Making 405 line NTSC - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

channel405 · 2nd Apr 2007, 12:06 pm

Quote:

Originally Posted by murphyv310

Not that simple I am afraid!
The easiest set to modify would be dual standard sets, 625 line stages do not like to run as slow as 10125 khz they produce enormous amount of EHT at those low frequencies Trevor

OK, so we saying then that LOPT's are designed to produce a certain EHT voltage at a given HT and line frequency and that anything out of these parameters will be unstable. So this would rule out designing a line out put stage to drive a colour CRT at 405 lines because no (modern split diode etc) LOPT will produce the correct/safe EHT at that frequency. What about modifying the tripler/EHT rectifier? Might be a bit difficult if its a valve one. Time for the return of the dreaded mains EHT using microwave oven transformers?!!

In which case might be easier to retime the drivers for an LCD display for 405 lines! I have seen that discussed on here too somewhere.

ppppenguin · 2nd Apr 2007, 12:23 pm

The modern line output stages used in PC monitors are designed to work over a wide range of frequencies but never as low as 10125Hz.

There is a lot of power circulating in a LOP circuit. Anything up to 3kW for a big colour CRT. Most of the energy is recovered so the consumption from the supply can be miniimal. All this energy means that LOPs are very critical and it's all too easy to damage components by incautious experiment.

It's not just the EHT. If you lower the frequency you will get too much width unless you lower the HT voltage.

Unless you are really dedicated to LOP design then start with a dual standard colour set.

murphyv310 · 2nd Apr 2007, 8:16 pm

Quote:

Originally Posted by ppppenguin

It's not just the EHT. If you lower the frequency you will get too much width unless you lower the HT voltage.

Unless you are really dedicated to LOP design then start with a dual standard colour set.

I am 100% in agreement here the only way to go is a dual standard set.
I would suggest a hybrid GEC dual standard but this is for my own preference.
I have an RT Russell Test Card generator and find this excellent for still images, it can be easily programmed for NTSC 405 colour and Fernseh has used this for his 405 colour set up to now, I have seen the results on this set and now cant wait to see the "full monty" with Darryls converter.
Trevor

Andy Green · 8th Apr 2007, 12:43 pm

HI all,

I have ordered one of Darryl's 405 line NTSC converters, as I have long held an 'ambition' to 'do' 405 line colour.
Not wishing to doubt the expertees of those who say that it is not possible to convert a 625 line set, (due to excessive EHT created by the slower line scan speed etc).
Out of interest, how does a dual standard set cope? Do they have a special LOPT, or is a transformer or potential divider used when switched to 405.

It would be nice to be able to convert a more commonly available set, as dual standard colur ones are very rare (almost unheard of here in Ireland, since RTE was 625 from the start, execpt for some 405 line relays for those near the border to Norhtern Ireland, and in Dublin where before RTE came on air, many UK 405 line sets were imported)
Theoretically speaking, what would be involved to convert a 625 line only colour set?

ppppenguin · 8th Apr 2007, 4:06 pm

Quote:

Originally Posted by Andy Green

Out of interest, how does a dual standard set cope? Do they have a special LOPT, or is a transformer or potential divider used when switched to 405.

In all dual standard sets there's quite a lot of switching around the line output too make it work properly on both standards. There's no particular difficulty designing a line output stage for any standard or any pair of standards. It's a lot harder to modify it from one standard to another. In monochrome sets, some 405 LOPs worked well at 625 with minimal changes. Others did not. In colour you have the extra complication of the convergence.

If anyone decides to have a go, I wish them luck! It won't be easy.

Andy Green · 23rd Apr 2007, 6:34 pm

Well, My Aurora NTSC 405 line standards converter is here, now all I need is to find a dual standard set to convert - ( I might be persuaded to lend it to anyone who has what they consider a working set , especially if it was to be be exhibited at a rally or show, and provided I was reasonably certain I would get it back OK! - I might need to check out any person so if you can provide other board memebers as refeernces , this would help put my mind at ease)

2nd Apr 2007, 12:23 pm	#22
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Making 405 line NTSC The modern line output stages used in PC monitors are designed to work over a wide range of frequencies but never as low as 10125Hz. There is a lot of power circulating in a LOP circuit. Anything up to 3kW for a big colour CRT. Most of the energy is recovered so the consumption from the supply can be miniimal. All this energy means that LOPs are very critical and it's all too easy to damage components by incautious experiment. It's not just the EHT. If you lower the frequency you will get too much width unless you lower the HT voltage. Unless you are really dedicated to LOP design then start with a dual standard colour set.

8th Apr 2007, 12:43 pm	#24
Andy Green Hexode Join Date: Jan 2005 Location: Waterford, Republic of Ireland Posts: 258	Re: Making 405 line NTSC HI all, I have ordered one of Darryl's 405 line NTSC converters, as I have long held an 'ambition' to 'do' 405 line colour. Not wishing to doubt the expertees of those who say that it is not possible to convert a 625 line set, (due to excessive EHT created by the slower line scan speed etc). Out of interest, how does a dual standard set cope? Do they have a special LOPT, or is a transformer or potential divider used when switched to 405. It would be nice to be able to convert a more commonly available set, as dual standard colur ones are very rare (almost unheard of here in Ireland, since RTE was 625 from the start, execpt for some 405 line relays for those near the border to Norhtern Ireland, and in Dublin where before RTE came on air, many UK 405 line sets were imported) Theoretically speaking, what would be involved to convert a 625 line only colour set?

23rd Apr 2007, 6:34 pm	#26
Andy Green Hexode Join Date: Jan 2005 Location: Waterford, Republic of Ireland Posts: 258	Re: Making 405 line NTSC Well, My Aurora NTSC 405 line standards converter is here, now all I need is to find a dual standard set to convert - ( I might be persuaded to lend it to anyone who has what they consider a working set , especially if it was to be be exhibited at a rally or show, and provided I was reasonably certain I would get it back OK! - I might need to check out any person so if you can provide other board memebers as refeernces , this would help put my mind at ease)