My SWR SM-900, circa 1996 is acting up and I could use some advice in diagnosing it. (I have both 1992 and 1997 schematics available) US made unit. First, the amp started making ‘egg frying” crackle noises through the cab while I was playing at medium volume, same volume as always. I swapped out basses, then input cords. No change. I unplugged the bass and cords and loud crackling continued, so it wasn’t the bass jack or the cord. When it crackles, both the preamp clip light and power amp clip light flicker on. I opened it up the next day and checked the passive input jack socket. Nothing unusual there. The amp has a 12AX7 tube in the gain circuit and I was thinking it might be going microphonic. I put in a new GT 12AX7 soviet made tube from GC. This didn’t cure it, but the symptoms changed slightly. Now, when turned on hooked to a 8 ohm cab, it made no crackling noise when no bass is plugged in. I plugged in the bass, and played. After about 1 minute the crackling returned, but only when certain notes were played. The E on the 7th fret of the A string and the A note on the 7th fret of the D string, approx. 80hz and 110 hz respectively drove the amp into crackling and both clip lights started flickering when those notes were played. Also, the sound seemed to oscillate a bit. I surmised the tube may have been part but not all of the problem. I had the limiter off and the amp top cover open. The fan did not fire up as far as I could see. I checked bridge mode and right and left stereo output modes, both channels. Same crackling thing happened either of the three ways I noted that cutting the amps parametric eq at around 100 hz reduced, but did not eliminate the crackling. I also found that if I turned the limiter up half way, the sound oscillated oddly as well. I typically use the amp in a rack bag, and the fan operates fairly consistently on and off, especially when pushed a bit in bridge mode. I use an 8 ohm SWR goliath 3 jr. 4x10. The flaps are open of course, it’s a 4u rack and the other 2u have short bodies, power conditioner and tuner, so there is air available in the back of the unit, it’s not ‘sandwiched” in. Is it possible the fan or the temp switch controlling the fan failed? In the schematic I have, it does not show the voltage the plastic fan runs at, just a block diagram notated ‘fan’. That was on the later schematic. The 1992 schematic drawn by Steve Rabe doesn’t even show the fan. I’d like to put current though the fan and see if it operates. Any one know what those typically run at? 3 volts? 5? 9? Is there a way to check the thermo switch without frying the output section? And finally, is there a quick way to check the tube using a voltmeter across any of the pins? I guess the new tube could be bad, but I kind of doubt that. Still, I’d like to eliminate that as a problem. I am starting to suspect fan failure and overheating damage resulting somewhere. I can’t see any fried caps or transistors, but that may not mean anything. Any thoughts on this? (Also fried an Eden WT-400 last week, Separate thread for that… ) Probably off to the tech shop unless I can figure something simpler.