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CTV [Sticky] 1974 HMV 2726 Coloumaster; Thorn 4000

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Cathovisor
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Well, you could do - what have you got to lose (apart from pads on the PCB)? 

If it's still faulty - well, you know what's next but geometry aside, that 4K is going to produce an excellent picture when completed. 

 
Posted : 05/08/2022 2:59 pm
Jayceebee
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Posted by: @cathovisor

Remember, you've changed one of the most critical parts in a PAL decoder. If that antiphase reflection has gone on TCF, I'd consider this a win... but you will be looking at a decoder lineup. 

Yes, although the looks red looks worse due to Hanover bars everything else is better and some realignment will sort it.

John.

 
Posted : 05/08/2022 3:06 pm
crustytv
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There are three sets of instructions for the Chrominance module set up, it depends on the module fitted. The modules are; PCA430, PCA438 and PCA428, mine has the latter fitted.

setup

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Posted : 05/08/2022 3:20 pm
Cathovisor
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Oh, that doesn't look too arduous...

I'm really enjoying following this thread. 

 
Posted : 05/08/2022 3:31 pm
Jayceebee
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Follow page 9 and I think it will be ok to skip step3. 

I'm also enjoying this as you can probably tell. 😀 

John.

 
Posted : 05/08/2022 3:37 pm
crustytv
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Exactly what I thought, I was sure it was far more complicated, I'm sure on other sets I've tackled, it has been, which is why I was initially reluctant. I'll tackle it this evening and post updates.

I think you're right, Mike, the 4K looks on track to give a very good account of itself. It will be interesting to see how the other 4K compares, especially being a 20" and much smaller cabinet. Will the confines of the compact cabinet be detrimental to the performance.

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Posted : 05/08/2022 3:38 pm
Cathovisor
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Posted by: @crustytv

Will the confines of the compact cabinet be detrimental to the performance.

Well, there's only one way to find out now you're on a 4K roll... 🙂 

 
Posted : 05/08/2022 3:44 pm
Jayceebee
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@Cathovisor Can you advise what are they referring to here. Vectorscope?

delay

John.

 
Posted : 05/08/2022 3:49 pm
Cathovisor
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@jayceebee No, not a vectorscope but some test signal generators used to put out some very specific colours for decoder alignment. One such was a sort of brownish colour? When you looked at bars on a vectorscope you adjusted the phase control so the tips of the burst sat on the marks on the graticule and then hopefully the bars sat in their respective boxes too. Any errors quickly made themselves known; a more critical test was to switch the vectorscope into a mode where it showed only one burst angle and you overlaid them with the phase control: if there were any "burst to bars" errors the dots of the colours would appear split.

If memory serves, you should be able to see the tops of the decoded colours "twitter" in  - as we called it - so you could see the difference in amplitude  on a line-by-line basis. You then just adjusted for minimum "twitter".

In studios we ran the monitors in Simple PAL: not only did this get rid of the "drop shadow" of the one-line delay but setting the decoder was a breeze - you just looked at the magenta bar and adjusted the phase control until the Hanover Bars disappeared. 

 
Posted : 05/08/2022 4:12 pm
Cathovisor
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I think I remember the signal now: it was in Test Card G or the PM5544.

If you look either side of the centre circle there are two vertical bars of an odd colour: if I remember rightly, in a correctly aligned decoder these appear as a solid grey when you turn the colour right down/press the 'mono' button.

Test Card W - the widescreen one - has these bars along the bottom and just up the LH edge. 

 
Posted : 05/08/2022 4:15 pm
crustytv
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Posted by: @cathovisor

some test signal generators used to put out some very specific colours for decoder alignment. One such was a sort of brownish colour?

I think my TEK TSG95 does this, here are two patterns.

A colour bar that appears to start with brown, called PAL mod Step 5

20220805 172805

The TSG has its own test card, and has all sorts going on in it.

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Posted : 05/08/2022 4:26 pm
Cathovisor
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Posted by: @crustytv

A colour bar that appears to start with brown, called PAL mod Step 5

"PAL mod 5 step" is a signal you'd use to check that you didn't have variations of chroma level with luminance: if you extract the chroma you should see a perfectly level envelope - any variation with luminance and you have diff. gain.

The burst is brown...

Incidentally, the level of your test tone is 0VU - which is +4dBu.

 
Posted : 05/08/2022 7:08 pm
crustytv
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Before embarking on this, I was familiarising myself with all the pots to set. I tweaked R226 and the colour bar looked good. However, when adjusting the user colour control, it behaved incorrectly. Hence, embarking on the alignment now.

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Posted by: @jayceebee

I think it will be ok to skip step3. 

OK step 1 and step two completed, however all colour is lost now, I see very faint "barber pole". Surely I have to now do 3?

20220805 200410
20220805 200618

 

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Posted : 05/08/2022 7:13 pm
Cathovisor
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Just follow the alignment process to the letter - it's only five steps after all.

The rewards will be worth it.

 
Posted : 05/08/2022 7:29 pm
crustytv
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Well, I tried step 3 and could not get locked colour, I went from one end to the other, it just stays grey with very, barely visible unlock bars.

I went through steps 1&2 again, setting 7V, then 1.2V. Then connected a .1uF cap as instructed, and still could not get lock when adjusting c207.

I set the pots back to the voltages before, which were 6V & 479mV

It looks fairly respectable now, but not right, I'm totally confused as to step 3 not working.

20220805 203554

I'm stopping for now as I've obviously been at it too long

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Posted : 05/08/2022 7:39 pm
Cathovisor
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You'll get there 👍 

 
Posted : 05/08/2022 7:50 pm
Jayceebee
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Not sure you get full lock when carrying out step 3 with the 0.1uF cap fitted as the 4.43MHz is running free. You adjust for floating colour, the more horizontal coloured stripes there are the further you are away from 4.43MHz. Think of it as shorting out the flywheel stage on a line oscillator and trying to get a floating picture.

You’re not far away now.

John.

 
Posted : 05/08/2022 7:57 pm
crustytv
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I don't think I explained myself well, there are no floating colours at that stage whatsoever, just grey scale. How can I adjust for floating colour when there is none?

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Posted : 05/08/2022 8:07 pm
Jayceebee
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No, my mistake. I took it those pictures of the unlocked colour were with the capacitor attached, i realise now you hadn't fitted it then. I'm not sure how you could get much colour at all with the cap attached as I suspect it will just about remove all the chroma info being fed into the ACC stage.

John.

 
Posted : 05/08/2022 8:17 pm
Cathovisor
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What I'm finding odd is that putting a 0.1uF cap across 13/1 and 13/2 would surely short the chroma going into the IC entirely to ground so indeed, how could you tweak the reference oscillator at this point - or am I misreading the circuit?

Edit - crossed with @jayceebee

 
Posted : 05/08/2022 8:18 pm
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