...

... the fault does not show up on B&W.

As the most prominent colour is affected, the fault is common to all three chroma channels, which might help location.

Check anything common - especially power rails, etc. Try tracing the signals back through the chroma circuits with a scope to see where the problem starts - preferably with a colour bar signal if you have one available.

When all else fails, read the instructions

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Posted : 02/08/2012 12:11 pm
Terry
(@terrykc)
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... Soft CRT I am afraid ...

My first thought, Trevor, but Aidan says it is ok on B&W.

If the tube was soft, I would expect similar symptoms - in B&W, of course! - if the contrast was advanced. I assumed Aidan had checked this possibility.

When all else fails, read the instructions

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Posted : 02/08/2012 12:38 pm
Refugee
(@refugee)
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I would check all the caps on the video panel before boosting the tube.
I have repaired VGA screens with something like that on them.

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Posted : 02/08/2012 12:40 pm
Anonymous
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i wounder if you could take a photo or two?
it could be flare from a a low emision tube or choma shift
what happens if you reduce the contrast ?
if the tube is low you could try shorting out the resistor that is in serries with the tube heaters,remember to remove the short if you replace the tube.
but like others i think if the tube is low it would be obvious on monochome as well.
rob t

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Posted : 02/08/2012 1:26 pm
AidanLunn
(@aidanlunn)
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i wounder if you could take a photo or two?
it could be flare from a a low emision tube or choma shift
what happens if you reduce the contrast ?
if the tube is low you could try shorting out the resistor that is in serries with the tube heaters,remember to remove the short if you replace the tube.
but like others i think if the tube is low it would be obvious on monochome as well.
rob t

Yes, it also goes down on reduction of contrast as well.

The colour is on max because that is the most acceptable - I've observed with all the sets I have with colour pots that the maximum on colour is about mid-range on later sets which had colour set electronically. So it's not "oversaturated", like you would get with colour turned right up on, say, something from the 90s. The focus is a little soft as well, so this may be signs of a tiring tube or it just needing an overhaul?

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Topic starter Posted : 02/08/2012 3:35 pm
AidanLunn
(@aidanlunn)
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Will a circuit diagram be helpful with these or are these as common as early 80s portables get? Nothing special about these apart from the on switch (press a main switch and then a standby button)

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Topic starter Posted : 02/08/2012 4:10 pm
Anonymous
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have you tried adjusting the focus and A1 controls ?
a picture showing test card c would be very usefull
rob t

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Posted : 02/08/2012 4:12 pm
Terry
(@terrykc)
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Wouldn't Test Card F be better ...?

A PM5544 (If I'm allowed to use such language here ...!) with its colour bars could be especially useful ...

When all else fails, read the instructions

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Posted : 02/08/2012 4:44 pm
Anonymous
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yes i ment test card f i realy think my brain is packing up i seem to get confused over the smalest
things nowadays.
rob t

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Posted : 02/08/2012 4:47 pm