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-   -   Help wanted with B/W TV Mod (https://www.vintage-radio.net/forum/showthread.php?t=153041)

Superscope 11th Jan 2019 10:41 am

Help wanted with B/W TV Mod
 
1 Attachment(s)
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

Studio263 11th Jan 2019 11:08 am

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.

Superscope 11th Jan 2019 12:35 pm

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

Studio263 11th Jan 2019 1:07 pm

Re: Help wanted with B/W TV Mod
 
Quote:

Originally Posted by Superscope (Post 1109645)
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.

Superscope 17th Jan 2019 3:35 pm

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

Semir_DE 26th Jan 2019 1:48 am

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

paulsherwin 19th Jun 2020 8:11 pm

Re: Help wanted with B/W TV Mod
 
Thread reopened as requested.

Superscope 22nd Jun 2020 8:09 pm

Re: Help wanted with B/W TV Mod
 
2 Attachment(s)
Thanks Paul,


Just a quick update regarding my Sharp 5T-20H Composite Video Conversion.

I was asked recently if I ever completed the Modification.

In short, Yes I did.

I took the easier route of using a Sony Modulator as suggested by Studio263.

But thank you Semir for bringing to my attention the extra work required with
injecting Composite Video in without using a Modulator.

I still plan to try this approach as a comparison, as I said, this was first and
foremost a Learning exercise, but for now, it is working and very useful.

The main purpose of this project was for converting the Analogue TV to Digital Freeview,
which has been very successful.

Apparently, the Service infomation for this Radio/Tv is apparently is impossible to
find, so I have attached a copy here in case anybody is interested.


Ian

zeoin67 26th Jun 2020 11:25 am

Re: Help wanted with B/W TV Mod
 
5 Attachment(s)
Hello,

Just wanted to do a quick post to say how helpful this thread has been to my own project. Especially the service manual which 'Superscope' kindly provided for me!

I came across this model myself while looking for a bedside radio and could not resist picking this up for its retro look (plus I am a big fan of vintage SHARP products). My only problem is that the TV would be inherently useless...unless I did something with it.

Following the theme that this would be used as a bedside radio I decided to create a 'Morning screen' which I would program in Python to display information and items useful to start my day.

What led me to this thread is how to connect up my Raspberry Pi (the tiny computer which would run my program). I ended up just buying an off the shelf modulator but am unhappy with this solution because it hums! Ever so quietly but incompatible for sleep. I will look into the Sony modulator but might need a bit more help as I am useless with anything hardware.

Anyway here are a couple of photos of my project so far. Still a few issues such as overscan and the occasional bad vertical line scanning but working on that now I have the service manual!

Functions implemented so far to the morning screen:
  • Train times (for my morning commute)
  • Local Weather
  • German word of the day (I am learning German)
  • Bin day (If the bin is supposed to go out...and what one!)
  • Podcasts (fully working and playing through the speakers)

Perhaps I will open my own thread if there is enough interest, plus there are a couple of things I will probably need help with! I can release my code open source too if anybody wants to do something similar. It has been a fun project so far.

Richard_FM 27th Jun 2020 2:07 pm

Re: Help wanted with B/W TV Mod
 
That's good to hear.

I've managed to use a modulator from an early Playstation by soldering a USB cable to a 2.5mm mono socket to provide a 5v power. It then just needs a 2 x RCA socket to plug lead for an input.

I've made another using a Canon camcorder modulator with the same connections, this had the bonus of being able to select the output channel between 30-39.

With a USB for the power supply you could run one from the Raspberry Pi without needing another power source.


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