Well thats a bit weird.
The MZ-700 has a great service manual, puts others to shame and gives these waveforms and tells you the test points to look for them:
Said test points can be found here on the schematic:
And the PCB has a decent silkscreen making part location and therefore test points, easy to find.
The trouble is, I get zero signal at TP 1. Just nothing. I did move on to look at TP 2 & TP4 , but given there was nothing at TP1, I wasn't surprised I found nothing, since TP1 was flat.
I will compare with another cassette deck to see what the resistance should look like for the head, as even connecting the scope directly to the head read nothing, so that feels suspect.
It doesn't quite answer the question of why the external cassette doesn't work, though if it is using the same circuit on the cassette and that is bad, then all bets are off.
There is 5v at the op amp, before anybody asks. Bit of a head scratcher.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...