GBS 8200 & LM1881 help

[mikey]

I have been using the 8200 for a while now on cps1 and neo geo hardware and it's worked fine, but I wanted to use it with some of my other consoles and so I added the 1881 to clean the sync signal. Problem is, now it seems like the sync is worse than before. The cps1 now intermittently drops sync every few seconds, and I don't get any signal at all from my Turbo and awful sync from my Genesis. I did this mod before to an 8220 and it worked fine, so I'm not sure what's causing the problem. I replaced the 1881 circuit completely with another one and I get the same result. Any thoughts? I followed the directions from here http://www.mmmonkey.co.uk/console/other/rgb2vga.htm Here is my board wired up:

 

[Xian Xi]

Use the auto scan feature. Hold the down button for like 3 seconds.

 

[mikey]

Just tried that, nothing changes. I went through every input to ensure I was being thorough and I don't get any picture. Before adding the sync cleaner, I would get sync that was unusable on my turbo, but at least something would come up on screen, I don't get anything now most of the time. Only system that seems to be happy is the snes.

 

[Hitokiri]

I tried to do the same with my sega genesis and I had the same problem, during the weekend I'll give it another shot.

 

[Xian Xi]

Just tried that, nothing changes. I went through every input to ensure I was being thorough and I don't get any picture. Before adding the sync cleaner, I would get sync that was unusable on my turbo, but at least something would come up on screen, I don't get anything now most of the time. Only system that seems to be happy is the snes.

Did you do the RGB mos for the turbo with the 4 amps?

 

[purple]

same problem here with gbs8220, i tried jrok sync cleaner, intersil el1883 and el4583. nothing comes up on hyper 64, genesis and cps1. i also have ISL59885 demo board from intersil but havent tried it yet. it seems like i will need syncstrike from arcadeforge. only one time i got partial picture freeze and it was looking good. try adjusting clamp settings that help a little. i ordered some lm1881's and 680k resistors i'll try again. By the way the power supply is another problem, you need 5v 2a regulated at least or 12v(?)

 

[Xian Xi]

I've personally used a GBS8220 with a Hyper 64 and it worked fine after using the auto scan button.

 

[purple]

Can you share your setup with us please?

 

[Xian Xi]

Can you share your setup with us please?

It wasn't anything special. Just my supergun powering the board and the GBS. I had a jamma breakout board and had the RGBS coming from that into the GBS. When I first fired it up it looked glitchy, held the "down" button for 3 seconds the first time, it cleared up most of the problem and then did it again and it fixed it.


Edit: Just to add, the glitchiest board I tried was a PGM. Even with holding the scan button it still didn't sync perfectly and that was with a sync cleaner, too.

 

[purple]

maybe gbs itself is faulty, i also think maybe its version issue. mine says v3 and there are v4 boards around, jammaboards also sells special software ones that works with mortal kombats etc..where did you get it from?

 

[Xian Xi]

Mine was from ebay from China/HK.

 

[mikey]

Mine is also from HK/China, it's a version 4 made in Feb. 2012. I'm powering it with a 5v 4amp supply, so powering it shouldn't be an issue. My turbo is also RGB modded, so it should be working fine. Like I said, I had an 8220 before, and with the 1881 added it worked flawlessly with everything I threw at it. What's really blowing my mind right now is that the sync cleaner circuit is making things worse than how it operates without it. Maybe I'll disconnect the 1881 and try holding down the auto adjust and see how things work out on it's own. I read earlier on the gamesx article about the 1881 that throwing just a composite sync signal at it can sometimes cause problems, as it's designed to take a composite video signal and strip the sync from it. It worked before for me though, so I have no idea what's going on.

 

[mikey]

Alright, so I spent the majority of my free time today working on this. I tried LOTS of things, moving the 1881 circuit before the turbo's rgb amp circuit, I built another sync amp based on the turbo's single transistor sync amp but made it with variable gain control so I could tweak it and inserted it both before and after the 1881 circuit, but that didn't work. Finally, I wired up a variable amplifier using an op amp with up to 100x gain (10k - 1m differential) and now I get a usable picture. With it cranked, it stays stable but the picture is still a bit "jittery". I guess some of the GBS 82XX series encoders need the sync signal to be pretty amped up to get a stable picture. I'll keep messing with it and then post results with a schematic once I figure it out.

 

[purple]

that would be very nice, thanks

 

[mikey]

Alright, here it is. I tested it with SNES, a Genesis Model 2, and a Turbografx, and they all work with this amp installed. The turbo's picture is a little shakey, but definitely usable. Not sure if someone can come up with a fix for this, I think it's because of the turbo's sync output. The picture from the snes and genesis are solid. You may have to adjust the pot for each system to get it to sync up properly. I also recommend installing a switch to enable or disable this circuit, I found my cps1 and neo wouldn't work with it, but they work fine with the GBS unmodified. Also, I tested a bunch of different op amps for this, including a bunch of dual ones, and the NE5534 was the only one that was able to get a stable sync signal from all three systems. I only had a bunch of common or audio spec op amps kicking around though, and there is likely one designed for video that will do a better job. Let me know if it works for you and if you have any questions.

Materials:
C1, C2 - 0.1uF
LM1881
NE5534 Op Amp
R1- 680K
R2 - 10K
VR1 - 100K Linear pot

 

[Yodd]

I wouldn't bother amping the Turbo's sync output. I rather use its composite video as sync. Especially since you are running it through a sync stripper circuit.


Also, which amp for the Turbo are you using? I hope not the single transistor per color as mmmonkey suggests. It's insufficient in my experience and missing other required caps and resistors.

Use the one at gamesx.com instead.


Sent from my iPad using Tapatalk HD

 

[mikey]

I'm using the single transistor circuit, so maybe I'll have to give the other one a shot at some point. Could the RGB amps be responsible for the picture being a little unstable, or is that just a sync issue? I just tried tapping the sync from the composite video signal and bypassing the single transistor sync amp and I get the same result as before.

 

[purple]

i believe this would be better together

 

[mikey]

i believe this would be better together

Very cool, I just ordered a couple of them to experiment with. You said you have the eval board for them? Have you had a chance to try it?

 

[purple]

i also ordered some samples, also i bought soic dip adapters. eval board is installed inside my supergun and it feeds both hsync and vsync to wei ya converter. hyper and cps1 syncs without problem, tried md1 and got rolling image slowly centers and locks. i think my gbs8220 is faulty so im ordering new one.

 

[purple]

i got my new gbs8220 and replicated your circuit today. tried with megadrive1 without success, i cant get stable picture. i can see sync change on screen but it cannot lock on it. at least i got picture from new converter. i wil try my cps1 and hyper64 and let you know.

 

[Xian Xi]

The LM1881 alone outputs the sync at the proper level, why would you apply an opamp on the output?

 

[mikey]

If I didn't use the opamp, it wouldn't sync up. Not sure why, I just know that with the opamp applied I am able to get stable sync on every system I have by tuning the pot. I tried multiple 1881's too, had the same problems with all of them.

Purple, are you using the NE5534 for your opamp? It was the only one I was able to get sync with, nothing else seemed to work (on all systems). Also, I found I had to be pretty "precise" when tuning the pot. In my mind, this is by no means a solution to this problem, just something to get it working for the time being. I also found that I didn't need this circuit to use my CPS1, CPS2, or Neo Geo, just consoles (minus the SNES, it seemed to work fine with just the 1881, but also works with the opamp tuned properly). I didn't try this on a genesis 1 though, just a genesis 2. I have a genesis 1, but I won't have time to bust everything out to try it anytime soon. If you're able to give the ISL59885 a shot, it may be more worth your time messing around with that.

 

[purple]

yes im using ne5534, i also noticed its very sensitive, have you tried measuring the pot value and replace with resistor?i think it would be better to use on/off switch instead. by the way my megadrive1 is ntsc modded pal machine, im using ''comp sync'' from 8pin din connector, i wonder what difference make if i use ''sync'' instead of ''comp sync''? i'll give another shot tomorrow. i also built el5483 circuit today because it has horizontal sync and you can use rgbhv option on gbs but its worse, cannot see any image, just garbled video. maybe i built it wrong, hey xian i noticed you had them on your page, can you help us build working circuit for el4583? anyone used rgbhv option? waiting isl's to arrive soon, i'll let you know.