The Atari 400 and the Atari 800 were conceived simultaneously and almost identical. One significant difference between the Atari 800 and the Atair 400 is the presence of the right cartridge slot. This slot was not a big commercial success and is absent in the Atari 400. It is also not there in successor models like the Atari 1200 XL, Atari 800 XL, and Atari 130 XE.
On pin "S" of the cartridge slots of an Atari 800, there is not the PHI2 signal attached as on all newer machines. Instead, there is the RAS signal connected. The falling edge of PHI2 and RAS is nearly the same, but RAS is much shorter low than PHI2. This is logical since there must also be some time for the CAS signal. This is the comparison of the signals on the scope:
This can lead to problems with modern circuits. Most modern circuits in flash cartridges or flashable cartridges sample the PHI2 signal for their logic. Such cartridges will run into issues on the Atari 800. They will either not work or run into errors during write cycles. This includes "The!Cart" and "MyIDE II".
The most viable solution is a minor modification that makes the Atari 800 compatible with the Atari 400 and the XL and XE series. This modification connects PHI2 instead of RAS to pin "S". No problems have been reported running any Atari 800 cartridge with the modification installed. Even original cartridges that write the cartridge like "Telelink II" still work fine. If you want to do this patch yourself, here is the easiest way.
Then connect pin "A" and pin "S" on the "right" cartridge slot together, as shown with the yellow wire. Then cut the RAS trace at the location marked on the right.
Alternatively, if you do not want to change something on the PCB on the Atari 800, there is also an alternative. You can use a 2nd cartridge in the right slot to get the correct PHI2 signal from pin "A". Then you can feed it directly to the corresponding pin of the PCB of the left cartridge. Of course, the connection of the pin on the left cartridge to the module port connector must be cut in this case. On the PCB of "The!Cart V2", you find two small pads prepared precisely for this purpose. It is labeled with "for 800 PHI2-MOD! cut here". Cut the connection between the two pads and solder a write from the pin "A" of the right slot to the upper pad.
That's all. Both mods are independent of which TV system you have. It works also on NTSC as on PAL or SECAM.
You can post in the related AtariAge thread if you have further questions.