One thing that is nice about a spectrum analyzer is that if you don't mind putting it on a machine that is either new or at least has no squawks from the flight crew, you can get a baseline over all frequencies for comparison when there are issues. I speak only to the 192 and 192a Chadwicks, although all spectrum analyzers could obviously fill this part of the bill.
As I recall, the only difference between the 192 and the 192A was a knob on the 192 that was labeled " broad band". On the 192A the knob in that spot was selectable between " track" or " balance ". I never even saw a 192 so cannot say what the broad band knob actually did. The knob on the 192A is obvious as far as function.
AFAIK, everything else was similar if not the same.
I'm old, so I have a soft spot for the simplicity of the 177, but when the 192A showed up, the advantage of a spectrum analyzer took much of the quess work out of finding was was actually vibrating. It's the difference between having a handful of data points and having a complete boatload of all the data. The other reason I liked the 192A was that it was not driven by specialized software that was someone's best guess for make and model. It did have belts and gears, servo motors etc. but 192A was a rugged and reliable unit. The lack of all the monkey motion with the newer all solid state units and being able to download your data are marked advantages to the units that are built that way. I did find, over 35 years of t&b that if you were familiar with the model it was faster to not tangle with software driven devices. Your milage may vary and not everyone ( anyone?) would agree with me.
Having spouted all that, for those who may not realize it from the conversation, the 192A is not supported by Chadwick as it was when it was in production. There are some proprietary integrated circuits that may be difficult or impossible to find. There are several companies that do work on them and you'd be wise to talk to them before buying a bargain unit on ebay from someone who doesnt offer full support. Make sure you can still get the special pen nibs from someone. Not much good without them. I also used it for propeller balancing for many years and would give the chart to the operator to put in a logbook and show them as proof that even if they couldn't feel the difference, the machine could. It's amazing how insensitive some people are to aircraft vibrations. It's a whole other thread.