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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 25th Jun 2015, 4:21 pm   #1
stuart_morgan_64
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Default Dim output 70s tv game

This is one of the very early TV games, battery operated. The composite video goes to a separate box with a UHF modulator in.

The display on a LCD TV is very dim. Brightness up full. Any suggestions please.
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Old 25th Jun 2015, 4:34 pm   #2
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Default Re: Dim output 70s tv game

Are you watching it via the RF modulator or 'pinching' the comp video pre modulator & feeding it into the TVs video input?

Many very early games were random interlace, 'block' field sync jobs then fed into a rubbishy "1 transistor taking off " UHF modulator. A modern set probably doesn't know what has hit it, but seriously, if you are watching the UHF output try fine tuning the TV and even scanning somewher else on the band to make sure there isn't another - 'better' image elsewhere. I've seen some of these modulators provide 3 or 4 tuning points on the band - in one case 1 of which appeared negative!

If you've an old portable 'proper TV' just try it with that first.
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Old 25th Jun 2015, 4:50 pm   #3
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Default Re: Dim output 70s tv game

Hya, i have tried both, I also have experienced the 2 or 3 with one good on UHF tuning. It is a very basic, early one.
Its is the most basic using AY-3-8500 chip. The image is the same, uhf or composite
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Old 25th Jun 2015, 4:54 pm   #4
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Default Re: Dim output 70s tv game

If you've a scope, check the sync height to video height; if the 0.3 sync to 0.7 video ratio is miles off it will horribly upset a modern TVs blanking etc.
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Old 25th Jun 2015, 4:58 pm   #5
stuart_morgan_64
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Default Re: Dim output 70s tv game

I have a scope, however not sure what I would be looking for. I do not have a old style tv here. Wish i did have a few video games to convert, including Atari's ect. I will google pictures of video signals.

Your help is much appreciated
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Old 25th Jun 2015, 5:08 pm   #6
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Default Re: Dim output 70s tv game

Scope the composite waveform using around 10uSec/div sweep - this will give you a full TV line (and a bit). The scope should -ve edge trigger in the sync pulse, the 'top' of the sync pulse being the biggest -ve excursion. Going 'North' you get to black level and the video information goes above this. have a look at the attached. (I hope)
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Old 25th Jun 2015, 5:31 pm   #7
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Default Re: Dim output 70s tv game

I can't seem to get anything like the waveform, it is a old scope, analogue.
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Old 25th Jun 2015, 5:53 pm   #8
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Default Re: Dim output 70s tv game

If the leads are X1 leads you need to set the scope to Vertical 0.1 V/div, Sweep to 10uSec/div; with a X10 probe 0.01 (10mV)/div, Sweep as before. This scope should be ideal for what you need. Earth clip on the chassis, -ve rail or case of UHF modulator then look at the video input pin to the UHF mod. If there's an image on the RF out ther must be video going in - even if the level is wrong.

Note, if you've a switchable scope probe (X1 or X10) make sure it is not one of the ones with an intermediate short circuit/calibrate zero/off positions and you've left it switched to that position (easy done).
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Old 25th Jun 2015, 6:34 pm   #9
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Default Re: Dim output 70s tv game

I wonder if this is like the early to mid version ZX81 video output problem, ie, no 'back porch' to define the black level immediately following the sync pulse. The symptom sounds exactly the same. The late version of the ZX81 ULA did finally fix it. Maybe the designers of the AY-3-8500 made the same error?

You can see if this is the problem by borrowing or unearthing a suitably vintage (ideally B/W) CRT TV, if you can lay your hands on one, and try it on that. You might be trying to fix a problem which wasn't really a problem when the unit was made.
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Old 25th Jun 2015, 6:48 pm   #10
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Default Re: Dim output 70s tv game

Hya guys,

lots to check and do, somewhere I have a b & e crt, tv radio cassette, sure we all remember them.

I will check waveforms following your advice too.

Thanks
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