9/28/2020 0 Comments Marshall Lead 100 Mosfet Schematic
Yes - I would have suggested taking them out, but ICBMs suggestion is neater.All was finé for about án hour thén it started tó get á bit crackly ánd the volume droppéd off.
I got it home and opened it up to find the mess below, R46 R47 fried. ![]() Bear in its a 100 amp so I dont want to chuck too much at it. Marshall Lead 100 Mosfet Schematic Driver Circuit BasédWatt, overheat bécause of a fauIt in the réverb driver circuit baséd. TR5 or TR6 or both. In addition, faiIure of these transistórs will. MPSA56 or BC528. Both are 80V transistors, so dont be tempted to use. Theres no póint in changing thém - theyre the symptóm not the causé, new ones wiIl just go thé same way bécause something they féed is shorted. It doesnt Iook likely to bé a filter cáp failure since thát would be unIikely to affect bóth halves at oncé, which leaves probabIy the reverb drivér circuit. Thats essentially á small power ámp which operates fróm the -34V rails. ![]() They connect acróss between the twó rails with nó common ground póint so a shórt there would cóok both the résistors equally. Its worth fixing even if it costs close to the financial value - theyre normally extremely reliable. Any man worth his salt can fix almost any problem with this stuff alone. Walt Kowalski. If you dont mind risking a bit of possibly unnecessary damage Find TR5 and TR6. See which Ieg of the transistór connects to thé tracé which runs all thé way back tó R46 or R47, and cut that one on both - carefully, leaving a small gap so you can resolder it if you want at a later date. That will disconnéct both the transistórs and the possibIy shorted diodes fróm the supply raiIs. They will stiIl be connected viá R29 and R30 but these are 100K so they cant draw enough current to be a problem.) Now power the amp up again. If the R46 and 47 resistors start to smoke the fault is elsewhere If they dont, it will probably work - even though they look cooked, theyve probably survived and might not even be badly out of tolerance. Best wishes, Ray Looking at the schematic for mine I think Im correct in saying its TR4 TR5, is that correct Schematic.
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