3S76/Repairs

Germanium transistor replacement

The 3S76 contains eight 2N1516 and two 2N2207 PNP Ge transistors in TO-7 packages. These transistors are prone to grow tin whiskers that tend to short random transistor elements to the case, eventually rendering it inoperative.

One way to remove shorts due to tin whiskers is covered on the Germanium transistors/Repairs page, but a more durable solution is to replace the Ge transistors with Si 2N3906s. This may require readjusting the Miller Levels and the Preamp Sensitivities for each channel, as well as the overall plugin gain. Dues to the long tailed biasing used in the 3S76, nothing else will need to be changed.

The original TO-7 Ge transistors have 4 leads in a straight line, arranged C__SBE, where S stands for Shield (the tin plated TO-7 case) and that lead has been cut off. The sockets for these transistors have 3 receptacles in a straight line, plus one offset to one side. The center and offset socket receptacles were factory wired together under the chassis.

A 2N3906 may be installed with its 3 leads installed in a straight line, with the flat of the TO-92 case facing the offset socket receptacle. Alternatively, the center pin of the 2N3906 can be offset toward the flat side of the package and inserted into the offset socket receptacle. The tripod arrangement may provide more mechanical stability, but its main advantage is that it assures correct installation.

Two of the TO-7 transistors to be replaced are under the delay line assembly and are extremely difficult to reach. I have found that these are best installed with the leads in a straight line, using a pair of surgical forceps. It will be virtually impossible to install these transistors with your fingers.

I made a tool from a popsicle stick with a small hole just big enough to hold a TO-92 transistor. With this tool, I could reach into the space under the delay lines and position the 2N3906s above their sockets. A second stick or tool was used to press the transistor down into the socket.

Make SURE that each 2N3906 has all 3 leads securely installed in their correct receptacles, flat side facing the offset socket receptacle.

-- Jim Adney