Dolby Board Overhaul
The unique feature of the 9090DB and it’s little brother, the 8080DB is their ability to encode Dolby noise reduction. Dolby was very new in 1977 when these units were introduced. Most tape decks didn’t have it. So Sansui provided circuitry in these receivers to allow it to add Dolby noise reduction through the tape output. These units can also receive and decode Dolby FM, a stillborn format. To calibrate the Dolby level, a test tone is built in.
Checking this tone is a simple test of the Dolby board. When activated by pushing the Meter cal/tone switch and choosing Rec 1 or 2 on the Dolby switch, you should hear a 1,000 Hz tone from both channels. In this mode the power meters are used for Dolby Calibration. In this test they should both come up to mid range.
The heart of the Dolby board is a 5 gang rotary switch. Like all switches in this vintage equipment, it is prone to oxidation. Since all audio signals must pass through this switch, it can cause symptoms like dropped or distorted channels.
The Dolby circuit board was very high tech in its day. I was one of the first PC boards used by Sansui to have circuit traces on both sides of the board. To connect circuits from one side of the board to the other, they pass trough holes in the board that are filled with solder. There are 64 pass throughs on the Dolby board. The electrical conductivity of these “pass throughs” can fail causing dropped or distorted channels and or failure of one or more of the Dolby functions.
At some point in the production of these models, Sansui recognized the shortcomings of the pass throughs and installed rivets in them. Boards with these rivets should be much less prone to pass through failure.
The Dolby board is wired to a gang of push switches. There are a total of 36 wires that run from the Dolby Board to the push switch assembly. Early production models used another “high tech” innovation of the day, flexible PC boards to connect the Dolby board to the push switch assembly. These flex PC boards get very brittle over time and have a tendency to tear. If that occurs they cannot be repaired and must be replaced. Sansui identified this problem too. In latter production units, the flex PC boards were replaced by Ribbon cable.
The picture below are mid and late production Dolby and push switch boards. Both of these boards have riveted pass throughs. The mid production board has the Flex PC board and the late production board has the ribbon cable.