Mackie_SWA1801_Service_Notes

Originally posted on music-Electronics-forum by nevetslabMackie SWA 1801 Service NotesAfter a recent failure on a Mackie SWA 1801,I thought I'd share a number of commonailments that occur when the Power Amp fails..In-rush Current Limit resistor R2 usually burns open before the HV power supply fuses F1&F4(10A)can react.,as well as the AC mains fuse on this same AC Mains PCB Assy F1(T8A).Maybe the fault current causes the relay to open(P/S sag),and the resistor becomesthe fuse.Once it's burned open,nothing powers up again.On this same PCB,invariably I find radial solder joint fractures on the IEC 320 AC mainsconnector...usually on all three solder contacts.Same goes with the Transformer Primaryconnector in use (whether 120V or 240V).De-soldering and re-soldering these is the propersolution...not just applying more solder.Often the leads of R2(22 ohm 20W)of this large cement filled bathtub WW resistor havesnapped off,unless it had been glued down.I add a small film of RTV when installingreplacement resistor,and fold over the leads prior to good soldering.It's only in circuit untilthe relay REL1 closes.I also find solder fractures on that relay.ALL the AC Mains current isflowing thru that,so it's a critical connection for the system.Also on the AC Mains board is J4,a tiny 2-pin connector,usually gooped with RTV to holdthe heavy cable in place.It provides the power for the AC Mains Relay,and has beenresponsible for abrupt output disruptions of the system from vibration if the solder jointsand/or connection from the power amp board isn't secure.A critical weak link in the system.I've been hoodwinked by that connection before,when everything ran proper until restoringthe power panel to the cabinet.Then,high level 'barking'on peak bass transients wouldoccur.The amplifier is very similar to the SWA 1501,except it has bipolar switched HV rails and noMosFET's returning the woofer output current back thru ground to the HV supply.Here,theSWA 1801 switches on the appropriate HV supply when signal dictates voltage swing needsto go above the +/-18V rails.Its'MosFET's Q6 Q14 are carrying the higher signal peakvoltages of the HV power supply thru 10A Fuses F1 F4 to feed the output stage Q4,Q5,Q15&Q16.I rarely find those fuses blown,BUT I often find the fuse clips broken or too weak to keep afuse in place.Replacing them is a PITA,as so much heat is needed to suck out all the solderin the plated-thru holes.I live by a well-maintained Pace Desoldering System,and in heavycopper areas,add additional heat with a wide tip on the soldering iron to extract the oldsolder.Those clips MUST be solid,as all your output current is flowing thru them and thefuses installed.Schruder P/N is 0751-0099When the power panel has failed,it's not uncommon to find MosFET's Q6 Q14 shorted,Output Xstrs Q4,Q5,Q15 Q16 shorted,driver Xstrs Q3 Q18 shorted,with their emitterresistors R8 R56 burned to cinders along with R7 R57 at the ends of the cascode stage.And,often the circuit traces between Q3