This is what you're looking for:
Sadly Richard42 sold all he built of those, but he posted instructions and diagrams so someone else should be able to build those again.
Basically this device converts the C128-digital-RGBI with 16 colors to an analogue RGB-signal which can be fed into converters like the GBS-8220 or the Ambery you own.
I own both the Ambery and GBS-8220 and before this small device came around connected the Ambery just like you did. Two problems:
- Signal levels are 5V when they should be 0,7V. Doing this may damage your hardware (so I've heard).
- you only get 8 colors because the I of the RGBI-signal is not converted.
The Ambery as well as the GBS-8220 claim to be "CGA"-converters, when in fact they are not. Sadly some time during the late 80s somehow the real meaning of CGA got lost and was used for arcade-machines monitors, which are in fact RGB analogue (like the Amiga). So in order to convert "true" digital CGA (= RGBI) you need the small device or some similar solution like this one:
I much prefer the GBS-8220 solution though. This produces a picture a lot better than the Ambery even with the RGBI->RGBA converter. Mine works with 50Hz as well. Also the GBS-8220 can be found on Ebay for about only $30-$40.