QL Video issue - Red color missing

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tofro
QL Wafer Drive
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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.

Tobias


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1024MAK
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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
                 |
                 |
               Signal
               from
               ULA
               to
               RGB
               socket

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.

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
Quickie
ROM Dongle
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Joined: Sun May 06, 2018 10:14 am
Location: Barcelona, Spain

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!


tinman40
ROM Dongle
Posts: 2
Joined: Wed May 20, 2020 4:18 am

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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Pr0f
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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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