the basic operation of water conditioners/softeners is simple:
raw water flows through a tank of "resin" aka "chemicals" which are designed to remove the offending "thing"
different resins handle different issues - pH, iron (and other metals), calcium (and other minerals), nitrates/nitrites, radon gas, etc and a whole long yadda yadda list of nasties.
not every resin can be blended/constructed to do 100% of the needed tasks - for example a previous house system had two 'resin tanks' - one primarily for pH and one primarily for the nitrate/nitrite problem.
after a short period the ability of the chemicals in the the resins to "capture" slash "remove" the nasties is "used up"
so a highly salty solution is "back flushed" through the resin tanks - the salt causes the "nasty ions" to "fall off" the resin bonds and they are flushed to a drain.
then some raw water is flushed through the tank so you don't get a batch of super-sea water at the sink - that flushing volume is also routed to a drain.
then the raw water is routed through the system to the house surge tank.
so there's a host of valves, timers, solenoids, seals, o-rings, plumbing and ports that eventually fail - virtually everything electric or mechanical fails, it's only a question of time.
after 20 yrs of heavy salt exposure, the internals of those components could be pretty dicey.
>>"replaced the salt"
does not need 'replacing' - the rock salt is used up by the flushing process - it's a 'replenishment' issue. if the system has been running for years with no salt, that's not a good thing.
>>no water in the glass
unless you are standing there as the system is going through its cycle, you won't see anything in the sight glass.
actually, the sacrificial anodes used in water tanks to combat bad pH can make the rotten egg smell worse. so if it had been eaten away, the smell would have "improved" not "got worse"
being on a fixed income doesn't make things easy, for sure - some things to put on into the decision making process:
pH - this is likely to be the biggest issue. if the pH is too acid it'll eat copper / iron pipes, and brass/bronze/pot metal faucets, fixtures. an out of whack pH can cost thousands and thousands to re-plumb every water "thing" in the whole house. that means tearing out walls, plaster, dry wall, etc. really really bad, it etches porcelain (sinks, tubs, toilets . . )
if you have a pH problem, it must be corrected.
metal/mineral content - there's "ideal" and there's "acceptable range" and there's "and so what difference does it make?"
organic contaminates - these are more insidious - do have a complete analysis run.
hydrogen sulfide - most ion exchange systems are ineffective - but there's always progress - make sure there's something in writing if the salesperson says the system cures cancer, baldness, bad politicians, provides instant weight loss and produces free energy to boot.
aeration - this process has to be accomplished in a tank "open" to the atmosphere - the "aeration" mechanically strips the hydrogen sulfide gas out of the water - but once stripped, the stinky stuff has to go away, or it is reabsorbed back into the water. so the water has to be pumped up from the well, goes thru x,y,z, then to an aeration tank - which has to be vented, then you need a second pumping system to pressurize the household supply tank.
activated charcoal is cheap - look for a filter system that does not entail proprietary 20 billion dollar per element costs.
oh, make lemonade before you compost the rinds (g)