QL Video issue - Red color missing

[tofro]

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

Tobias

[1024MAK]

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

[Quickie]

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]

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

[Pr0f]

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.

[chriskgnr]

Yes. Sometimes, shocks can travel from the unprotected RGB socket directly to the 8301 and blow an output.

5.6V zeners to ground from all five lines would be prudent, if you're comfortable to retrofit them.

wouldn't be better to use 5.1V zener diodes?

[Pr0f]

For static it makes little difference - as the voltages are that much higher, but the danger of a lower voltage zener is it starts to conduct a little before it's rated zener voltage

The idea is to try and prevent too much voltage at the chip pins to avoid the parasitic diodes / thyristors and goodness only know's what that will become apparent when the pin voltage is more than about 0.65 volts above VCC

[Popopo]

Hi!
Now I am making some tests with some RGB2HDMI converter, I have seen this thread about how dangerous is to use RGB connector of the QL.
I would like to protect the only ULA I have. So that thread is a good way to do it.
Just now I hesitate how to do it in the best way to do not damage the QL board.

Did someone implement any of those suggestion and have photos to share?

And the end... the best way to protect the ULA is using Zener & FastSwitch diodes 1N4148?

Thank you a lot

[Peter]

And the end... the best way to protect the ULA is using Zener & FastSwitch diodes 1N4148?

I don't think so. The breakdown voltage of a Zener diode is not precise, the forward voltage drop of the 1N4148 is unnecessarily high, and both are relatively slow.

If I wanted the RGB signal "as it is", I would use Schottky diodes which are suitable for ESD protection to clamp versus +5V and GND.
With that, one gets very low forward voltage drop, fast switching, and over 15 kV ESD rating.
Also available as arrays with several diodes in one package. For example, the NUP4302MR6 has 8 diodes and common pins for VCC / GND.

Or ask yourself, if you want a VGA signal in the end. If you add a converter behind the RGB outputs, it also protects them.

[Popopo]

I don't think so. The breakdown voltage of a Zener diode is not precise, the forward voltage drop of the 1N4148 is unnecessarily high, and both are relatively slow.

If I wanted the RGB signal "as it is", I would use Schottky diodes which are suitable for ESD protection to clamp versus +5V and GND.
With that, one gets very low forward voltage drop, fast switching, and over 15 kV ESD rating.
Also available as arrays with several diodes in one package. For example, the NUP4302MR6 has 8 diodes and common pins for VCC / GND.

Or ask yourself, if you want a VGA signal in the end. If you add a converter behind the RGB outputs, it also protects them.

Thank you for your advice Peter,
I am not sure what I would like to do. Mainly to connect the RGB output from QL to a TV with SCART connectors, but in some days also to connect it to a converter RGB2HDMI from Macerlo design. Also the another one that is great and simply to make posted here (perhaps it is your design, I am not sure).

So, I prefer to do not assume any protection from the other side. This package (NUP4302MR6) sounds nice, but I need to know or to see how to place it in the most elegant way (and that fits the original case).