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Old 4th Nov 2018, 1:47 pm   #1
Colourstar
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Default Bush CT187CS - The colour problem....

This monster 25" Bush A823, rescued some years ago from a derelict flat (with a tree growing in it) above a closed fashion boutique in Wales, has always had colour issues. To get it this far I am greatly indebted to the knowledge and generosity of two Davids- First was DavidH of this parish who very kindly went over the decoder board and had it running perfectly in his own A823 with photographic evidence to prove it! When re-fitted to my set however, things were still awry. Blue was fine, but red and green were rusty orange and a murky ochre respectively, with heavy hanover blinds.

Enter the second David, in the shape of Mr Boynes who had an enjoyable (?) afternoon recalling the days when this chassis was his bread and butter, tweaking coils and trimming trimmers with the result that we now have correct colour. Reds and greens and colours derived from them are still rather prone to hanover blinds although further tweaking has improved them considerably.

Considering the set is still working on 95% of it's original components, it's doing very well, particularly having sat unused for maybe 15 years or more. Some cap changes are probably wise now, particularly in the frame circuit which is a bit prone to bounce. Some electrolytics are dated June 1970.

I'm sure Fernseh will be along shortly to explain the problems further, but here are some photos showing all the right colours in the right places....

Steve
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Old 4th Nov 2018, 4:02 pm   #2
Welsh Anorak
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Default Re: Bush CT187CS - The colour problem....

I hope the tree wasn't growing in the TV...mind you it is a Bush!
Just goes to show that these one-chip decoders could give a good picture, far better IMHO than the 2-chip ones. Like the Bradford, they never seemed to be set up quite right at the factory, with slight Hanover bars and a brick-red colour. Both problems could be easily corrected.
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Old 4th Nov 2018, 6:10 pm   #3
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Default Re: Bush CT187CS - The colour problem....

That's a really cracking picture. I've had a 20" A823A for about 12 years, but the colour has never been great on it, brick reds and dirty yellows. I have been meaning to make up a set of extension leads for the decoder for some time.

I never thought I would come across another, however I have picked up another 2 in the last month, the 20" A823B in the other thread and a 22" A823A with legs.

I'd love to get a single chip decoder panel to see the difference.

John Joe.
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Old 4th Nov 2018, 8:10 pm   #4
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Default Re: Bush CT187CS - The colour problem....

Ah that's interesting then, as those problems are pretty much what mine has been suffering from. Orange reds, dirty yellows amd hanover blinds. The colour bars looked horrendous! Careful adjustment of the trimmer just below TP6 on the decoder board cured the incorrect colour, although the hanover bars are still problematic to some extent.

One oddity is that when the user colour control is advanced, the colour bleeds in from the top of the screen downwards, thus you can have a picture that is colour at the top and b&w at the bottom. The servicing article in the Nov and Dec 1977 editions of Television mag has lots of good info.

Steve
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Old 4th Nov 2018, 8:29 pm   #5
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Default Re: Bush CT187CS - The colour problem....

Hi.

I read in the Television magazine that it can be a real problem correcting Hanover blinds on the A823 series. If the Chroma delay line is out of spec then this is a very likely cause of blinds that can't quite be adjusted out. I think it affected the ITT made delay line as opposed to the Mullard unit.

Regards
Symon
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Old 4th Nov 2018, 8:40 pm   #6
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Default Re: Bush CT187CS - The colour problem....

Hi Steve,
I enjoyed working on the set, it must be over thirty-five years since I last repaired a Rank A823.
Getting started. The oscilloscope confirmed that the switching bistable was working and the burst gate was correctly adjusted.
The Rank A823 differs from most other colour TVs inasmuch it employes a passive sub-carrier generator. The swinging burst is converted to a constant phase and used to "ring" the sub-carrier crystal. Having confirmed the 4.433Mhz sub-carrier was present it was time to move on to getting the colours right. The blue was easy, just a tweek of the blue phase trimmer produced a perfect blue colour bar. But the red bar was some horrible yellowy colour. The R-Y sub-carrier was present at the SL901 integrated circuit pins 17 and 18. I was becoming convinced that the fault must be in the delay line circuits. Adjusting the direct R-Y gain control 3RV3 solved the problem, or it appeared to do so but when the PCB was flexed the fault reappeared. A dry soldering joint perhaps? All the PCB solder connections are good so I decided to take a look at the internals of the DL20A delay line. Re-soldering the connections on the delay line's PCB seems to cure the fault.
The is still some evidence of "Hanover Bars" but despite that the pictures are very good indeed.
I got my suspicions about the delay line, so we're on the lookout for another DL20A
The set has an excellent Thorn "New-Life" CRT. The A823 has the clever convergence board which simplifies adjustments.

There are lot of aspects of good design in the Rank A823 although electricity supply companies did not like the thyristor power supply very much.


DFWB.

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Old 5th Nov 2018, 11:01 am   #7
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Default Re: Bush CT187CS - The colour problem....

An excellent day out indeed. Great to see the set working so well.

Brian
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Old 5th Nov 2018, 11:19 am   #8
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Default Re: Bush CT187CS - The colour problem....

The automatic colour control (ACC) in the A823 needs some explanation. The only part of the chroma that can be considered constant is the burst signal. The burst is gated and rectified and supplied as a positive going control voltage to the to the emitter of ACC transistor 2VT7, see attached circuit diagram. The amplified ACC voltage is supplied to the base of chroma IF amplifier 2VT8. The BF196 has the forward gain control characteristic, an increase of current through the transistor reduces the gain.
The user colour colour control varies the voltage applied to the ACC transistor 2VT7. Turning the control clockwise increases the positive voltage to the base of VT7 which in turn reduces the collector voltage thus cancelling out the gain control voltage from the rectified bursts. So in a sense the colour control is inside the ACC loop.
In the early A823 the user colour control always worked in a strange manner, only a rather limited range of adjustment. it's as if the ACC is trying to defeat the adjustments of the user colour control.

DFWB.
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Old 5th Nov 2018, 1:40 pm   #9
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Default Re: Bush CT187CS - The colour problem....

I have an old single chip decoder I salvaged from a pile of junk. It will have the correct delay line, i suspect. The board's yours if you'd like it. Though it's beyond saving it will yield some spares.
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Old 5th Nov 2018, 9:33 pm   #10
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Default Re: Bush CT187CS - The colour problem....

Thanks very much Glyn, I think I may be taking you up on that offer...

Steve
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Old 6th Nov 2018, 1:54 pm   #11
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Default Re: Bush CT187CS - The colour problem....

I may have spoken too soon. It's all there, but the delay line has been knocked over pulling the connections out of the case. I've had a look, and the small PCB is broken. However, the glass line and the coils are OK so could be transferred to your board without too much trouble. The rest of the board is a bit battered, but is complete.
PM me if you'd like the lot r just the delay line - just the postage costs, of course.
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Old 10th Nov 2018, 11:19 pm   #12
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Default Re: Bush CT187CS - The colour problem....

Hi Steve, If Glyn's delay line doesn't work I've got several scrap A823 decoders that should provide a decent working unit.


Cheers
Neil.
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Old 12th Nov 2018, 9:03 pm   #13
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Default Re: Bush CT187CS - The colour problem....

Hi Neil, thanks very much for the offer. If I get stuck for a panel, I'll message you!

Steve
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