As was mentioned, there is a bandpass filter section for each band the radio can tune. After the BP filters, there are the bands RF sections which contain the 1st and 2nd RF amp transistors in simple terms.
Some older radios had an RF stage before the bandpass filters but then all bands would be affected if your 1st RF amp went out.
My guess is the 1st RF amp failed in yours. It could also easily be the BP section but those contain more passive components along with a couple varactor or pin diodes. They usually also turn on/off the needed RF section and 2nd mixer stages needed for the band you are tuning.
Unfortunately, those sections of the RF board are also the part of the board that is covered by black (usually) epoxy. When they forced manufacturers to block the old AMPS cell band, people found they could alter the BP section and regain cell reception via images in other ranges of the scanner. So the rules regarding blocking cell phone reception were rewritten and now forced the manufacturers to also restrict access to the BP section so the radios could not tune cell band via images.
The easy (and usually only) way to achieve this new rule was to limit access to the BP section and its wiring or copper traces.
The downside is that it makes component level repairs almost impossible.
Sometimes you get lucky and can still get at the RF amp transistors.
But... about the same time as the cell band hit the manufacturers, Uniden also stopped supplying service manuals so it would be really hard to determine what is what in the radio.
As n3617400 also said, you could try reverting back to an older FW version in hopes it will reset something. I think there also may be a reset button in the battery compartment but not positive on that. It's been a while since I used my HP-1!
That is worth a try though.
I guess the other question that comes to mind is your saying that Uniden did not repair it during a recent trip to them for service. I assume that trip was for this problem?
If so, I'd send it back with a nice note saying you sent it in for this problem but it was returned unrepaired along with a copy of the original repair ticket showing they only checked the radio with P25 systems and likely only in the 800 MHz band to boot.
In the worst case, Uniden offers a flat rate repair service that is just under $50 dollars. So even if the problem is in an epoxied area, they would repair it for the flat rate fee. I'd imagine they just swap the RF board.
If you paid them for the 1st repair attempt, they should not charge you again as they did not fix anything.
I know Uniden just moved in the last couple weeks so many are complaining of unanswered calls into their service center. Try searching the threads here in the Uniden forums as I think someone posted a better or new service center phone number.
If they charged you but did not repair the problem, as far as I'm concerned, they should also cover the shipping costs to send it in again but good luck with that.
Have you made sure you have not turned on a global attenuator? Your mention of the air band still working makes me think it is not on but it's still worth checking.
I think UPMan once mentioned the majority of the units they receive back for repair are fine and the user had enabled the global attenuator.
UPMan may also see your thread here and offer assistance in getting your unit back in for a proper repair.