Decca CTV22/C 405line NTSC Mod Project

[murphyv310]

Hello.
A very rewarding hours work this afternoon!
I refitted the decoder this afternoon with the new subcarrier crystal fitted, a 100pf cap across the osc tuning coil an the chroma delay ine removed with a bridge across the signal path. On switching on I was amazed to have unlocked colour straight away, albeit at a low level. I now need to concentrate on the band pass transformers tuning to the lower chroma frequency this should improve the chroma level and allow me to work on the osc and phasing for the r-y, b-y etc.
trevor.

I will need to replace the CRT & converge the set!

[murphyv310]

Hi.
After a quick word with Fernseh I locked up the PAL switch, this removed the venetian blind effect, and with the xtal osc set as close as possible to the correct frequency the colours look ok,
I will need to modify the burst tuning coil tomorrow, so hopefully the osc will lock up and give an NTSC 405 picture.
I gave the crt a good boost which has made a huge improvement to brightness and focus.
watch this space.
Trevor

[FERNSEH]

Originally Posted by murphyv310

Hi.
After a quick word with Fernseh I locked up the PAL switch, this removed the venetian blind effect and with the xtal osc set as close as possible to the correct frequency the colors look ok,

Hi Trevor,
If you think about it, on most dual standard colour TVs the PAL switch must be still be in operation on 405 lines. I have never checked a set to see if that is so. The switching rate will be 5.0625Kcs. As the set is to be converted to 405NTSC colour only, one of the switching transistors can turned off by short circuiting the base to emitter, or removing the bistable components alltogether.
The choma bandpass will require retuning to 2.66Mhz as will the sub-carrier tuned circuits.
I sense that this project is going to be a great success.

DFWB.

[Andy Green]

How will you approach the implementation of the NTSC 'Hue' control? or will you not bother?

[murphyv310]

Hi.
3 steps forward and 10 back to-day I am afraid. I disabled the PAL switch and modified the bandpass transformers and also the burst tuning coil, this resulted in locked colour but somewhat out of phase, I reverted the pal switch to the opposite state this resulted in near perfect colour, this was cured by setting up the r-y & g-y settings but the best improvement was obtained by introducing a core into the 90o phasing coil, the results were excellent but very short lived. By the time I put new batteries in the camera the colour dropped out.
I checked the chroma bandpass amp and found the BC109 had gone u/s after fitting a replacement and powering up there was no x-tal lock up (see photo)
there now is no burst pulse to lock up the osc, plenty of line pulse but no burst something else has gone u/s as well. A couple of minutes later the boost cap went bang! A new .47mf 1000v brought the line back to life but again a few minutes later the convergence went wildly out accompanied by smoke from the convergence board, time to abandon ship and wait till next weekend for more time.
Trevor

[murphyv310]

Hello
Good results today with the convergence board repaired, it had a couple of duff pots and some bad dry joints that had started to burn.
I then paid attention to the burst gate that was giving wrong timing on 405 this was due to the burst gate coil tuning at 7.8 khz rather than just over 5 khz, I tried tuning the coil with a slug but this just widened the pulse rather than shifting it so a shunt cap of 225pf put the gating pulse right on top of the burst, I still needed a little core in the coil to widen the pulse enough to give a good lock. I now have an NTSC pictiure (see photos) although some further work is needed. I have a colour shading effect redish to the left and greenish to the right of the picture but this is for next weekend if the missus lets me!!
So far then:
Disable the colour killer o/c D603, s/c D600
Remove Chroma delay line connect pins 5,6 to pins 1,2
Disable PAL switch by connecting D605 cathode to chassis
Remove l602.
Retune and increase caps on L604, L601 burst tuning and burst gate as above.
Replace subcarrier X-tal
Introduce core into L605 (affecte hue)
We now know Darryls NTSC converter works and to a very high standard, I will do some program pictures during the week and post them.
Thanks
Trevor

[Andy Green]

Well done Trevor!

The red to green hue shift is intriguing - it certainly looks to me like a gradual shift of the colour subcarrier phase, (rather than a degaussing type problem) - it appears to be related to the picture width, so maybe something to do with the line timebase sawtooth perhaps slightly varying a voltage in the colour decoder - might need better voltage regulation, or better screening / decoupling of the power lines - perhaps in PAL mode this wouldn't have been noticable. What are others thoughts on this?

[Tazman1966]

Originally Posted by murphyv310

I have a colour shading effect redish to the left and greenish to the right of the picture but this is for next weekend if the missus lets me!!

Hi Trevor.

Is this shading evident with the colour down? If so I would suspect that one of the 12k wirewounds from pin6 of the PCL84's to the HT line has died or is dry jointed, probably R674, in the green channel.

The Pye hybrids had a similar thing happen and depending on the colour shading showed which channel it was in. Hope that helps.

[murphyv310]

Originally Posted by Tazman1966

Hi Trevor.

Is this shading evident with the colour down? If so I would suspect that one of the 12k wirewounds from pin6 of the PCL84's to the HT line has died or is dry jointed, probably R674, in the green channel.

The Pye hybrids had a similar thing happen and depending on the colour shading showed which channel it was in. Hope that helps.

Hi.
All OK in this area the problem only showed up after the subcarrier osc locked up, I think its in the gating to the PCL84 triodes as Andy suspects. The only time though I will have is next weekend unless I can get some time off! Watch this space.
Trevor

[FERNSEH]

Congratulations Trevor. I knew you could do it. The picture looks great. The interesting thing about this project is that it shows that really good results can be achieved by employing a very simple NTSC decoder. It has always been my belief that the results from the NTSC colour system should be better than PAL when operating under ideal conditions.

Originally Posted by Tazman1966

Hi
Is this shading evident with the colour down? If so I would suspect that one of the 12k wirewounds from pin6 of the PCL84's to the HT line has died or is dry jointed, probably R674, in the green channel.
The Pye hybrids had a similar thing happen and depending on the colour shading showed which channel it was in. Hope that helps.

Also, the PCL84 triode sections which act as driven clamps of the CDA amplifiers should be checked.

DFWB.

[murphyv310]

Hello.
I have been slowly tidying up the set over the last two days since I was released from hospital.
I have felt rather cold in my shed even with the heater on so I will work for half an hour or so then go in for a cuppa!
I have removed the IF deck, the wiring from the tuner and all loose cables no longer needed, further cracks and dry joints were repaired in the decoder and the shading has now gone.
The subcarrier osc is still very touchy with insufficient burst amplitude to make the PLL lock tight, changing from the aurora to the rt russell genny means a readustment to the osc setting. I think the level of pulse from the burst gate is too high causing the PLL transistor to saturate, so I am going to experiment with a clamp circuit and see what the difference is.
Progress though is a lot slower now though as I still don't feel that good, and run out of energy and patience very quickly.
whatch this space though.
Trevor

[FERNSEH]

Originally Posted by murphyv310

The subcarrier osc is still very touchy with insufficient burst amplitude to make the PLL lock tight, changing from the aurora to the rt russell genny means a readustment to the osc setting.

Hi Trevor,
My 405NTSC converter has arrived in England, in fact it is waiting to be collected from the Parcel Force depot in Gateshead. I'll collect it on Monday.
Also, on that day I'll buy a new digital camera to show the results on the forum. My old one got trod on and is badly damaged. The victim of a Size 10 safety boot.
The Pye does not employ a crystal in the sub-carrier generator, there is a front panel adjustment of the oscillator frequency.
The set locks up on the R.T. Russell 405 colour generator. It will be of interest to compare the sub-carrier characteristics of the two units.

I've been studying the sub-carrier generators of the Pye CT70 and the Decca CTV22 and 25. The circuits are very similar except that the Pye employs a DC amplifier between the phase detector and the sub-carrier oscillator.
The BRC 2000 series also has a DC amplifier to control the oscillator.

DFWB.

[murphyv310]

Hello.
Here are some photos off screen via the Aurora NTSC 405 line converter. I have deliberately included some photos that include the line structure. Of course there is still work to be done.
Trevor

[murphyv310]

And The rest!

[FERNSEH]

Hi Trevor,
Very impressive. It just shows the results that can be realised by using a simple colour decoder circuit.
NTSC 405 might have appeared in the UK if ABC television had got their way. Imagine a real dual standard TV 625PAL and NTSC405!

DFWB.

[ppppenguin]

Well done. I think these may be the first published pictures of 405 NTSC in modern times.

With suitable engineering care NTSC can give pictures that are every bit as good as PAL. The problem is that the world isn't perfect (and certainly wasn't back then) so that the almost inevitable chroma phase shifts gave hue errors. A 625 version of NTSC could have given very good results, directly comparable to 625 PAL, except for the sensitivity to phase errors.

If you can get the decoder working well the colour reproduction on 405 NTSC should be excellent. There shouldn't be any obvious colour errors. You may be limited by the IF response which may introduce unwanted effects at chroma frequencies. Although I can't formally verify this I believe the Aurora gives an NTSC output that is near enough perfect for all practical purposes.

[murphyv310]

After a few problems with a freeveiw box I was using, I am now going to fit a small plug in freeveiw box inside the set and also fit the converter in the set as well. I also need to fix the audio amp.
A NOS A56-120X will be fitted and set up. I am also rewinding the chroma bandpass and burst tuning coils to resonate at the correct (new) frequency rather than upping cap values.
When all this is done I think the set at the moment could be called the only true 405 line digital colour set in the uk?
What do you think?
Trevor

[murphyv310]

Hello.
Things are imvroving all the time I have now mounted a Freeview box inside the set along with the Aurora, built a seperate PSU for the Freeview box and Converter so I can use them independant of the set, 405 line NTSC records and plays back on VHS without any chroma circuitary although there are some hue changes. I have also replaced some faulty pots in the decoder and gone through the convergence, I also now have sound. So its time to put the back on and move the set.
I will possibly on a new thread let you know how the set performs for reliability etc.
Its been an interesting project and done really quickly, I would like to do another but a much smaller sized set.
Thanks all for your interest
I include some last photos
Trevor

[murphyv310]

the last two

[murphyv310]

Hello.
I had a nice visit from "flyingtech55" this afternoon. I showed him the collection of sets including the Decca 1000 & the Decca CTV22, after some teething troubles I was able to demonstrate 405 line NTSC. Infact he said "is that 405"? I think that confirms that 405 was a good medium for colour. The set has been acting up but I am sure its down to the fact the set has been idle for some time.
My intention is though once all the faults are corrected for the set to be put into my daughters bedroom.
I think that once I am fully recoverd from my recent illness and back to work I will get it up the stairs, I will then report on its reliability.
Trevor