QL Video issue - Red color missing

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Re: QL Video issue - Red color missing

Postby tofro » Thu May 17, 2018 12:21 pm

Your picture quality is incredibly good considering you are using the modulator.


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Re: QL Video issue - Red color missing

Postby 1024MAK » Sat May 19, 2018 1:11 pm

Protecting the 8301 ULA
Rather than just using 5.6V Zener diodes, adding some fast signal diodes can improve the protection. Use 1N4148 or similar. Connect a diode between 0V/GND and the signal pin, then another diode between the signal pin and the +5V power rail. Do this for all the signals that go to the RGB socket.

Code: Select all

       A  K             A  K
0V ----->|-------+------->|------- +5V

You can of course retain the 5.6V Zener diodes.

The signal diodes will clamp any out of range voltage spikes within their current ratings.


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Re: QL Video issue - Red color missing

Postby Quickie » Mon May 21, 2018 6:23 pm

Hi all,

Thanks again for all the provided information and suggested mods and improvements. This really increases my interest on the QL! :)
Unfortunately I am currently trying to fix a couple of Dragon32's and I would prefer repairing them before retaking on the QL (which I currently consider fixed, but misses the improvement) :)

Best regards!

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Re: QL Video issue - Red color missing

Postby tinman40 » Wed May 20, 2020 11:48 pm

without a scope is there a way to check the 1377? my green is gone.

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Re: QL Video issue - Red color missing

Postby Pr0f » Thu May 21, 2020 10:31 am

It's not so easy without a scope, but the following *may* work.

The outputs from the 8301 ULA are TTL level, and go directly to the 8 pin Monitor connector on the back of the QL. If you display a white image which drives all three colour signals most of the time, you could check using a multimeter on 20V setting and DC input - you should see the same voltage on R G and B signals - if your green input is lower than the other 2 it does suggest a possibly defective 8301

Be careful not to short anything at the 8 pin connector - you could do more damage, also a good idea to earth yourself too, before doing the testing.

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