Recently, I have also made some time to work on some of the electronics projects that have been gathering dust.
I recently repaired a Sansui AU710 Integrated Amplifier. This is an export unit and is essentially the same as the US spec. AU717 except the AU710 has a dual voltage power transformers and is derated by 5 watts for 50Hz operation. These are really nice Direct Coupled amplifiers with dual power supplies. One of the nicest designs from the Japanese stereo craze of the 1970s. They are conservatively rated at 90 watts per channel on the AU717 and 85 watts per channel on the AU710. One issue that is common to any Sansui products from this period is glue rot on the printed circuit boards. Sansui used a brown adhesive to stabilize the larger capacitors on the boards so they would better withstand the shock and vibration of shipping. Unfortunately as the adhesive continued to out-gas and dry to a brittle state, it becomes hydrophilic and begins to absorb moisture from the atmosphere creating galvanic corrosion of the surrounding components.
The good news is the boards in the AU710 are all single sided, so none of the PWB wire traces are affected. I carefully scraped away the offending adhesive and replaced any damaged components. This was required on both power amplifier driver boards and the power supply/protection boards. Once all that was taken care of, the unit was reassembled and tested. After allowing the amp to warm up by playing a cd for about an hour, I set the bias current in the outputs and reattached the cover.
The most recent project on the electronics bench is a MCS-3275 stereo receiver. This particular unit was made by NEC. The black face NEC version is model number AUR 8075.
While not as nice as the Sansui, this is still a formidable unit at 75 watts per channel, plus it is a big fancy silver dial unit that most of us remember from the 1970s. The final PA power transistors were shorted in the left channel. These are obsolete NEC devices; 2SDA588 and 2SB618. I replaced these with ONSemi MJL3281A and MJL1302A. These are much more rugged, but it did require some modification to the heatsinks and the mounting scheme. I also replaced the driver transistors and recapped both boards. If this repair is successful I will then recap the rest of the unit.
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