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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 4th May 2005, 11:26 am   #1
grindrod
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Default TV36C Success!!

Good news everyone! I modified the field timebase sync circuit to incorporate a pulse shaping diode and now the interlace is very good

If anyone wants it I might part with this set as I have acquired a TV22 to keep my TV12 company and the TV36 hasn't yet gained the approval of the 'station manageress' and therefore is forced to reside in the workshop, DOH!

I am off to watch George Formby in 'No Limits' again .

Cheers, Martin
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Old 4th May 2005, 11:35 am   #2
oldeurope
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Question Re: TV36c Success!!

Hi Martin,
without a schematic I am not able to understand what you are doing.
Are you able to show the original and the modified schematic?

Kind regards
Darius
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Old 4th May 2005, 8:12 pm   #3
grindrod
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Default Re: TV36c Success!!

Hi Darius, (hopefully) I have attached before and after circuits, hope this helps!!

Cheers, Martin
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Old 5th May 2005, 9:41 am   #4
oldeurope
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Exclamation Re: TV36c Success!!

Good morning Martin,

I had a look at trader sheet 1175. Is this the right one?
There is a R77 interlace adjust. So why do you have problems
with the interlace?

The modification you made is a big one. The crystal diode was more
expensive than a valve and they found a way to save it in the TV36.

The most trouble with the V-sync is caused by the DC- restorer
in most modulators. (not mine of course!)

Look with the oscilloscope at the plate of the video amp and trigger
on V. Is there a DC change in the sync tip level?
If so you have a modulator problem.

The interlace depends more or less in the V- hold adjustment.
Maybe you do remember what Malcolm (flyback) said.
You can see the quality of the V-sync stages of the set
and some make a good and others a poor interlace.
In 405 the equializing pulses are missing and this makes it
a bit difficult for the set.

But in 625 sometimes you have interlace trouble too.
A clever person (does anyone know who?) found the following solution:
A simple RC is added in front of the g1 of the sync sep valve.
From this time on every service men added this
and the TV manufractuers used it too.
But nobody knows exactly why this makes a better interlace

Kind regards
Darius
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Old 6th May 2005, 1:15 pm   #5
grindrod
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Default Re: TV36c Success!!

Hi Darius, I use a Domino converter/modulator and have very good results with my TV12a.

The interlace control is not very effective and interacts with the field hold control, I guess it was added after Bush found the interlace wasn't very good on some of the TV36's. The problem with a 17" screen is that poor interlace is very obvious!!

With my set it was only 5 minutes work to modify the circuit which is the same as used by Ekco on some of their mid 1950's chassis, Cossor used a similar circuit as well.

I have used a field blocking transformer from an Ekco set as the original Bush part was O/C on one winding, the ratio is the same but there will be other differences that might explain the poor interlace.

Cheers, Martin
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Old 6th May 2005, 2:38 pm   #6
oldeurope
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Default Re: TV36c Success!!

"I have used a field blocking transformer from an Ekco set as the original Bush part was O/C on one winding, the ratio is the same but there will be other differences that might explain the poor interlace."

Ah, yes I agree.
An other blocking transformer needs changes in the values of C73/74/77.
It is much work to find out the perfect values, so your modification is the
easyer way.

Kind regards
Darius
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