This is a detail shot of the DD control board… the black chip just off screen is the pre-amp. This processes the magnetic pulses from the tape into audio signal. This is fed to the DD control chip, a Hitachi HA12134AF. From this chip, the audio is sent out.
If you were to zoom in on the left most edge, you can read the letters MBPFGLD9R. This is not a model number, but rather this is the pin-out for the connector on the under side. The only letters I’m concerned about are the G, L, and R. Tracing the green traces on the board, the G seems to be a common line on the board, indicating possibly a GROUND path. L and R…? I wonder… So, I trace them back, and they terminate under the Hitachi chip.
I removed the Hitachi chip rather crudely, just by clipping the surface mount leads with a pair of scissors on my pocket knife (I use that knife EVERYWHERE).
Sure enough, the lines for L and R terminate under this chip. I went out and Googled the chip to find a spec sheet, and sure enough, the third pin from the right, on both sides of the chip, are the PB out. The chip is designed as a DIP (Dual Inline Package), and each side is identical and linear. Signal comes in one end, is processed, and outputs from the other end.
So, I followed these tracings to the pins, and then through the plug to the control board. Here is the front of the control board. In the upper right corner, is a small horizontal connector from the DD signal board.
Turning the board over, I follow the traces on this side. The L and R lines do not go through the control chip or any other components on this board, they go right to the ribbon cable connector on the other side of the board
I followed the traces again on the ribbon cable and up to the main board. From the main board, they do not go through any components, and go right to the main signal processor board. Here they pass through several components, and through some IC chips. This is where my trace of them ends, but signal on this board eventually passes to the amplifier.