The principal problem with maintaining amplifiers like my Krell FPB 300 is that the power output transistors, MJ1302 and MJ3281, have long been unavailable.
Motorola discontinued these transistors while Krell was still making amplifiers with them, so Krell bought the entire remaining inventory in the mid 1990's. They used them in many different amplifier designs, and according to legend, Krell freely installed new ones during amp servicing. That was wonderful while it lasted, until about 2009 or so.
Now, to keep the amplifiers running with authentic parts, it is necessary to scavenge transistors from parts unit amplifiers, similar amplifiers that might be even more valuable than the one you are trying to fix. There is no public place where such scavenged transistors can be safely purchased, maybe you know someone you trust (and, do you really trust them).
MJ1302A and MJ3281A are sold on Ebay and through Asian distributors are almost certainly fake parts. I bought some of these in the hopes they may be OK now that Chinese fabs have been making them for well over a decade.
Similar silicon is still available in the TO-247 case in the MJW1302AG by ON Semiconductor, the Motorola spin-off which still manufactures basic transistors for linear applications.
The problem is that the FPB amplifier heatsinks were designed for TO-3 transistors. To use TO-247 would require complete re-engineering of the chassis and heatsinks. One might as well buy a D'Agostino amp.
Meanwhile, ON Semiconductor does still make TO-3 power transistors, including the MJ15024G, which others now use in power amplifiers.
Motorola discontinued these transistors while Krell was still making amplifiers with them, so Krell bought the entire remaining inventory in the mid 1990's. They used them in many different amplifier designs, and according to legend, Krell freely installed new ones during amp servicing. That was wonderful while it lasted, until about 2009 or so.
Now, to keep the amplifiers running with authentic parts, it is necessary to scavenge transistors from parts unit amplifiers, similar amplifiers that might be even more valuable than the one you are trying to fix. There is no public place where such scavenged transistors can be safely purchased, maybe you know someone you trust (and, do you really trust them).
MJ1302A and MJ3281A are sold on Ebay and through Asian distributors are almost certainly fake parts. I bought some of these in the hopes they may be OK now that Chinese fabs have been making them for well over a decade.
Similar silicon is still available in the TO-247 case in the MJW1302AG by ON Semiconductor, the Motorola spin-off which still manufactures basic transistors for linear applications.
The problem is that the FPB amplifier heatsinks were designed for TO-3 transistors. To use TO-247 would require complete re-engineering of the chassis and heatsinks. One might as well buy a D'Agostino amp.
Meanwhile, ON Semiconductor does still make TO-3 power transistors, including the MJ15024G, which others now use in power amplifiers.
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