New RGB Bypass for Megadrive/ genesis/ SMS/ SNES/ Famicom

[VajSkids Consoles]

Hi guys ,

I’ll try and keep this short and sweet.

I’m from Victoria and we are in massive lockdown (again) because of Covid.
I found it really hard to get SCART cables and THS series amps (for bypasses) from overseas take ages to get here

I searched a local electronic store for suitable amps and found the analogue LT6557, and found some 6 RCA to SCART bidirectional cables that seem to be of high quality.

It’s a great RGB amp but I want it to be perfect

Hoping an expert can help sort this out as I really can’t be bothered tinkering anymore

Here’s the quality of output (albeit on a CRT)

https://youtu.be/1umFdpofZ9s

The issue I’m having is my modded master systems are working flawlessly on any CRT I hack along with the Euro Loewe with factory RGB scart... it’s a 100hz model and I couldn’t play zapper games with my NES / sega systems though so I started hacking CRTs in some of my spare time (and have a few more to do)

I’ve now hacked an old Hitachi (philips main chip) a recent flat screen Trinitron and today an old Sharp
They’re all fine with my modded master systems, a scart cable for my 1chip SNES arrived today and I confirmed that’s also fine (though not super impressed when compared to my 2 non 1 chip SNES and my famicom through the LT6557, nor the fact it came wired for sync over composite)

Anyway my LT6557 prototype (external) wouldn’t work until I removed or adjusted resistor values used as voltage dividers.

I’m not sure on the maths and measurements required to achieve perfect VPP levels on each channel, I have ordered some PCBs matching my prototype from oshpark to play with which should arrive over the next few weeks. The quality of picture is too good for me not to want to get it “just right “

The Loewe television isn’t fussy on VPP and I can just adjust colour temp/ brightness etc via OSD but on the linked video I’ve used 75 ohms on red and blue and 20 ohms on green with factory default settings and no changes .. for some reason using the exact same components on the input and output and measuring VPP at the output of the amp (after the in series resistor and cap) green was around .5v under red and blue (though never measured on a completely white screen)

Any input on how to achieve the correct attenuation for maximum compatibility would be much appreciated as I’m planning on running this through an OSSC

Edit: I’ve tested with pure white screen on sms test suite , and without any terminating resistors on the amp side (they probably shouldn’t be there anyway) 2vpp red and blue, but green 1vpp , I think I have a dud amp ? 75ohm to ground sliced this to 1.5 on R/B. For reference my master systems which are flawless on everything , including direct driving CRT guns were 1.6vpp at scart head @ tv end each channel.


https://oshpark.com/shared_projects/xxUPtrbn

The Untested board which is my prototype copied over to PCB, take it easy on judgement as it’s first PCB I’ve ever made, learning the software as I go