Hello and welcome back to a little addendum about my RGB mod.
Now, it was brought to my attention several times that there might be a problem with the
design of my mod.
So, first I’m going to show you what the problem was, then I’m going to show you
how I’m going to fix it, and then I’ve got a little surprise for you!
You may recognize this rough schematic from my last video.
However, I just want to focus on the part here that deals with the blanking pin.
So, the idea behind this was that when I flipped the switch, it would send 5 volts to the blanking
circuit over at the jungle chip.
And it does that just fine.
However, it also sends power back into the micro controller as well.
And this can be a problem because when the micro controller tries to bring the line low
while the switch is on, there is a direct short circuit.
Obviously the chip has survived so far, but long term it may end up burning the chip out.
So, I’ve taken the board back out of the TV and I’m going to remove this wire from
the blanking pin.
OK, so looking at the board here, the trace on the other side takes the signal from this
chip right over to this resistor.
And so when the micro controller wants to send the blanking signal it sends it through
this resistor and onto the jungle chip.
So, here’s what I’m going to do, I’m going to heat this resistor up and pull it
out just like I did the other ones.
At this point the blanking line is now disconnected completely.
The plan is, since my toggle switch is a double throw, it has 3 pins.
So what I’m going to do is run the blanking signal from the jungle chip to the middle
pin on this switch.
Then I’ll connect the rest like this.
So the idea being that when you flip the toggle switch it will connect the jungle chip to
the micro controller or the 5 volt line, so it will be one or the other, but not both.
This should protect the microcotroller as it will no longer have 5 volts being forced
into that pin.
Nothing will change in the way the TV or the switch operates as compared to last time.
So I’ve attached one wire to this resistor here, and I’ll put some heat shrink on it.
And the other wire will go back in the hole the resistor used to be in.
And since I’m in here, I guess it’s time to cut this connector off.
It was a temporary fix anyway.
I got the new part in I needed, which should make this look a lot more elegant.
However, I had originally removed the extra wires from the ribbon cable that I didn’t
need, so now I’ll have to add one back by sliding an extra wire in here.
It was either that or re-solder the whole ribbon cable.
There we go, so I’ll just clamp this down.
Then, of course, I’ll need to connect this extra wire to the other empty connection on
this toggle switch.
I’ll just use a couple of zip ties to help keep the extra wire together with the ribbon
cable.
Yeah, I guess that will work!
And now, I can go ahead and re-assemble the two halves of the TV again using the new connector.
OK, so now that that is fixed, I have something else I want to show you.
So, in the last video, I had commented that theoretically it should be possible to display
a VGA video signal.
Now, a lot of people were intrigued by that and wanted to see how that would work, so
I’m going to show you how that works!
OK, so now it’s time to build a cable.
But how will I do that?
Well, I’ll start by chopping this extra VGA cable in half, and then stripping back
the insulation, and then untangling this shielding.
Now we have a million wires, where do we start?
I’ll use my multi-meter in continuity mode to track down which wires go to which pins.
Now, take a look at this diagram I found online a long time ago showing how to wire up a VGA
signal to connect to a composite television.
Now, I should preface this with the fact that this design assumes the VGA port is outputting
the correct frequency for an NTSC television.
This part of the circuit essentially takes the 3 different colors and combines them with
different resistor values to convert it to a nice grayscale.
After all, there’s no way to get color on a signal like this because composite color
is generated in a fundamentally different way.
Technically you only need one of color signals, but if you used the RED signal, for example,
green text would never show up.
So combining them like this probably works best.
Now, this portion of the circuit combines the horizontal and vertical sync into a composite
sync.
Now, I’ve actually used this design before years ago and it does in fact work.
But, I’m going to make some modifications.
First of all, I’ll get rid of this section here.
The bottom part will now become just a sync signal.
And then I’ll pipe the red, green, and blue right into the TV.
And so here it is.
This is just a quick and dirty way to make this cable in the interest of a demonstration.
Obviously this could use a lot of improvement.
Anyway, if you plug this into the TV and power on the computer, you’re going to see this.
Yeah, there’s obviously a video signal here, but it’s at the wrong frequency for the
TV.
So how do we fix that?
Well, there’s this little DOS program that will change the output frequency of the VGA
card.
Of course, when I do this on my VGA monitor, it looses picture, and then the only thing
it says is out of range, and that’s because the VGA monitor was not designed to work at
NTSC frequencies.
So, lets swap the VGA cable back over to the TV and see what we get.
Yep!
There it is.
VGA text mode is displayed on the TV and its actually pretty clear.
Sure, it’s not as clear as a VGA monitor, but most certainly readable.
And of course, it is in full color.
Most any text-based program should work fine like this.
But what about some games that use graphics mode?
Well, that’s where things get interesting.
So, a lot of games will work just fine.
LHX is a good example of one that works flawlessly.
Everything is perfectly readable, which makes sense being that this game is only running
in 320x200 graphics mode, which is plenty easy for a TV to display.
However, let me show you what some other games do.
I’m going to start up Lemmings.
And everything goes crazy.
That’s because Lemmings obviously fiddles with the timing on the VGA card and so there’s
no real way around this, it just flat out isn’t going to work.
Now here’s another game that does something weird.
Populous stays in sync.
However, it is compressed to half the screen.
I mean, other than that, it’s perfectly playable and everything is very clear.
But, it’s just annoying that it is squished like that.
I wondered what an EGA game would do, so I tried Maniac Mansion and it does more or less
the same thing.
I thought I’d try one more game, Planet X3.
After all, I know it doesn’t mess with any of the VGA registers.
And of course, it works just fine.
So this solution is not optimal, mostly due to lack of proper software support, but it
does in fact work.
And it’s actually remarkably clear.
Looking at these graphics on this type of CRT reminds me a lot of looking at something
from an Amiga or Atari ST.
And so hopefully that wraps up this mod.
Now, I must admit it’s still not perfect, and if I were to start all over again there’s
probably, you know, a few things I might do different.
But, overall, I’m pretty satisfied with this mod and I’m sure you’ll be seeing
this TV again and again and again whether it is being used in composite mode or now
with the newly added RGB modes.
Anyway, stick around for those videos and thanks for watching!