That is a crystal oscillator radio. If I am not mistaken, the basics of the circuit is a DC power supply stimulates a transistor which in turn amplifies the crystal. Those crystals are your RX channels. Back in the 80s-90s, most police and fire frequencies (at least in the LA area) were in the 150-160 mhz range (most being in 155 area). Nowadays, it is not unheard of but much more rare for police/sheriff not to be on countywide digital systems nowadays (along with fire and EMT and even security and public works - the channels and radios are digital so they can utilize the bandwidth more efficiently). I have never used a crystal oscillator radio myself, but I used to have a programmable scanner when I was young (talking early 90s). My grandma had one which got me interested in electronics. However, getting back to your questions....
- How do I determine if the crystals from the 70s are still valid at the labeled freq rate?
I think there is a device you can buy to test them (like https://medium.com/@rxseger/crystal-oscillator-tester-frequency-counter-kit-review-586ea0bac2f9) but honestly I think as long as they give static when you turn the squelch on/off (I forget which one makes you hear static sorry) then your probably fine. It is noteworthy, however, that the frequencies of those crystals may not correspond to a transmitting station in your area. What I mean is that the crystals may work fine but be worthless to you because there is no one using its frequency, so it would probably be best to simply plug them all in, scan for a while, and keep the ones that you can hear analog transmissions on (not sure if you could convert digital transmissions with that thing or not). A good resource for you to use to determine what frequencies of crystals to purchase would be radioreference.com, which is a website that holds all the FCC databases (technically so does the FCC but RR is much easier to navigate and much more organized). NOTE: I am not affiliated with RR or the FCC beyond my Ham Radio license.
- If I just want to play around, what crystal frequencies should I try to obtain? I remember reading that most police, emergency radios have gone digital, so that I would not be able to receive that. What frequencies would be interesting to listen/scan?
Okay so just because your radio is analog does not AUTOMATICALLY mean that you can not listen to digital signals through a audio cable hooked up to your android/apple/pc. For the ladder there are a plethora of applications to choose from depending on the type of digital frequency you want to decode. Some of the common ones are DMR (Digital Motorola Radio - used mostly by businesses and security companies), P25 Phase 1 and 2 (These systems are used mainly by law enforcement/countywide systems that incorporate all their radio users into a cell phone like system - these are sometimes not encrypted but the trend has been going towards encryption), POCSAG (remember pagers? that's POCSAG), and many, many more. For P25, DMR, and some others, DSD+ (digital speech decoder plus) is the program you want. For POCSAG you want PDW. Check out this website it will help you learn all the digital protocols along with the frequencies, locations, and decoding software - https://www.sigidwiki.com/wiki/POCSAG
Okay so as a retro thing I get using a crystal oscillator radio, but unless you insist on using it for your airwave-surfing needs, I would suggest you check out what is called an SDR, or Software Defined Radio. There is this TV receiver that has a "realtech" chip that someone hacked and was able to open the full spectrum from 1kh - 6 ghz or something like that and these devices sell for something like $10-$20 (not including the HF end of thing). They are USB devices and you run them with a program called SDR# (SDR sharp). Because digital signals are not always the same bandwidth (meaning that it uses more than one frequency - any transmission is using a frequency RANGE - for example, a cop on his walky talky transmitting on 155.895 mhz with an analog radio uses 25khz of bandwidth, meaning that he is actually transmitting on 155.895 - 155.920. That's all fine and dandy for a analog radio, but what happens when we see a narrow banded P25 signal? Let's assume for a second that the narrowband signal is actually analog. Would it sound different that the 25khz one? Yes. Narrowbanding only uses 12.5khz bandwidth which means some of the frequencies in your voice are not transmitted, so people tend to sound "hallow" on narrow-banded frequencies (this is an over-simplification but you get the idea). Anyway, because of the variance in the bandwidth it makes it hard to get digitals signals that are decodable on an analog radio, but it is possible. Although it is much easier with a RTL-SDR. As for the frequency allocations, back in 2008 all TV went digital. All persons holding a FCC license of almost any voice type must go narrowband now, so the FCC can take half of the spectrum they sold these entities and resell it again. That's the FCC, that's what they do. On a side note, you asked about what you CAN still listen to with that thing. NOAA weather radio is usually around 162.55 or 163.45 where I live (although I think they have 5 or 6 different frequencies), Airplanes and Airport towers use the 120-130ish mhz range, Railroads have 100 channels in 160-161 mhz range, fast food transmitters are always fun to listen to although the actual frequency is sometimes hard to find. Also there are GMRS, FRS, CB and Ham radio frequencies that can be listened to via an analog radio, and sometimes even P25 systems have a VHS patch to their channels for fire and law enforcement that are unencrypted and analog. All of this varies greatly by region, so radioreference.com is your starting point for that. To answer the last part of your question about interesting frequencies to listen to - I used to live in La Mirada, CA, which is about 20 miles from the coastline. On a VHF frequency with my RTL-SDR I actually heard the entire call from an ocean rescue. It just so happened that one of the helicopters only had a VHF radio on board that worked, so they switched the whole operation onto a frequency I happened to be monitoring. Police channels are ALWAYS interesting, but as previously stated, most are going encrypted. If you live near Los Angeles, try 483.7625 mhz (that's dispatch 13 for my old area). You can only hear dispatch unless you are close enough to the transmitting unit to hear the uplink frequency (uplink=dispatch freq + 3 mhz) so dispatch 13's uplink is 486.7625 mhz.
Do y'all have any suggestions of suppliers, since I be buying onsy-twosy?
AMAZON.COM is where I get most of my stuff related to that
Do they still make them in that style that would fit in this radio?
Is this radio even viable with all the changes in the frequency allocations?
Yes, as long as you are listening to analog signals. As stated above, digital signals on an analog radio can be tricky
Lots of changes to account for new technology, and old frequencies are being recycled, etc.
Yes, especially with Narrowbanding.
I hope I gave you enough information to be mind blown :) Let me know if you need any clarification or have any other questions in this realm.