Long explanations to answer several questions at once and anticipate some others:
Back in 'the olden days' pagers like Minitors were tested in a Motorola RTL-1005A Universal Radiation Test fixture;
Buy Motorola RTL-1005A Univeral Radiation Test Fixture in New Berlin, Wisconsin, United States. This is a test fixture for servicing pagers and probably other similar size receivers and other electronics. It measures about 8" long, 6" wide, and 5" high. It is sturdily built with a heavy brass base.
www.tzsupplies.com
You would connect your signal generator/ service monitor to the BNC connector at the right hand side, and lay the pager face down in the fixture, and generate a very low-level RF signal to it. Ideally, just enough to set off the pager, and while it was in there, adjust the RF tuning controls in the Receiver section for best sensitivity.
The little toggle-clamp assembly would flip down and contact ground and a receiver metering point to measure from. Keep reducing the RF output of your service monitor, tweak the coils & caps in the Receiver, 'til you can't make it any better.
Really really well set up radio shops had the pager test bench in an RF screen room, so no outside signal could get in and interfere with your on frequency tests, and no signal from the generator inside could get out and set of pagers unwittingly.
In a pinch, you can put a rubber duck antenna on the generator's output port, do a very low signal, and lay the pager right alongside the duck; not as precise, but a good quick check.
All this assumes you have a proper piece of RF signal generator (and most for the last decades also include two-tone encoding). Trying to use a portable programmed with frequency & tones will tell you the pager receives, but not the basic sensitivity.
Along the US-Canadian border is a thing called "Line A" roughly paralleling the border and about 75 miles from it, (It's Line B inside Canada), where radio frequency coordination for business and public safety licenses have to be also cleared thru the opposite country's authority, and
Here's a good page from WWARA:
And another mapping effort, zoomable:
And here is finally some meat of the matter, again from WWARA:
Down near the bottom of that document, you'll see a footnote that Amateur operation cannot be in 420 MHz to 430 MHz, north of Line A. Because that range is used for their (business? public safety? what else?) radios.
If you go over to Buffalo & Erie County NY, (
Erie County, New York (NY) Scanner Frequencies and Radio Frequency Reference) you'll see lots of Public Safety frequency use between 420 & 430 MHz. Buffalo, Rochester, and such are within Line A, so they can get coordinated for frequencies in that range and hams must stay out.
I looked at several counties in Washington that are within Line A, but I don't see them using this range.
Since you're in King County, Line A runs right thru your county. Down in the rest of U.S., your 423.7750 would normally be in the ham band, but not where you are.
In spite of growing up south of Buffalo, I only got my ham license after moving here in North Carolina, so I've never learned the particulars of Line A operation, rules, etc. When I've visited back home, I typically don't do much ham time, esp. on 70 cm, and even then it would generally be on established ham repeaters listed in the directories.
All that said, don't lose your gung-ho for this radio stuff. (aside: very funny movie, when Michael Keaton was young. 'Between a rock and a hard-on')(but I digress again). Up in your neck of the woods, this Line A stuff makes life complicated. And somewhat last, Minitor II's are I think the last of the crystal-based pagers; if you want to change frequency, it's gonna cost a fair amount. You could move it into the ham range where you are, but that would take a list of expensive or almost unobtainium equipment and a long learning curve.
I've probably raised more questions than I've answered but I figured I would try to answer some of the ones you're heading towards anyway.
