I recently got to fix up another Bose 901 Series 1 equalizer which I received for repair. These are some of my favorite electronics to work on – they’re easy to work on and each one has its own history. Every one of these I’ve seen has been slightly different and this one was no exception.
This one in particular has 4 separate repairs. One is especially interesting.
The last one is somewhat clever. A 10K resistor, probably 5W, across those terminals is the modification to run this equalizer on 240V in Europe or similar. It’s been jumped with a solid piece of copper bus wire taking it out of the circuit but still leaving it in the equalizer if conversion ever needs to happen again.
Top-off testing was next. The neon indicator lamp in the power switch was flickering badly – it had likely been losing neon through the metal-glass interface very slowly over the past 40 years. It’s a neon lamp attached directly across the AC mains with a voltage dropper/current limiting resistor in series. The total power consumption is a few mA at line voltage.
Here it is removed from the circuit. The lamp/resistor combination is actually a single component – they’re welded together. I replaced it with an NE-2A/150K resistor combination, I believe the resistor is 1/8W the draw is so small. The envelope size of the new bulb is about half that of the old one, but it fits in well from the bottom to let wire tension keep it in place better.
After burn-in testing, the equalizer checked out perfectly! It has incredibly clean switches. The others I’ve serviced are much improved after cycling but can hang up the first time they’re used and these didn’t even need cleaning.
This one is going to be a great performer for a long time, and these are a lot of fun to work on.