The wording on the original diag bios is kinda crappy for explaining this. On MV1B and MV1C board in order to use the diag M1 cart you need to power-on with D+B held down. This lets the diag bios know to expect the diag M1 but to not do a slot switch. A slot switch is needed on all other mvs boards because at power-on they will use the SM1 rom on the motherboard and have to be told to switch to the cart's M1. MV1B/C don't have an SM1 rom on the motherboard and directly uses the carts M1.
No matter what is done (nothing, holding D, or holding D+B) at power-on the diag M1 is going to run on those boards so at a minimum you should be getting the "i'm running" jingle.
Given what you see with stock/unibios I suspect the diag m1 is not even running. I would check the following things
1. Using logic probe verify you see pulsing on the z80's clock pin
2. Using logic probe verify a low then high signal on z80's reset pin at power-on
3. Using multi-meter verify continuity between Z80's address and data lines to SDA0-SDA15/SDD0-SDD7 on the cart slot (CHA board)
There are additional M1 related signals on the cart edge (SDMRD, SDROM, SDRD0, SDRD1) that you should check for continuity for, but I don't know where they come from on the MV1B.
