250W amp - Mark Bass , Little Mark 250, of 2008, Italy

Just checked again The polarity pip of the original was not ground off and p1 is connected to the dropper from the + of the HV dc caps , ie Vcc and p4 is connected to the zero/negative of the HV caps (not ground of course) . So someone getting his pin convention wrong I think, overlay "pip" is obscured by the chip and agrees with the original orientation of the chip

Thinking back to the original failure mode. Solder joints all looked ok , but this is definitely PbF although absolutely no mention anywhere. The holes in the pcb, for the powerfets , although plated through, are 3 times the required diameter of the width of the powerFET pins and no eyelets padding out. If bias to a gate failed then that could easily short circuit that mosfet, would the other one fail soon after, if the amp is in use at the time.

With the IR2153 and 1N4007 and 10R dropper in each of HV dc supply and hot side of hottest mosFET and 57 percent of mains there is life Blue power LED on, SMD LED D20 on, fan on, +/- 31.6V on main rails , 7.7V on

12V 7812, -9.6V on 9812 and 9.2V on 7814. Then hot side , 12V on new 10uF C67 and 13.5V on new 100uF C69. But a minute in and that 4007 smokes and mains current draw rises from nothing readable on variac AC analogue meter to about .15 amp and I switch off. 4007 survives as does the IRF740. Is this a result of me trying to run on too low a HV dc and removing droppers and taking mains to 75 percent or so will remove this or something else wrong. I cannot believe that bootstrap diode would be more than 1 amp inside a D suffix 2153 DIP8 package. I have the fall-back option of refitting the original driver chip but think I'll carry on with the replaceable 2153 for the moment, monitoring the voltage on the R78 Vcc dropper , first I think, in htat first minute

Where can that power be coming from to heat the bootstrap diode? Not the Vcc dropper , far too ohmic, the 4148 D40, 150mA or so is fine . I will disconnect my added 1N4007 and see what happens to those main rail voltages

A bootstrap diode is required, without it and its back to no output and that low level descending pitch audio at switch off Must be passing too much HV ac , ie too much leakage. Tried 2x 1N4007 in series and no problem over 2 minutes, and no heat to finger touch soon after switch off. Enough time to put a sniffer coil next to the SMPS Tx and stable operating f of 430 KHz. For 330pF and 18K the IR2153 shows expected about 120KHz, influence of T24 or low Vcc perhaps, 11.5 and climbing above 11.6V at switch off So do you need a high frequency , and high voltage diode, here if only a few tens of mA of DC required to pass but need to block 100s of hf V ac in reverse. Or just low leakage and high voltage silicon diode here?

From IR application notes it looks as though the internal bootstrap diode is actually a gated powerFET in recent manifestations of that IR215. series. The accepted external diode for a IR2153 seems to be MUR160 so placed a BYV96E in there and it is comfortable. Removed the droppers Raised supply to 66 percent mains and voltages coming up to +/- 36V, presumably between 50 and 55 V main rails with 100 percent (this amp is 250W rated, that schematic is 500W) Same oscillator frequency Time has come to replace the IRF740s with pair of 20 amp /600V powerFETs

Had it running under load, osc f 380 KHz, taking temp readings of both heatsinks until stabilised without fan cooling, top cover removed , but monitoring the fan DC . Can mosfets go ohmic with age as I cannot see that ps heatsink getting all that hot. Did a single powerFET going D-S randomly short then drive too much power into those 2 caps to make them fail and knock out the second FET? Could there be a flaw around the design that with bad ESR caps , instead of going into protect mode could the driver IC go into a fast cycling restart/protect mode and somehow overdrive the powerFETs to knock them out. Both shorted original powerFETs had a discoloured area on thge heatsink face as though overheating, Infuriating having so few clues, and having to conjecture failure mechanisms. Shorted mains fuse from 2 D-S shorted powerFETs and 2 bad caps and no other obvious overheated Rs etc

Got word back on this and its survived a number of gigs including one where it was pushed hard. So a pair of footprint-converted SMD STB20NM60 and that IR smps driver + diode are an adequate replacement. Thinking back about it the owner reckons it may have been a song sheet sloppily laid next to the air inlet was its downfall.