Here you see the TR-66 circuit board. Specifically, it is the board in most TR-66 units I've encountered and the one shown in the original Sony service guide as well as in period magazine articles. It also happens to be the same R-6D circuit board found in the late (final?) production TR-6 radios. In contrast, note that first-production TR-66 radios appear to have used the earlier, different TR-6 I~1 circuit board more commonly found in the TR-6. Untangling these words now, the first TR-66 units were built using the earlier "TR-6" circuit board, while the last TR-6 radios produced used the "TR-66" board.

Only one transistor in the unit pictured above is labelled with its part number, and that's the "top" one (converter), which is a 2T513. The greenish oval varistor is also labelled; it's a 1T41 (and not the 1T90 specified in the period reviews). The lack of labelling suggests that Totsuko was still testing each transistor and sorting them by their performance when this radio was made. These transistors were marked with various red or black dots, stripes, etc. depending on how they tested. Using a period magazine review as a guide to the transistors used goes like this: 2T51 x 1, 2T52 x 2, 2T64 x1, 2T63 x 2 [1T23, 1T90]

I purposely took this scan with the battery tube holder removed to offer a look from behind at the embedded panel of a different colour, (in this case red.)