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Re: QL Video issue - Red color missing
Posted: Thu May 17, 2018 12:21 pm
by tofro
Your picture quality is incredibly good considering you are using the modulator.
Tobias
Re: QL Video issue - Red color missing
Posted: Sat May 19, 2018 1:11 pm
by 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
Re: QL Video issue - Red color missing
Posted: Mon May 21, 2018 6:23 pm
by 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!
Re: QL Video issue - Red color missing
Posted: Wed May 20, 2020 11:48 pm
by tinman40
without a scope is there a way to check the 1377? my green is gone.
Re: QL Video issue - Red color missing
Posted: Thu May 21, 2020 10:31 am
by 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.
Re: QL Video issue - Red color missing
Posted: Mon Jun 08, 2020 11:16 am
by chriskgnr
Dave wrote: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?
Re: QL Video issue - Red color missing
Posted: Mon Jun 08, 2020 11:32 am
by 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