I had a case like this, beautiful Onkyo receiver but every time I would run it for more than 20 min. I would be on the edge of throwing up.
Smoker owned for sure, heated smell was combination of a mold infested basement and cheap Karaoke bar on Monday morning...
I tried everything, list would be too long to mention here, but that included spaying with alcohol, rubbing with all kind of refreshing stuff - nothing helped.
One of the craziest attempts was to cut two fresh onions and seal them under the top cover for two days!
Long story short, when I started restoring it I would spray/wash/rinse every single PC board with mix of 80% water and 20% dishwasher liquid. And after each finished board smell would get
less and less intensive. Finally, when I recapped both power amp driver boards the smell was completely gone. Not to mention that water running
off the washed PC boards was dark brown/grey sludge...Not an easy fix but at least it is possible.