![]() ![]() The Bose 901 Series III/IV speakers have foam surrounds which can degrade, so that’s worth watching out for, but this equalizer is back to top shape. The Series III/IV share an interchangeable curve and it is very different from the Series I/II curve.įully serviced, this should last a long time to come. I have them run with their own equalizer and a 400 Watts RMS stereo Technics SA-700 power receiver. The 901 Series IV is the first of the active equalizers to use op-amp chips instead of discrete transistors. There we go! With it checking out on the scope, now I moved on to a frequency response and live listening tests. I own a pair of bose 901 series 4 speakers since 1979. This capacitor, though, was the specific one which caused the channel to go out. These were definitely causing some issues – most likely, noise and distortion, although since they would have degraded over time it might not have been obvious. Every single one of the originals had high ESR and was changing in value. I use a variety of manufacturers for premium replacement parts depending on what’s in stock at any given time in this case it’s getting all Nichicon Gold capacitors. The electrolytic capacitors in this equalizer were about a decade better condition than the ones on the S1/S2, but they were universally bad. Very nicely, they’re all socketed – these early op-amps weren’t that reliable and often developed noise and distortion over the years. In this case they’re LF353N op-amps Bose used the LF335N, TL072, or 4458 op-amps interchangeably and the schematic and service manual allow for any of those chips. The 901 Series IV is the first of the active equalizers to use op-amp chips instead of discrete transistors. Other parts are replaced as needed, but Bose used stable resistors and high quality, stable, true film bypass capacitors in this device so there’s no benefit to a wholesale parts replacement like on the Series I/Series II. The standard service for the Bose 901 Series IV Active Equalizer includes replacing the electrolytic capacitors and op-amp chips by default for best performance. This unit came to me with one channel dead. The PCB on this model is single-ended, so can be completely removed from the chassis. The Series IV was sold from 1978-1983 and its construction reflects this. A bit different from the run of Series I and Series II that I usually see, I got to fix up this Bose 901 Series IV Active Equalizer which was sent to me from Austin, TX. ![]()
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