I attached a copy of my 043899 board for you to look at. You can see that the 028 transistor has a date code of 912 and the 038 and 039 transistors are original with the diode under the left. I am thinking that in the late 80's I may have replaced the carbon resistors because I have a box of them I kept over the years that look like the parts from the two boards. I have no issues with the boards, in fact the amplifier works well and sounds good. I say good because it is not as good as the Marantz 16B I went through driving my Martin Logans. This is what got me started on this MC2105.
What is funny about this amplifier is half of the output has the older 132-042 yellow transistors and the other side the newer 132-070. The date codes on the 132-043's are 1969 also as is the TO-66 on the pc board.
Now the fun part with this amplifier and where I am headed. I have gone through a number of the TO-3 transistors of both varieties and observed their operating characteristics on my curve tracer. I drew the same conclusion that got me started on the Marantz amplifiers doing some well deserve updating on them. You could almost hear these transistors on the curve tracer, just kidding, but having removed a number of SJ Motorola from the Marantz amplifiers and examining them with the curve tracer I drew the conclusion that most semiconductors from 50 years ago are fair at best.
The yellow 132-043 Mc transistors have low gain, you would hope for more, but they are linear in operation. The green 132-070 Mc transistors have much more gain but they are not flat, rather crappy observing the steps.
Are you still with me? I believe way too much emphasis is placed on the fT of the transistor and whether it should be used as a driver or a final in an emitter follower configuration. The talk about possible oscillation or not sounding like the "factory original" because of not having a high enough fT transistor as a driver to me is way over exaggerated. Yes, some amplifiers will have issues when replacing transistors. fT and gain are important. So is local and overall feedback in the design.
I need to go to Kroger. I am making tuna salad for Good Friday. I keep it short because I will be going back and look at those two boards and decide what I will do with them after looking at the powered up on the bench. If you get to those outputs and decide to use the MJ15003, it is a good choice. Dead transistors (low gain) put a little more strain on the drivers and their operating point. I need to look at the three available choices I have for drivers: MJ15018, MJ15024 and MJ21194. Some people perhaps will comment that the MJ21194 is a final transistor. From what I observe, it is far better than any of the two choices the factory supplied in these amplifiers with good gain (equal to the 132-070), low saturation Voltage and very, very linear.
What is funny about this amplifier is half of the output has the older 132-042 yellow transistors and the other side the newer 132-070. The date codes on the 132-043's are 1969 also as is the TO-66 on the pc board.
Now the fun part with this amplifier and where I am headed. I have gone through a number of the TO-3 transistors of both varieties and observed their operating characteristics on my curve tracer. I drew the same conclusion that got me started on the Marantz amplifiers doing some well deserve updating on them. You could almost hear these transistors on the curve tracer, just kidding, but having removed a number of SJ Motorola from the Marantz amplifiers and examining them with the curve tracer I drew the conclusion that most semiconductors from 50 years ago are fair at best.
The yellow 132-043 Mc transistors have low gain, you would hope for more, but they are linear in operation. The green 132-070 Mc transistors have much more gain but they are not flat, rather crappy observing the steps.
Are you still with me? I believe way too much emphasis is placed on the fT of the transistor and whether it should be used as a driver or a final in an emitter follower configuration. The talk about possible oscillation or not sounding like the "factory original" because of not having a high enough fT transistor as a driver to me is way over exaggerated. Yes, some amplifiers will have issues when replacing transistors. fT and gain are important. So is local and overall feedback in the design.
I need to go to Kroger. I am making tuna salad for Good Friday. I keep it short because I will be going back and look at those two boards and decide what I will do with them after looking at the powered up on the bench. If you get to those outputs and decide to use the MJ15003, it is a good choice. Dead transistors (low gain) put a little more strain on the drivers and their operating point. I need to look at the three available choices I have for drivers: MJ15018, MJ15024 and MJ21194. Some people perhaps will comment that the MJ21194 is a final transistor. From what I observe, it is far better than any of the two choices the factory supplied in these amplifiers with good gain (equal to the 132-070), low saturation Voltage and very, very linear.
