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Miguel,

At the time I used my HP 8560A spectrum analyzer which has a tracking generator as the nanoVNA didn't exist yet.

The setup is very simple. There are test points at which you inject the signal from the nanoVNA S11 port and test points at which you attach the S21 port to examine the log magnitude display and adjust until the display matches the manual. This is Scalar Network Analysis. Never having done it before, I did it twice. The stages interact a bit and so after I adjusted everything, I went back over it to make sure all the passbands looked correct.

I assume you have the factory manual. It's not much, but it's enough. I'll scan it if it's not in the Files section.

As you remark you don't have a signal generator I'd like to suggest that you buy a 1 MHz square wave output crystal oscillator module and set it up as a 0 dBm (1 mW) signal source. That will give you markers every 1 MHz. You'll need to add a resistive matching network with 50 ohm output impedance that lowers the signal level. Put it in a neat box and you'll have a very useful piece of test kit. That said, just building it on a breadboard with no connection will work. Just not homebuilt test gear which is the best kind. BTW If you box it, label it. I have a box of such things I built but never labeled. It's a real puzzle to figure out what they are 40 years later.

The tinySA is an excellent signal generator if you add a harmonic filter, but you can't use it as an SNA as you can the nanoVNA.

In the absence of other indications, I suggest you start at the antenna and look at the signal with a nanoVNA as you work through the circuit section by section. Connect S11 to the antenna and start tracing through the schematic with a probe connected to S21. Make sure twisting the knobs produces the desired result The mode switch has been a continuing annoyance on mine. I need to take it apart and remedy the intermittent contact.

Have Fun!
Reg