What is the VU board you are using? Most voltage regulators need higher voltage then their output, for example for 7800 series or LM317's they drop out when the source supply is less than 2v above its output voltage so +4v is usually a good target to aim for. Going too high, like feeding a 12v regulator 24v will cause it to run very hot as it has to dissipate the extra voltage.
A hot transformer could indicate the load is higher than the transformers current rating. Generally those small PCB transformers do run hot so its usually a good idea to fit one rated for more current than it needs to supply, assuming you have the physical space.
They have changed the layout of the board since I bought mine, but it's something like this:
http://www.ebay.com/itm/2pcs-Panel-VU-Meter-Warm-Back-Light-Audio-Level-Amp-One-driver-board-/121468037573?pt=LH_DefaultDomain_0&hash=item1c480f29c5You are right, they are using 7812 regulator on this board, and I measured, it does not run very hot in my application. It gets 22V DC input from the rectifier bridge and then regulates to 12.0V. Right now I have installed a 18V AC transformer that is rated for 0.16A, whereas the board consumes 0.8, so fine here.
I then run the transformer outside of the case for several hours to understand if it's overloaded or something - it doesn't get hot at all, like 30-40 degrees maybe 50 tops. Inside the case, hanging upside down, it does get pretty hot, I presume because the overall temperature inside a Crack case is high, everything dissipates a lot of heat. It's been working for a week for me, sometimes 8-10 hours non-stop, with no problem so far.
