ULA (ZX Spectrum 16 48k)
The starting point of the system is the ULA chip.
If this chip is bad, you will get zero activity on the oscilloscope anywhere. and absolutely nothing (not even black screen) on the AV output.
A good check if the ULA chip is ok is to probe the Z80 CPU address and data pins (any of them) to see if you get any activity. If you do, it usually means the ULA is ok.
Presuming you do not have a faulty AV system (LM1889N) or RF/Composite output preventing you seeing output, then inserting the DiagROM into the extension port will load at least something on screen even if you don't have a Z80 CPU installed.
If you get no activity at all, its worth investigating the ULA.