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Old 11th Jan 2019, 10:41 am   #1
Superscope
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Default Help wanted with B/W TV Mod

I am looking to add a Composite Video Input to an old B/W Sharp 5T-20H TV.

Now, I'm not overly familiar with TV circuitry and this is all new territory
for me, so I'm treating this as an educational project.

I thought I would ask advice from those far more knowledgeable than myself on the subject.

The TV works perfectly. I just wish to Mod it to make it useful again.

Now, I am guessing, that an external Composite Video Signal, would need to be injected
in somewhere before the TV's Sync separator circuit?

But, not being familiar with TV circuits, I am struggling to find this point.

Could anybody with good TV Circuit knowledge point me in the right direction?

I have attached a copy of the Schematic, which I hope somebody can help me with.


Any help gratefully received.

Ian
Attached Files
File Type: zip Sharp 5T-20H Schematic.zip (470.0 KB, 65 views)
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Old 11th Jan 2019, 11:08 am   #2
Studio263
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Default Re: Help wanted with B/W TV Mod

Somewhere near the base of Q801, but the bias for this stage comes from IC201 so you'd have to disconnect R262 and make your own arrangements to set the operating point. You'd also have to organise AC coupling to stop the bias source being upset by the 75R source impedance of the video input. As for the audio, that could go in at C403 but you'd have to organise your own volume control. The audio signal path is complicated by the radio section in this model.

All in all, this is a much easier approach:

https://www.vintage-radio.net/forum/...ad.php?t=73500

You could hide it somewhere inside and power it from the rest of the set if you felt strongly about it.
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Old 11th Jan 2019, 12:35 pm   #3
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Default Re: Help wanted with B/W TV Mod

Hi Studio263,

Thanks, I wasn't expecting a reply quite that fast.

The Base of Q801 is quite convenient to get at, and disconnect from R262, so I will hopefully
have time tonight to experiment on that.

Thanks for reminding me about the Bias, I'd not thought about that, but it should be fairly straightforward to set up a temporary adjustable Bias for testing to get the final required level.

The Audio side of things I don't have a problem with, that's fairly simple.

Regarding the UHF Modulator, that would for sure be a simpler solution, but I want to avoid up converting
the Video Signal just to have it down converted again with the associated loss of quality.
I have also had trouble with interference between Modulators and some Freeview Boxes in the past as well.

Most importantly for me though, is if I can successfully convert this Set, I will move on and try the same type of mod on one of my other portable Sets where there really is no room for a Modulator inside.

I bought this particular Sharp 5T especially for this experiment to gain some knowledge as it has plenty of room inside and all the Boards are quite accessible.


Ian
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Old 11th Jan 2019, 1:07 pm   #4
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Default Re: Help wanted with B/W TV Mod

Quote:
Originally Posted by Superscope View Post
Regarding the UHF Modulator, that would for sure be a simpler solution, but I want to avoid up converting
the Video Signal just to have it down converted again with the associated loss of quality.
Those Sony modulators are pretty good, you'll be doing well if your converted circuitry gives results that match their performance. The tubes used in that type of set don't focus to pin-sharpness in any case, so there's no point trying to make up bandwidth that can't be displayed. A really good domestic mono portable (like a Rigonda / Vega 402, super sharp focus and no chroma notch filter) shows that the modulator passes the full HF end of the signal with no problems.

If you have a number of sets the modulator really is the way to go, so much less hassle.
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Old 17th Jan 2019, 3:35 pm   #5
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Default Re: Help wanted with B/W TV Mod

Update to this Project.

I have managed to feed the Composite Video in at the Base of Q801
and have set up a 3v operating point which gives me a basic Picture of sorts which
is an excellent start.

However, the Vertical Hold is quite weak and the Horizontal looks quite
distorted. So I am assuming I don't have the necessary Sync.

On looking at the Schematic, it looks like the Video and Sync are already normally
separate at this point, Sync on K5 connector and Video (where my new Signal is injected) on K4 going to Q801's Base.

If this is the case, can anybody suggest how I go about inputting the required Sync, if indeed that is the problem?

Also,
I have ordered an old PS1 modulator to try out the other suggestion in parallel.

Ian
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Old 26th Jan 2019, 1:48 am   #6
Semir_DE
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Default Re: Help wanted with B/W TV Mod

Hello Ian,

it seems that IC 201 has a built in sync separator. This kind of complicates want you want to do. There are two options:

1) find an input on IC201 that accepts a composite video signal, so you can inject your video signal before it is being processed inside IC201. I have not been able to find the data sheet for this chip so it is unclear whether it has a video input. From the looks of it it only processes the video IF RF signal.

2) Build a small circuit that strips the sync from your composite signal. This could be done with an LM1881 IC. You would have to feed the incoming signal to both the transistor Q801 and the input of the LM1881. From the LM1881 you would get a composite sync that you could feed into the connection point marked "F5". Make sure to disconnect pin13 of IC201 before feeding in the sync.

Please also note that you will need a clamping circuit for your video signal before feeding it into the base of Q801. If you just AC-couple the signal into Q801 via a capacitor without clamping, the average brightness of your images will change depending on the content of the video signal.

All things considered I would go for the UHF modulator approach. The loss in image quality will not be that significant if even visible and you can preserve the original state of your TV.

Just my $0.02

Cheers

Semir
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