scan converter - RGB signal too weak?

[chrisdanford]

Hello.

I have Averkey Imicro scan converter (pinouts below) and an arcade monitor.  The Averkey has a very funky connector labeled "RGB" that is identical to VGA except for the following:
 pin 4: RGB=NC, VGA=ground
 pin 9: RGB=select PAL, VGA=NC
 pin 15: RGB=compos sync, VGA=NC

First, I spliced R, G, B, Csync, and Ground from the Imicro onto the corresponding wires running to the monitor.  The result was an almost entirely black image.  However, I could see that there was some signal reaching the monitor because the ghostly image moved when I played with the position controls on the converter box.

Next, (being a bumbling electronics idiot) I tried hooking this connector up the RGB connector to my JPAC.  The following strange very strange thing happened:  The image on the monitor was perfect (normal color, perfectly centered, no double image

[Dave_K.]

Sorry for the lamo question, but why aren't you using pins 13 and 14 (H/V sync)??  Don't bother with the LM1881, and run these syncs directly to your monitor.  The only reason why I'd guess you are using csync is because maybe you are trying to hook up to a jamma connector?  Well you will be better off bypassing it and going direct to the monitor.

I don't have a JPAC, but it may do some boosting of the RGB lines in the circuit that were charged up before you had it plugged in (perfect picture) got charged again in the jpac circuit (super bright) and then bled off (down to dark).

If hooking the imicro direct to your monitor with r/g/b/h/v is still dark, put some 220 uf non-polorized capaictor in front of the monitor RGB inputs.  Works for brightening console rgb to arcade monitors.

-Dave

[chrisdanford]

Thanks for tips Dave!

You're right that there was an the error in my post.  When I hook RGB up to the JPAC, the JPAC is using the H&Vsyncs and ignoring the Csync on the cable.  By the way, my monitor takes R/G/B/C and not r/G/B/H/V.

The JPAC site says that it "amplifies the VGA 1 volt signal level to approx 5 volts peak-peak which is the arcade monitor spec."  The color signals from the RGB connector are "0.7Vpp +/-0.2 Vpp 75ohms", so why aren't they being amplified by the JPAC in the same way that it does with the VGA connector?  Is the JPAC just picky and won't amplify unless it is a 1V input?

Should I put the 220 uf capacitors between the RGB cable and the JPAC, or between the JPAC and the monitor, or bypass the JPAC completely and put the capacitors betweeen the RGB cable and the monitor?

Sorry for all the questions.   I can only hope that my mistakes will save some other poor soul a lot of hassle  

[Dave_K.]

What type of arcade monitor are you using?  I've never herd of one that only has csync input only.  

I'm suggesting you try bypassing the jpac, and run directly to the monitor.  Put 3 220uf caps in line with the monitor RGB inputs to brighten the picture (if needed).  If you can't find non-polorized caps, polorized will work, but I forget which way they should be oriented (I'd have to look at my circuit).

[chrisdanford]

Aren't most arcade monitors Csync only?  These are relevant pins on a JAMMA connector:

12 = red  
13 = blue  
14 = Video ground  
N = green  
P = composite sync  

I don't see any other place that I could hook up separate Hsync and Vsync signals.  Anyway, I don't think it's a big deal because my scan converter outputs H, V, and Csyncs.

I'll try to find these capacitors.  Thanks again for all your help.

[Dave_K.]

No, as far as I know all arcade monitors take H/V sync (with optional csync on H).  What you are talking about is the Jamma input connector.  The Jamma spec calls for csync only.   If you look at the back of your monitor (where the jamma cable hooks to the monitor) you should see inputs for H and V.  Just disconnect the jamma wires and run your r/g/b/h/v/gnd directly to it.

Yeah, first give the csync a try through the jamma connector if your box outputs csync (with the caps to brighten if needed).  Otherwise run directly to the back of your monitor.