Most older style thermos are just switches, pretty tough to trash, it can be done if you try hard enough. Like Red Green says "If it aint broke your not trying hard enough."
The 28 vac should be fine. You need to find why its not getting to blower and AC control relays. Understands (draw it out for yourself) how you system is wired and try to bypass thermo. Jumper 28 vac to control side of blower relay. What brand are we talking about?
Check site on thermostats in earlier post. Broke or miswired most likely.