By now, the AVR would go into an Overload condition about a second or two after turning it on (when the main output relay would normally click closed ... which wasn't happening ... no output at all).
It turns out the schematics for the Panasonic SA-HE200 are easy to get on the internet.http://www.eserviceinfo.com/download..._SA-HE200.html
There are 2 large MOSFET power transistor ICs mounted to the main heatsink. The 6 channels are split between them. The chances of both of them going out at the same time are slim (it's probably one or the other). Panasonic made it easy to unsolder them from the bottom of the main board without pulling the board out.
If you compare the voltages on these 2 chips, you should see one with unusual voltages (around the middle) PINS 12,13,15,16. Those deal with the Internal Overload condition of the chip and if ok, giving the ok to turn on the main relay on the speaker outputs.
Pin 10 just helps turn on the fan as needed. Pin 12 of ic601/602 is the real Overload signal. They go to pins 3-4 of ic901 (the main CPU).
In my case, it was IC602 with the weird voltages there. It was also the cooler one to the touch (even in Overload) while IC601 (the other MOSFET of the pair) was hotter.
I know that you can sometimes remove the bad transistor or regulator and it will stop blowing fuses. Keeping that in mind, I removed IC602 and powered it up. Now, instead of immediately going into Overload state, the amp powered up more normally.
I could play my MP3 player into various inputs and the left channel of the headphones work. This is because they split the main L/F channels between the chips. I could change inputs, DSP modes, use the radio tuner, etc.
The main speakers did not work yet, but this is not really surprising because the main speaker output relay doesn’t click on. I think it can tell that one or more channels are down and refuses to activate (which is expected).
So, now I knew a lot of boards were working fine (particularly the Power Supply, Main Logic/DSP board and the main CPU/Display/Control boards). I also tried various inputs (some on the main Power Board) and they all worked properly as well.
I HAD NOT seen Overload since I removed IC602. I tried not to push it too hard because the output it “unbalanced”.
I replaced ic602 with a new one. I had to get it from Canada.http://www.richtechparts.com/i-1550-RSN310R37A.html
It works great now
Of course, I had to clean off the old and replace the silicone heat sink compound on the new MOSFET, but while I was in there, I also replaced the silicone on the un-blown or still good ic601 MOSFET. I'm glad I did and here is what I noticed. With the old (but still pliable) silicone the front of the MOSFETs themselves would get very hot. When cranked up, the fan would come on but they and the large heatsink would still stay very hot.
With the new silicone on both ... the MOSFET fronts stay cool and all the heat is properly transferred to the heatsink (which still gets really hot). However, now when the fan comes on it cools the heatsink down drastically.
It amazing how much better the thermal management works now with the new silicone. I think that the inability of the old silicone to dissipate the heat from the MOSFETs, might have lead to the premature failure of the one MOSFET. It also seemed to have also kept the heatsink/ fan system from working properly. This turns out to be similar to how old silicone can allow your computer's processor to overheat and even burn-out (even if the fan is still working).
I posted this in case it might help someone one day. However, if you are not familiar with soldering or board level repair of complex electronic equipment, do yourself a favor and get some professional help (or even a friend that knows how). I have seen repairable gear turn into unrepairable junk many times.
As for the silicone, the next time you send in your nice high-end amps for servicing, you might ask them about re-siliconing the power transistors. It's like maintenance on your car. It's all about being pro-active ... before something blows up.
This is as far as I had to take it apart. This picture was taken right after I had taken it apart (old silicone still on all the parts). Don't run it or check voltages without the heatsink installed. I had to move the transformer and PS Board to get to the screws on MOSFET ic601. I reinstalled them temporarily before turning it over for soldering. Silicone it and screw the new MOSFET to the heatsink half before installing it for soldering. Apply the silicone thinly and evenly, but don't get any on the pins (only the metal backing).