I just got a SINAD Meter at a flea market; as usual no manual.
If someone is familiar with the controls could you give a brief
description on the adjustment of the front-panel controls:
1. Tuning
2. Balance
3. What response should I get in the SINAD position?

A SINAD meter meters the difference between signal and noise
(gee, what a surprize).

Typically, these things have an audio input and a notch.
They measure the audio level before and after the notch.
The difference in levels between these two signals is the
signal to noise ratio or SINAD.

You use them with a RF signal generator to adjust receivers.
Modulate the signal generator with 1000Hz, 3kHz deviation
(did I mention SINAD is an FM thing?).

Assume that the 1000Hz signal is 20dB stronger than the background
noise. The complete signal has a level of X; the signal after the
notch has a level that will be 20 dB down (as it will only have
the background noise). The SINAD will be 20 dB.

Your box is adjustable; I suspect tuning sets the notch frequency,
balance sets the signal levels and SINAD is the signal level difference.

These are actually pretty useful devices if you want to squeeze
the last dB out of a receiver. If you don't have one, you can get by
by using a _good_ comms speaker and aim it directly at your ear,
then fiddle for minimal noise, reduce RF signal, fiddle receiver again, etc.
That's what I did before I got one and you can get reasonable close
this way too.

73,

Geert Jan PE1HZG