Supergrafx RGB Image Shearing

famiac · 10/20/2015, 02:13 AM

Hi all.

I modified my SGX using the amp on the gamesx website. Much like Chris Covell, i experienced image shearing after the initial mod, but only on my BVM 20F1U. I removed the amplifier as Chris suggested here, but i'm still getting the same problem.

Any idea what might be causing this?

Edit:

terminated every single BNC output. Still no dice.
I also get the exact same behavior when i use a t connector to combine the sync and green and set the bvm to use internal sync.
I tried using an LM1881, but i also got the same behavior.

I removed the LM 1881, and fed the pvm pure sync from pin 44, using this circuit:
http://console5.com/wiki/File:Hu6260_RGB_Amplifier_Schematic.png
Same issue.

I discovered that the shearing happens on all other channels as well! even when the supergrafx's input card isn't selected. This could be a clue. Note: this only happens with my supergrafx.
I don't have any other pc engine variants to test with.

Note: I get a nice and stable picture on my two PVM monitors.

More pics:

thesteve · 10/20/2015, 02:52 AM

your monitor is expecting C-Sync and your feeding it composite video instead

famiac · 10/20/2015, 03:01 AM

I'm pretty sure, if not positive, that i tapped the signal from the correct c-sync pin. Also, my pvm, which does display the image properly, would not display it if i was actually feeding it composite video. I will double check, though.

Thanks for the tip. I'll open it over the weekend. My gamebit is at my parents' house.

K-2 · 10/20/2015, 03:31 PM

Have you properly terminated the BVM'S outputs with 75ohm BNC terminals?

I have the same monitor and I had to do it to get my Duo to work.

You can find them on Ebay, just make sure they are actual 75 ohm BNC terminal caps and not just dust covers. Hope this helps

thesteve · 10/21/2015, 12:49 AM

its also possible that your sync is just too low

Vimtoman · 10/21/2015, 07:29 AM

It's definitely Sync tearing. Try placing a cap in series with the C-sync signal. Try a 220uf .

famiac · 10/22/2015, 03:04 AM

Ok. I will try all of your suggestions. I bought some 75 ohm terminators. I only have one right now.

Thanks guys

famiac · 10/23/2015, 10:52 PM

Ok, so it's definitely connected to c-sync.

I added a 220uf cap in series. I'll report back tomorrow to see if it worked.

thesteve · 10/24/2015, 01:29 AM

remember the 6260 only puts out 0.3V sync which doesnt make most displays happy

famiac · 10/24/2015, 03:44 AM

The sync was amplified, but i disconnected the amp after i saw the image shearing because that's how chris covell got his to work. It was part of the trial & error process. I could reconnect it.

thesteve · 10/24/2015, 04:15 AM

the amp i suspect you used would not have produced any voltage gain
the easiest way to deal with it is using the LM1881 from composite video

Vimtoman · 10/25/2015, 10:51 AM

The image may shift if using the LM1881.

famiac · 10/25/2015, 04:14 PM

Ok, i've tried adding the 220uf cap and terminating the outputs. None of those worked, so i'm gonna try the lm1881 as a last resort.

Vimtoman · 10/26/2015, 03:28 AM

0.3v is the standard level for the sync pulse.
Have you tried the sync pulse direct from the expansion port via a capacitor?
Not through the amp.

famiac · 10/26/2015, 01:31 PM

Yeah. That's how it's configured right now

Vimtoman · 10/26/2015, 03:14 PM

Ok then Steve idea using the LM1881 may sort it out.

or just use composite.

famiac · 10/28/2015, 11:21 PM

The LM1881 did not help. I still have the same problem. Could there be a problem with the composite circuit? I replaced all of the caps, and as far as i know i didn't lift any traces. The image does work on a different monitor, after all.

It may be worth noting that R159 is missing. I think that normally it should be there. Can anyone check its value?

here's the location

picturepush.com/public/14519585

This image is not mine.

thesteve · 10/29/2015, 12:12 AM

if you are using the din plug, your missing a signal
looks like composite
also looks like 23ohms

famiac · 10/29/2015, 01:05 AM

Yeah i thought it looked like 23ohms too, but i wasn't quite sure.

I use a modified a din socket. There are only 4 signals. Red, green, brue, and sync. The RGB signals are definitely present, and the sync is there. It's just not working properly on my BVM.

About terminating the outputs:
does every single output on the BVM have to be terminated, or just the ones on the slot i'm using?

Vimtoman · 10/29/2015, 03:59 AM

Looking at the manual for the monitor you need to terminate the out BNC on all the connections.

I think this should sort it out.

famiac · 11/22/2015, 09:23 PM

Alright, an update for you guys.

I terminated every single BNC output. Still no dice.

I figured a couple more things out.
I removed the LM 1881, and fed the pvm pure sync from pin 44, using this circuit:
http://console5.com/wiki/File:Hu6260_RGB_Amplifier_Schematic.png

Same issue.
The shearing jittered a little bit back when i had the lm1881 in, but now it's back to how it was before.

I discovered that the shearing happens on all other channels as well! even when the pc engine's input card isn't selected. This could be a clue. Note: this only happens with my supergrafx

More pics:
https://imgur.com/a/atmSd

Edit:
I also get the exact same behavior when i use a t connector to combine the sync and green and set the bvm to use internal sync.

Vimtoman · 11/23/2015, 05:06 AM

Can you try the composite sync

thesteve · 11/24/2015, 02:48 AM

a few points
1 the amp you used doesnt increase the sync pulse, so it wont help
2 have you tried composite video on that screen (can you)

famiac · 12/06/2015, 01:54 AM

It was resolved. The BVMs sure are picky

The details are here:
http://shmups.system11.org/viewtopic.php?f=6&t=55732

Vimtoman · 12/06/2015, 02:16 PM

Well done.
I guess it's down to dicky VCR's in Ye old days. E ayre missus.