STA-10 Car Amp Repair

[edPP]

Hi, first post so I'll try to do this right.

I am trying to re-teach myself some electronic repair as a hobby, I play too many video games but am getting old and bad at that. I got a cheap broken car amp to repair - a PowerBass STA10, class D amp.

This is despite not having a car, or interest in bass  .  Symptom is no audio output. Scope shows signal l going to -ive driver terminal, none to +ive. Signal responds to gain control where present. Putting scope aside due to lack of qualifications...

I know the first thing to check are transistors and rectifiers, so opened the lid on her and count:

• 1x TIP41C - NPN - right hand side - https://www.st.com/resource/en/datasheet/tip41c.pdf
• 2x IRF540N -   N-Channel - right hand side - https://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/IRF540N.pdf
• 2x PTP16N06N - N-Channel   left hand side
• 1 (each) x MUR1220CT/CTR- - rectifier - left hand side

I don't see any charring, but have not removed the PCB yet to check the underside. No definitive magic smoke smell either (I am an expert on breaking things so would stand to know).

The right-hand side heatsink has some white paste that, to my untrained eye, seems to have spread out over the bar - as if it has melted, as attached.  I have a cheap multimeter - a Ragu 81D. I don't trust it, because it is cheap, so I have a Fluke on the way - yay, a collection. But owing to my ill-informed suspicions I have probed ahead anyhow around this area.

TIP41C - tested per this site https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07 Whilst still in the circuit .

Step 1: (Base to Emitter): 0.67V 
Step 2: (Base to Collector) 0.67V 
Step 3: (Emitter to Base) 2.7V 
Step 4: (Collector to Base) rises slow to 2V 
Step 5: (Collector to Emitter) rises slow to 2V

The IRF540Ns also look dodge, but I'll save typing that up until a good person tells me it might be useful.

Does a kind soul have input to offer and ease my learning experience? I am tempted, on the basis of the mess and above, to desolder the TIP41C and measure in isolation. But don't have so much time that I want to do unnecessary stupid's, because then there would be far too many in this regard.

Ed