White coating over part of surface mount amp

The 500 class D amp shown component level on eservice is near enough the same as the 300 with scaling down of ratings of components. Overlay numbering 7... for transistors, 6... for diodes, 5... for inductors. The large lump that looks like a transformer is 30uH output filter choke. Will replace the powerfets with mica'd IRF740 and 0.056R "fuses" with a small piece of resistance wire, previous such repair of this model of amp never bounced back but otherwise unknown . I see the replacement FETs for the 2SK3607 is IRF640 which gives me more confidence. Will try and find from the owner how they knew and why they replaced the output fan. But will do my previous retrofit fudge of bending up the corner of the chassis top cover over the fan ouput and a 5mm nylon standoff under the fixing screw to at least double/halve the "choked" fan . Otherwise the ridiculous air flow design? of this amp is to try and make a compressor out of these little 12V fans. I will assume heat build up was failure mode as quite a bit of dust inside, even under the 1/4 inch sockets etc where air prefers to ingress rather than the designed? row of tiny holes in the chassis cover. The crack in the FET gate solder was just a partial PbF crack , bending the leg to force the "crack" open failed to do so , so not through to the pad/trace. Plus usaul suspects of PbF dealt with JIC.

"N_Cook" wrote in news:inph8p$h89$ snipped-for-privacy@dont-email.me:

the fan may have been replaced because it's bearings wore out and got noisy.

No reply from owner on that yet. Previous repair of this model, that owner had noticed the exhaust air was hotter than normal and some time later it failed. Presumably spread over some time rather than immediately prior to failure. Someone else , in that situation, may have thought a fan problem and got it changed.

T'was a noisey fan. Still I find it suspicious the same fault outcome, both powerfets shorted all round but just one 0.056R "fuse" blown in the negative rail supply. The fan must be overloaded as well as poor amp ventilation. Allowing for central core of fan but not the fan blades themselves, area across duct within the 2 inch fan is 420 sq mm. Ignoring aerodynamic braking effects of forcing the outcoming air to immediately do a confined right angle, a wall 10 mm beyonfd the face of the fan, and vortexing effects from forcing througth 2mm wide slots, the combined area of the outlet slots is 170 sq mm. So more like a compressor than ventilator. Not only will I be lifting the outlet grill to double or treble the area for the outlet air, but drilling out every other inlet hole as well perhaps.

both

negative

the

hole

There must be a ventilation problem with these tiny amps

The outlet grill , on this presumably later version, is under the master control in the pic orientation. This pic shows about twice the number of slots as the one I have here, so in recent years they have punched more slots. This one is just the slots in the black area of that pic, not the continuation towards the master control. I'm not in the position to retrofit slots, but can lift that corner with a standoff. The inlet holes are a line of 1.5mm holes , directly under the top handle in that pic, so obscured.

Just as well I checked the ps underside for PbF problems, ok but one pcb screw loose, no star washers or lacquer and the 1.5 inch fan although turning, in its attitude you cannot see it was at half-c*ck. Another oddity with this. One only of the main ps caps has a cable tie over a strip of rubber wrapped around the cap. Cable tie just keeping the rubber in place not tied to anything else. Some sort of anti-vibration damping? its the one nearest the centre of the board.