Help with Composite out

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bwinkel67
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Re: Help with Composite out

Postby bwinkel67 » Sat Jan 25, 2020 12:00 am

janbredenbeek wrote:I haven't used my BBQLs much with composite colour, but I remember the picture quality was quite poor. Lots of dot crawl and definitely not suitable for a 80x25 screen. On the other hand, the monochrome picture is rock steady. I'm still using my green monochrome Philips monitor of almost 35 years with the BBQL! Only drawback is the screen size which is a bit on the small side at 12 inch ;)

Jan


Ok, Thank you Jan! You may have just saved me a lot of debugging. That's exactly the picture I got so maybe it's normal. If so then no need to try and run pin 9 of the MC1733 out to pin 1 of the RGB connector. It doesn't give me much other than maybe being able to connect it to projectors since if I'm connecting it to a TV I use RF (which is just as good as color composite) and if a monitor then I would be using some sort of RBG to VGA/HDMI upscaler.

I will still investigate why my monitor said PAL but Martin might be correct that it just got confused.

This was a big help! Sad to think the QL couldn't perfect a good color composite picture. Still, maybe doing that tweak with the resistor then Adrian Black (YouTube link I posted in an earlier part of this thread) showed on improving the color of composite might be worth a try.


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Re: Help with Composite out

Postby Martin_Head » Sat Jan 25, 2020 11:03 am

bwinkel67 wrote:When I use pin 9 of the MC1377 and ground off of the RGB connector (pin 2) I seem to get a color PAL composite signal (when using the composite connector on the TV, my TV seems to briefly display what type of composite input it gets) that basically looks pretty close (as crappy) to my RF signal -- this was the surprise as I expected composite to be better. But it is composite as it plugs into the yellow RCA jack on the monitor (not the RF coaxl)
I was misunderstanding your problem. I thought you had no colour, or the wrong colours.

It sounds to me like there is nothing wrong with your QL. I think the Microdrives create a lot of interference into the video circuitry . There is usually a metal plate on the back of mdv1_ to try to reduce it. Also I don't expect that the MC1377 is a very high quality device for encoding the colour signal. If I remember correctly, without checking the circuit diagram. The monochrome signal doe's not go through the MC1377. So that's probably why it looks so clean.

EDIT I wrote this before seeing the above post.


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Re: Help with Composite out

Postby bwinkel67 » Mon Jan 27, 2020 6:36 am

So I finally did the mod. What I discovered is that color composite is indeed NTSC. What gets confusing is when I use color composite and press F1 for monitor my cheap monitor can't handle the signal, gives me a crappy monochrome picture and says PAL. So Martin was correct, the monitor was just getting confused.

Looking at the back of the motherboard the blue wire goes right to pin 1 where it connect to the motherboard (pal color composite -- or in my case NTSC color composite). When I unsoldered the blue wire on the top end and took a multimeter, it was still seeing connections so it must be on the other side of the motherboard under the RGB connector. I then decided to see if pin 9 on MC1377 was connected to pin 1 of RGB and indeed it was. So then I just tried to see if I could finally get a picture. I got no picture anywhere. Looks like that blue wire supplies the 5 volts for the video and just gives a high to pin 1. I have no idea.

back-of-board.png


In any case, there was no way I could get to pin 1 of the RGB connector and somehow cut it to bypass it to pin 9 of the MC1377. The only pin on the RGB connector that seems to be isolated was pin 3 (the monochrome composite) which is likely why it's such a crisp picture since it's not connected to all the other pins (btw, when I say they are all interconnected, there is resistance on those lines but they do let signals through...it's like 5-10K ohms).

Pin 3 is connected to a trace that goes to a resistor. I pulled the one end of the resistor and put a 2-way switch that bridges either the resistor to pin 3 or mc1377's pin 9 to pin 3. I placed it with a glue stick on the modulator as it was a perfect place to access it near the vent in the back (I didn't want to cut the case and I like to mod things that are completely reversible).

switch.png

back-of-ql-switch.png


After I got it all back together the picture was a bit washed out color wise. So I followed Adrian Black's advice and stuck a 47 ohm resistor on in and it actually improved it a ton...now it's as good as the RF.

switch-plus-resistor.png


This turns out to be better than getting pin 1 working because this way I only need one DIN-8-to-composite cable that can show both monochrome or color by flipping a switch.
Last edited by bwinkel67 on Mon Jan 27, 2020 6:58 am, edited 2 times in total.


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Re: Help with Composite out

Postby bwinkel67 » Mon Jan 27, 2020 6:54 am

Finally question. I've read that it's important to add some sort of protection (zener diode?) to keep zx8301 chip from unwanted monitor feedback (static) as it can blow the chip. I've only ever used RF and I'm guessing that it somehow protects the chip since I've unplugged the rca connector for decades (not anymore). So question, is composite included in that needed protection? I had some shoddy connections when testing so that didn't do anything. Can a composite connection to a monitor also send surges back on those two wires?

I had planned on making a DIN-8 male-to-female cable about 2 - 3 inches long that protected each line (like an external buffer) but am wondering if I need to have that for my DIN-8-to-composite or if those lines are like RF and I don't have to worry about it.


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Re: Help with Composite out

Postby Martin_Head » Mon Jan 27, 2020 10:27 am

All those resistors tacked onto the board under the video socket is not normal on a UK QL. It may be that a previous owner has tried to reduce the 5 volt RGB signals to 1 volt signals. Which are required by some monitors or TV's. This is usually done by a couple of resistors in each line in a QL RGB to SCART cable ( I don't know if SCART was used in the US).

I think that black wire may be normal. As for the zener diodes, I have never tried using them. Although I did once patch a TTL logic chip into the signal lines to act as sacrificial buffer. I think the main thing, is to never plug or unplug the DIN video connector when the QL or the monitor are switched on.


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Re: Help with Composite out

Postby bwinkel67 » Mon Jan 27, 2020 3:59 pm

Martin_Head wrote:All those resistors tacked onto the board under the video socket is not normal on a UK QL. It may be that a previous owner has tried to reduce the 5 volt RGB signals to 1 volt signals. Which are required by some monitors or TV's. This is usually done by a couple of resistors in each line in a QL RGB to SCART cable ( I don't know if SCART was used in the US).
.


I wish I had actually looked when I had the motherboard off but I blew the picture up and it looks to me to be blue/silver/black/gold which is 68 ohms. The resistor at the bottom of the picture could have a brown instead of black band but I think it's just lighting since they are all likely the same. Plus, I have some spare resisters in the 100's and the brown band looks much lighter, esp compared to the gold band. You be the judge (if you click on the picture you get a much, much magnified picture).

Resistors.png


From what I've read the resistors need to be higher (330 ohms for the color and 390 for the sync). Also, no traces cut just bridging so does that work? My US board looks a lot like the German boards. I have a capacitor on my 68008, I have diodes on the ZX8301 (btw,what do they do???) and a daughter-board on top of my RAM and I've definitely seen those mods on pictures of German QL's posted. So lots of mods. Plus the blue wire on back seems to supply a crucial voltage.

On the back it says "Sinclair QL US 5" so does that mean I have an issue 5 board US?

BTW, when working on it, the motherboard looks so robust that you forget how old it is. I put some blue painters tape on the ASTEC label to protect it from the glue gun since I like to do no harm and it actually peeled some of the printing off. I've never had painters tape damage anything since it has such a light adhesion but I guess after 35+ years things deteriorate. Wonder when even wiring will start to become brittle.

RF-before.png


RF-after.png


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Re: Help with Composite out

Postby bwinkel67 » Mon Jan 27, 2020 9:31 pm

Now that I"m running off of NTSC color composite, the picture is better than RF. At least some of the artifacts do go away (like most of the shimmering). It still has the shadow effect where, for colors, the image is echoed a millimeter or two to its right (and there is no adjustments on the monitor to fix that). But I haven't built a solid cable yet so maybe that will fix it (right now it's two small nails with each having a wire coiled around it and the other ends on a cheap RCA audio cable. Maybe once I build the plug and use a more shielded cable the shadow/echo artifacts will diminish or disappear. Still not as great as the monochrome composite (which uses the same cable hack obviously). I'm guessing the color signal has to send way more info over the line.

I will say that the RF on the QL does a pretty descent job. I've always wanted to get composite (not realizing I had monochrome already) and now that I have it I didn't gain all that much. Hopefully when the upscaler I ordered arrives, it will create a decent HDMI picture.


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Re: Help with Composite out

Postby Derek_Stewart » Mon Jan 27, 2020 11:23 pm

Hi,

I hope you have better luck with the HDMI upscaler. I could not get an upscaler to display all the QL screen.

I gave up and got a refund. Now using a GBS8220 to a VGA screen, all perfect no missing columns.


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Re: Help with Composite out

Postby bwinkel67 » Tue Jan 28, 2020 1:25 am

Derek_Stewart wrote:Hi,

I hope you have better luck with the HDMI upscaler. I could not get an upscaler to display all the QL screen.

I gave up and got a refund. Now using a GBS8220 to a VGA screen, all perfect no missing columns.


Oh good to know. Looks like they are around $22 with shipping right now on eBay (and Walmart and Amazon have them for a few $ more). I was thinking of getting that $24 SCART-to-HDMI that some QL'ers have said is good if my el-cheapo one for $7 doesn't work well (which it might not). Right now I can get a workable F1-monitor-mode signal in monochrome on the monitor I have. It looks better in "monochrome composite" mode then "color composite" mode but either work .

Also, from time to time it shows color briefly in the "color composite" mode as the entire picture flickers and jumps for a second, just for a split second so my cheap monitor is trying to adjust itself struggling to display it correctly. Actually, in "monochrome composite" when I hit reset and it does the memory test, I also sometimes get a quick color glimpse before it goes back to monochrome. Must be something the MC1733 is doing during reboot.

Maybe on a better monitor my F1-monitor-mode picture in "color composite" would show color. It does miss 2-3 columns on each side but I can live with that (write a BASIC boot script that re-adjusts the main console and two windows). If I can get of similar quality with the upscaler but with color then I'd be happy...getting all the columns would be a bonus.


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Re: Help with Composite out

Postby Martin_Head » Tue Jan 28, 2020 11:26 am

bwinkel67 wrote:Now that I"m running off of NTSC color composite, the picture is better than RF. At least some of the artifacts do go away (like most of the shimmering). It still has the shadow effect where, for colors, the image is echoed a millimeter or two to its right (and there is no adjustments on the monitor to fix that).

The way analogue PAL and NTSC colour televisions work, is a high resolution black and white picture, and low resolution colours on top of it. If you just look at the colour parts of the television picture. You would just see coloured blobs. The coloured blobs have to overlayed onto the black and white picture in the correct place. Otherwise you would see a left or right mismatch, like you describe. I was never very 'up' on the NTSC system. And it's been a very, very long time since I did any theory on the PAL system. But the PAL system (I think it was PAD-D in the UK) has 'delay' lines to re-time the video signals to ensure that the colour, and the monochrome signals overlap correctly.

So this effect may be down to the cheap monitor. It could also involve this issue about the monitor thinking it's a PAL signal. Where it's adding, or not, the required delays to the signals.

For the resistors, take a look at http://www.dilwyn.me.uk/docs/hardware/scart.zip One of the examples uses 680 ohm resistors.



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