Fixing an LED Home Cinema display caused whistling and a spark, is it salvageable? I was given an LED Cinema Display 1920x1200 from December 2008. I was able to fix a similar one because of a software issue on the owner's computer.

I was told that
it was working intermittently, showing the computer screen and turning off soon
aftewards. It stayed in a slightly damp basement for one year: when I opened it
apart, it had a bit of mold on the metal.

I connected it to my mac computer and the display showed nothing. I took it
apart and inspected the two boards, which seemed fine, although some of the
metal on the power supply board had a layer of rust. I tightened the internal
cable connections, left the cable for sensor-2 disconnected because it was too
short to be connected when the screen was loose, and plugged it to power without
a connection to a computer: the logic board whistled, had a spark, and produced
smoke for a few seconds until I unplugged the power cable. I checked the logic
board again and the damage was not apparent. I plugged the cable again, with no
whistling, spark, or smoke this time, and connected the mini-display cable: the
monitor displayed nothing but the fan was on high gear. I tried with several setups for the
sensor-2 cable: disconnected, connected one way, and connected the
other way. The result was the same: no display on the screen and high speed on the fan.

Does the whistling and the spark mean that one of the two boards is fried? If so, Apple has discontinued the product and does not seem to
sell the parts, so I'd need to buy from a reseller with the risk of
incompatibility. If so, Apple has discontinued the parts, so the cost and risk of second-hand parts to fix a 10-year old monitor seem large compared to the
more modern options available.

The display is also missing a glass screen: does it play a role beyond aesthetics, e.g. in filtering UV from the backlit display?

I read this thread and this
one,
and I think I have a different problem.
caused whistling and a spark, is it salvageable?  Does the whistling and the spark mean that one of the two boards is fried?

This sounds like you shorted something to ground.  The whistling is most likely from a blown capacitor.  I've intentionally shorted them for demo purposes and some did in fact whistle.  While it could be a different component, my bet is a blown cap.

the logic board whistled, had a spark, and produced smoke for a few seconds

This is a classic case of &quot;you let the magic smoke out.&quot;  In all seriousness, a spark and the sign/odor of smoke is a clear indication that you shorted something causing it to blow.
Now, there should be two boards - the power supply and the logic board.  It's entirely possible that the damage is to the logic board but the descriptors used point to the power supply.  Either way, without putting hands and eyes on it, we can only &quot;best guess&quot; it at this point.

It stayed in a slightly damp basement for one year: when I opened it apart, it had a bit of mold on the metal....although some of the metal on the power supply board had a layer of rust

Oh, oh.  This is a very good indicator that there are shorted connections.  Water may cause mold which in and of itself is easy to deal with, but moisture causes corrosion which is conductive.  Rust?  That too is conductive and should have at least been removed.
Can it be salvaged?
Technically yes.  But is it worth it?  I doubt it.
Logic boards and power supply boards can have components replaced quite easily. However, being that there is corrosion and rust with an unknown amount of time &quot;infecting&quot; the PCB; it's impossible to tell how much damage and how many layers are workable.
You may have to get replacements.  They are available on second hand markets like eBay.  (Apple no longer sells or supports these monitors).  However you're ultimately spending money and time on what amounts a 10 year old FHD monitor.  I have three in my garage right now (19 to 22&quot;) that I can't even give away.
This is probably something you want to sell as parts given the likely damage and the missing glass (which protects the LCD, prevents dust and contaminates from getting in, and enhances the colors).  There are simply too many better (new) monitors you can get for a few dollars more.