I recently had this Bose 901 Series I Active Equalizer, serial number 50911, in the shop for repair. These are a lot of fun to work on and very rewarding for the great sound they produce when they’re working properly. This particular one looks to me like it had seen some service in the past, and the left corner of the front cabinet looks like it had cracked. The Early Production versions of these cabinets were made from hardwood, but the later ones merely MDF with a veneer applique on top.
Inside, it looks like there was a series of repairs which replaced most of the electrolytic capacitors and a few of the signal capacitors in the middle. Either that, or it was a production stock changeover – but I find that unlikely. On all of the original components, the lead ends are slightly crimped in addition to being soldered – you can’t pull a desoldered bare lead which is sticking through the board back out the top side. On every replacement part I’ve seen, the wire end is not crimped. It’s not a foolproof method, but it seems to be pretty accurate.
You can see the capacitors starting to leak:
This is the first time I’ve seen this, although it’s a familiar technique: the transformer’s electrostatic shield, which is connected to the outer mounting lugs of the transformer on the core cover, is joined to circuit common. Tube radios used this technique as well to reduce interference. These Series I Active Equalizers are very resistant to interference in the first place – especially compared with the Series II and Series III – but this will theoretically help even more.
This equalizer’s owner requested I make a small modification to the circuit. These were originally fitted with electrolytic output capacitors – and that’s what I usually use as an OEM-level replacement. Electrolytic capacitors both then and now do have a fairly high dissipation factor compared with other capacitor materials. Hi-fi audio makes use of a wide array of film capacitors which offer a much lower dissipation factor – about 100x lower in many cases – to improve the output quality and lower distortion. This is one area where materials science has advanced significantly and we can make a real performance upgrade.
The specific audiophile capacitors I use vary depending on what I have in stock, but all of them offer the same advantages. In this case, I’m using a 5.1 uF capacitor to replace the factory 5 uF capacitor. This is well within the +80/-20% tolerance rating of the original capacitors which would consider any capacitor 4-9 uF correct.
Looks great all finished up! This one will run beautifully for many years to come, and the upgraded output capacitors will really help bring out the dynamics in the music. After cleaning the switches and a burn-in period it’s all boxed up and ready to go home.
I recently had a pretty rare piece of equipment in my shop and just finished bringing it back to life – the Series I Early Production Active Equalizer. Bose build the 901 Series I from 1968 through 1972-1973, but they very significantly changed circuit designs after the first year. About the only thing kept the same between Early/Second Production models is the equalizer curve: they have different circuits, different components, and even have a few extra Zener diodes. They’re only a little more challenging to service, but this one in particular required a lot of detective work to get it going again.
The first thing to notice is the chaotic, hand-laid PCB with no silkscreen.
This early production variant used a few 3.3 uF 10V capacitors in part of the low-frequency circuit that aren’t present on the late production model. In the same circuit there are a few 6.8V Zener diodes. In this case, though, they were very interesting – standard transistors had been used with the third leg left disconnected and the C-E joint reverse-biased to get the 6.3V. I think the transistor was a 2N3393, although it was marked over so it’s tough to tell.
After component replacement was complete, I was still only getting signal from one channel. The other channel had the negative edge of the waveform clipped.
After extensive troubleshooting with the oscilloscope, multimeter, and schematics checking each trace I discovered a cracked solder joint on one of the jumpers. With that repaired, the signal started coming through cleanly.
This equalizer is going to Germany. I outfitted it with new, gold-plated jacks to replace the damaged originals, and added the 10K 5W dropping resistor and European plug to convert it to 220V operation. This is the method Bose originally used.
With this set, I’ll be boxing it up and shipping to its new home where it should serve faithfully for many years to come!
A change of pace from the Bose equalizers and hi-fi I’ve been working on a lot of lately, I had the pleasure of working on a 1936 GE Model A-52 antique radio.
This is a nicely designed and straightforward table radio with 5 tubes, AM and one Shortwave band. Back in the ’30s, RCA and GE shared chassis and designs quite closely and it’s no surprise this one uses all RCA metal tubes, 6A8 6K7 6Q7 6F6 5Z4.
This radio had been serviced in the past but was due for another go-around. Most of the capacitors had been replaced in the ’70s or ’80s, although there were a few that still needed to be replaced. I swapped the 4 capacitors which were definitely in need of replacement, but the other units tested fine and are recent enough I’m not too worried about them.
The radio power switch, though, had been bypassed. The radio’s owner reported the switch was sparking in the back. I tracked one down after several weeks and was able to get it installed and it functioned perfectly after that.
The radio’s alignment was already spot-on so no adjustments needed there. I re-assembled the radio and let it play for several hours of burn-in testing before sending it back to it’s home where it will continue to play beautifully for years to come.
Once in a rare while I’ll run across an old Bose Active Equalizer that I buy for myself, but they never last too long – I get a lot of requests to purchase a complete Bose 901 Series I equalizer to go with a set of speakers which long since lost the matching controller. I did just recently have one in stock and it went quickly; once they’re purchased I repair them on demand before sending them along.
This one’s all original as far as I can see. It was reported to have a dead channel when I purchased it, and the resistors had certainly drifted out of their tolerances with age. The case was in decent shape for being 40+ years old, too, although the light doesn’t quite catch it all very well.
Component replacement was pretty straightforward.
Some Bose 901 Series I and Series II equalizers used BC239C-labeled transistors, others used 2N5088s. They’re nearly identical – indeed, the rest of the circuit is identical – but they have a slightly different gain spec. Practically, this just translates into a slight difference in the volume control on your receiver – the generated curve is the same and both are identically factory specification compliant. When I need to replace transistors, I use all 2N5088s – but in this case, all transistors were good, so no replacement necessary! The neon bulb was flickering, though, so I replaced it with a brand new NE-2A and current limiting resistor. Then, a good solid control cleaning so all the switches moved freely and made good contact.
This one went to its permanent home next week where it should perform for many years to come! With precision metal foil resistors and new electrolytic and film capacitors, not to mention the very light duty cycle experienced by the equalizer (which draws only 1.5W total power consumption), mean it will be a long time until this needs service again.
I can repair your Bose 901 Series I, Series II, Series III or Series IV Active Equalizer for a low flat-rate with some optional upgrades. Most every one of the Series I equalizers needs to be reconditioned at this point. The majority of Series II do as well, and even the later series are coming up with defective capacitors and op-amps more regularly.
I recently had another Bose 901 Series I equalizer on my bench for repair. It was in for the standard service, all resistors and film/electrolytic capacitors included and evaluation of the transistors and other components for replacement as necessary.
This one is in pretty decent physical shape for its age.
This equalizer’s owner reports it has never been previously serviced, so Bose must have mixed capacitors from two different parts buys together in this one; typically the filter capacitors are all the same model. It’s not a functional problem, though, and it obviously worked this way for many years!
The early film capacitors have the dreaded dark center spot indicating they’ve degraded.
The old carbon composition resistors had badly drifted and were throwing off the curves produced by the equalizer, further adding to the distortion introduced by bad capacitors. This one, 47K 10%, has drifted by over 32% – well exceeding its tolerance.
Component replacement proceeded uneventfully.
I finally replaced the mail filter capacitors and boxed it back up after burn-in testing. I cleaned the switches, which were quite crackly and didn’t make good contact, and now they work nearly good as new. This equalizer is heading back to its owner where it will serve for many years to come. I just love the wide sound produced by the Bose 901 speakers, especially the early series. If you haven’t heard them, I’d encourage you to give it a try!
I’m happy to announce that starting soon, I’ll be offering professionally restuffed vintage capacitors for historically accurate repair and restoration of your antique radio! In my rebuild process, the old capacitor is carefully stripped of the old wax coating and the innards carefully removed. The body of the capacitor is lightly cleaned and a new high voltage axial lead film capacitor is installed in the cylinder. The ends are then filled with a medium density clay filler to provide stability, followed by a fresh coat of bee’s wax. The end product is nearly indistinguishable from one in original condition, perfect for performing a historically accurate repair of a valuable antique radio.
If you’re interested, leave a comment! Pricing is expected to be $3-5 per capacitor depending on type and value.