Consolized MVS: RGB from JAMMA (PCE Amp mod)

Tried to find a better solution for handling RGB out on a MV-1FZ unit to avoid overblown colours etc.

This thread often pops up when looking into such issues.

To summarise: attenuate the RGB signals to 0.7V with a voltage divider, then put them through a video amplifier with 75 ohm back-termination resistors.

The PC Engine RGB amp already has most of these features on board and only a couple of modifications are necessary to handle this task.



Firstly, we need to attenuate the signal, rather than amplify it further. On the back of the board, there is a trace that connects the resistor array to the THS7314 amp.



Scrape the trace to the right of the cut and solder on +5v to supply the amp directly.

Tie the original +5v pad to Gnd. We now have a set of voltage dividers over the RGB input lines.



To we need to attenuate the output from the Neo Geo DAC down to 0.7v as input to the THS7314 amp. Output from the DAC is 4.1v, so use this calculator to choose suitable resistor values.

I have chosen 6.8k ohm (R1) and 1.4k ohm (R2) which gives 0.7v output.

Finally the resistors on the output side should be 75ohm (Some PCE Amps use 100ohm here).

This is the unit installed:



(I managed to rip off the +5v trace on the underside - hence the yellow wire up top. Don't do that).

Here are the results!:



I have only tested this on a MV-1FZ so far.
I hope it should be suitable for other models, but that will require more testing.

Thanks to mikejmoffitt for his help fine tuning this and mickcris for initial input.

Cheers!

I appreciated the effort on this, but I'm not sure I'm satisfied with the results you posted. In the "After" image, the black still doesn't match the black of the unused portion of the screen like it does with other consoles on the Framemeister. So why go to the trouble?

The 'after' shot clearly looks better with regards to the way the colours fade. Just checked my 1FZ and it looks the same as your 'after' photo, but I haven't added any amp!?! Just the basic mod using resistors (same way the supergun boards work), and it looks great! Maybe I just lucked out with the resistor sizes?

Neo Alec said:

I appreciated the effort on this, but I'm not sure I'm satisfied with the results you posted. In the "After" image, the black still doesn't match the black of the unused portion of the screen like it does with other consoles on the Framemeister. So why go to the trouble?

Click to expand...

That's...not really the issue being addressed here...? Colours are overblown as the input voltage from the MVS is too high for a standard TV to handle.
That is an issue that is unique to the MVS. In the After image, the issue is the black level. That is an issue related to the TV brightness setting.

Gadget - what resistor values are you using? I'd like to compare!

nam9 said:

That's...not really the issue being addressed here...? Colours are overblown as the input voltage from the MVS is too high for a standard TV to handle.
That is an issue that is unique to the MVS. In the After image, the issue is the black level. That is an issue related to the TV brightness setting.

Gadget - what resistor values are you using? I'd like to compare!

Click to expand...

390ohms on the RGB lines!

The only issue I'm concerned with regarding this is the non-black blacks. If the amp doesn't fix that, I'm not sure why it's necessary. I've had no complaints with contrast on my MV-1C otherwise.

GadgetUK, how do your black levels look, and what black setting do you use on the Framemeister?

I made something similar just to have an RGB buffer. I use it in my RGB only units.