Greetings. I apologize for the long post that follows, but I'm new here and I must start out begging for help. If anybody is familiar with the AMP BH420 head, I sure could use some advice in troubleshooting. I'm pretty much a "tube guy" and have worked on a number of them, and done lots of mods to them as well. I once fixed my solid state Sansui stereo(blown power transistor), and I think that I have much the same problem here with the BH420. My brother asked me to take a look at a friend's bass amp. being stupid, I assumed it was a tube head. The problem with it was that it occasionally "cut out" while being played. I figured it was probably a loose jack or bad power cord. However, when I plugged speaker and bass into it, and turned the power on, the red indicator lights only lit up for a split second, and the amp died. I had the cover off so I could observe the circuitry, and sure enough, the instant it "died", I noticed just a tiny "whiff" of smoke or fume come from the board/heat sink, that has the big power transistors mounted to it. I got the schematic for it at : www.epanorama.net/sff/Audio/Musical/Products/BH-420.pdf. I should have never fired it up because I noticed right off, before firing it up, that R 65, and R 86(both 800 Ohm, 5Watt) had been overheating enough to "char" the board underneath them. They set right behind the tone balance control, opposite side of the output section, in the head. I guess that my question is: Does anybody have any such experience with this head, that could narrow down my search for what is evidently open on the power board/heatsink. The only thing I can think of to do, is to remove ALL transistors form this "board" and test. I've already checked the 4 diodes on that "board", and they are good to go. Any tips or advice is greatly appreciated in advance ! tonequester.