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CTV 1967 Baird M702: 700 Series Chassis: Dual Standard

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sideband
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Finally sussed it. :bba

For some reason C273 was missing from my board.

That's the problem with sets of unknown history. You never know what has been done (or undone) in the past. Excellent results Chris :aad

Now what about 405.....?

 
Posted : 28/08/2015 11:13 am
crustytv
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So what's the story on 405 line?

Now what about 405.....?

As I said the other day I had all manner of weird things going during that lock problem. I briefly looked at 405 and it worried me. I also stated I wanted to be sure of things before committing to post. When I switched to 405 briefly at that time I had a 1" high about 10" long defocussed raster. The line whistle from the set was deafening and didn't sound right at all. With 625 finally sorted I was more confident the tide was turning on this set and its protest at being awoken. Switching to 405 brought up a full raster but the line speed was way off. Adjusted R478, L431 & R402 405-line is working fine, just a little fold-over on the right, other than that, this set is done. The RCA tube seems to be getting better with prolonged use.

The black and white is superb on 405, I think I now need to adjust the controls to balance B&W on both 405 & 625, There is a procedure for this.

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Posted : 28/08/2015 11:58 am
Cathovisor
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Finally sussed it. :bba

For some reason C273 was missing from my board.

That's the problem with sets of unknown history. You never know what has been done (or undone) in the past.

Precisely - it might have been removed because it was diagnosed as being faulty, and was awaiting the fitting of a replacement part...

 
Posted : 28/08/2015 12:35 pm
crustytv
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One thing I have not done yet is check the EHT since first obtaining a raster.

During the troubles and the unregulated EHT with the arcing a spitting going on, as advised I fixed the "Set EHT" slider to minimum. I have not since measured EHT with my wand, frankly on three colour sets now I had flash-overs using them so its proved too risky. What I needed was a simple and reliable way to measure EHT without using the wand type meters.

Thankfully, in steps a good friend who had a professional Brandenburg EHT monitor going spare. After an exchange of beer tokens, said EHT meter arrived this morning. This meter is the dog's danglies when it comes to testing EHT on a colour TV as its in-line. You plug the input lead into the final anode of the CRT, the sets final anode lead then plugs into the top of the meters input lead. Safe testing with no risk of flash-over and the ability to constantly monitor EHT.

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Already I've noticed on TCF EHT is mainly static but does fluctuate by 1 or or even 2kV. Not sure I thought valve stabilised EHT was supposed to be rock solid. Its possible the Shunt valve needs replacing, I will dig a new one out and see.

The main revelation which leaves me stunned, I've been getting this cracking picture with just 15kV!!!!

I wound it up to just a little over 20kV and checked the EHT current was still set to 1.2V. This 20kV will do nicely and gives the old RCA tube lopt and tripler a gentle life. I've read its good practice to run with reduced EHT so all my TV's are set around 22kV.

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Posted : 28/08/2015 1:09 pm
Cathovisor
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15kV? Might be worth just double checking the EHT reading on the Brandenburg against a 'conventional' meter, just in case. These should read high by comparison due to the minimal current draw.

 
Posted : 28/08/2015 1:44 pm
sideband
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15kV? Might be worth just double checking the EHT reading on the Brandenburg against a 'conventional' meter, just in case. These should read high by comparison due to the minimal current draw.

Agreed on that! You have no idea of the accuracy or calibration of the new EHT meter. There are only a few milliamps to play with and it won't take much to pull the EHT down and give a false reading. I suggest you check another set where you have already set the EHT, measure it on the new meter and see what the difference is.

Those Brandenburg meters may be for high voltage, higher current measurements. As an aside, Brandenburg are well known for making insect killers, the type with UV lamps with a high voltage grille behind them so the unsuspecting insect flies towards the UV tubes and gets zapped as well. Those grilles run at about 5 mA......they bite! The meters may well be suitable for the insect traps but not for TV.

 
Posted : 28/08/2015 2:20 pm
crustytv
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Nothing wrong with the Brandenberg.

I built an extra large rubber shield for my wand from a hot water bottle. I know the wand to be accurate having used it on many B&W sets and a few colour. I checked and EHT with the wand is 20kV. Then fitted the Brandenburg and that read 20kv.

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Posted : 28/08/2015 2:48 pm
Jac Janssen
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The Brandenburg 88M is a very, very good meter.
Input resistance is 45 GΩ, so the load is negligible.

Jac

 
Posted : 28/08/2015 3:14 pm
Cathovisor
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That's excellent news, Chris. I used to use exactly these ones at work - for setting the EHT on colour monitors, oddly enough.

And Jac - thanks for confirming what the input resistance is; I knew it was pretty high!

Brandenburg are famous for everything HV: I remember their power supplies in caption scanners and the like.

 
Posted : 28/08/2015 3:49 pm
sideband
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Well worth double-checking though.

Excellent results. :thumb

 
Posted : 28/08/2015 4:42 pm
Doz
 Doz
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Stunning job Chris.

:bba

 
Posted : 29/08/2015 4:58 pm
Brian Cuff
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You've done it again, Chris. A real demonstration of tenacity considering the number of steps backwards you were forced to take - excellent work and a learning curve for most of us.

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Posted : 29/08/2015 6:11 pm
PYE625
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You've done it again, Chris. A real demonstration of tenacity considering the number of steps backwards you were forced to take - excellent work and a learning curve for most of us.

Couldn't agree more.
A very interesting set and quite a different kettle of fish to my easy Pye CT72. :aad

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

 
Posted : 29/08/2015 6:45 pm
Dr Wobble
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Nice to see you got the Baird working Chris, well done.

Andy.

Curiously curious

 
Posted : 03/09/2015 7:40 am
Jayceebee
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Hi Chris, I hope I'm not going to put he mockers on it but that really looks very good indeed now. it just goes to show that following the complete setup procedure in the manual really pays dividends. A excellent result :thumb .

John.

John.

 
Posted : 04/09/2015 11:33 pm
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