If you have 20VDC across the speaker terminals then (as
Enzo has already noted) the chip is likely fried.
Disconnect that speaker until you get zero volts on the output otherwise you could burn up the voice coil.
Check to see if the Zobel network on the chip output has signs of over heating, a sign there is instability,, possibly bad layout, circuit design.
If so then others here may know what is wrong.
The Zobel is the series resistor and cap across the output. It should be as close as possible to the output terminals of the Power chip.
If there are signs of discoloration (overheating) then check that resistor is within tolerance.
To the best of my understanding the LM1875 will work,, though you may have to replace the zobel.
TDA2050 data shows 2R2/2Watt and 470nF,, while the data for LM1875 shows 1R and 220nF.
I just did a search on TDA2050 and spider 2 and 3 seem to have a habit of blowing.
so maybe bad design or maybe counterfeit chips.

hope that helps,, Phil.