Bose® 901 Series II Active Equalizer #64364 Overhaul

This Series II Bose® Active Equalizer came through the shop. It had suffered age-related failures in its electrolytic capacitors, and the resistors had drifted with age and moisture to produce the wrong curves. Fortunately, it’s nothing some maintenance can’t solve!

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Inside, there was clear evidence on the inside where the capacitors had outgassed as they died, leaving a chemical residue inside showing their failure.

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Inside, it has seen some service, with a few caps having been replaced previously.

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After replacing all the electrolytic and film capacitors and all the resistors, the performance was improved but still uneven and distorted in one channel. Tracing the signal, the transistors themselves were causing the problem. Transistor failure is pretty rare, but it does happen, and it’s covered in the standard service!

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After a set of 10 new 2N5088 transistors, the problem was corrected and this equalizer was back to producing the right set of curves. A new power lamp to replace the old one which was dead, and it was all set!

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