I am 15 miles away and getting them loud and clear tonight on 471.1875 with a PRO-96 and a ham 2m/440 rubber duck. I'm not familiar with the psr-500 but you should check your settings.Make sure you don't have dcs or ctcss turned on.
Lexington has never had a strong signal to my quarters (which isn't too far away); I suspect it is site-related, as back when Lexington PD was on 155.535, they would sometimes be wiped out by Hillsboro County on 155.52.
That said, their Astro signal is loud and clear here. The reason why one doesn't hear much is that they use the MDT for most routine dispatch.
I get them Loud and Clear in Lowell, Also when i got to Reading they are overwhelming. Like said earlier, make sure CT or DC isin't turned on, and make sure you have the correct NAC, i beleive it is NAC-775
Sorry to bring this thread back from the crypt...I downloaded the M'sex county PS freqs from the RR database and didn't notice that Lex PD was digital. I heard them on the scanner and thought "darn, that has that digital sound." I looked it up and sure enough, P25. It really does sound different - and I'm only two towns away with a good strong signal.