I can see why you are pleased with that.
So, what's going on with "however when I insert the MDA card in and power on, all I get is a very short high pitched and sometimes low pitched beep and nothing else so I'm not really sure what to make of that."
To me, it is now sounding like a situation of 'multiple variables were changed, and a resulting symptom attributed to the wrong changed variable'.
The first time I plugged in the MDA, was for use with the RDR - assuming that it's onboard RAM might be needed for things like serial access. This is when that occurred and before I stopped trusting the breadboard adapter. The RDR appeared to be executing as expected at the time (ie I got tones and relay click) as did the landmark ROM. I've since tried the MDA with the original 5150 ROM in the socket and I do not get the high or low pitched tone. However as expected still, the display doesn't work so the POST is failing somewhere - likely the RAM stil I guess
Agreed. Re the continuous beep. See my hypothesis at [here]. It sounds like you have one of the subject motherboards - with no U33, or a compromised U33, a continuous (or intermittent) tone is heard.
The symptoms sound exactly like what you describe in that article and potentially adding the MDA card while I was using the ROM adapter was preventing the ROM from being read, leading to that continuous tone also