(Need your Active Equalizer repaired? Visit Rain City Audio)
While some other projects have been pending information and parts, I’ve had a chance to work on the Bose 901 Series 1 equalizer that’s been on my bench for a couple of weeks.
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are an array of small drivers designed in a way that requires the signal to be equalized and pre-amplified, and if you don’t use the Active Equalizer they’ll sound pretty bad, lacking much in the way of bass or treble response. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, using the 901 system without one is likely the origin of the derogatory slogan “Bose: No Highs, No Lows”.
This particular model came to me from a client complaining about distortion and eventually signal loss in one channel, and general sub-par sound. He had the equalizer for about a year after purchasing the set from a collector, and it never sounded quite like it should and rapidly degraded from there. The capacitors in the unit had probably been going bad for a while but only crossed the threshold to completely dead after some time in use.
The Active Equalizer offers 30-some combinations of curves to select and enough pre-amp gain to maximize the speaker’s output.
The equalizer is fairly simple construction, using a single-sided PCB with hand routed traces, ten transistors (five per channel) and an assortment of capacitors and resistors and a few inductors to do the work of shaping the frequency curve.
With the top cover off, you can see inside clearly. This particular equalizer came to me with reports of distortion and low gain. Obvious immediately are the large orange and red capacitors. The red models especially have visible discoloration at the very top. I also noticed that many of the resistors are the original carbon composition type, which is known to absorb moisture and change values. As the resistors in the signal path, any drift can change the equalizer’s effectiveness. I spot-checked a sample of the resistors, and found that very many of them had drifted past their stated tolerance and were also going to need replacement.
To start, I began replacing the electrolytic capacitors and resistors from the top down. I’m using high-precision metal film resistors instead of carbon composition resistors, all rated for 1% tolerance to ensure long-term precision and stability. In addition, metal film resistors have a much lower noise figure than carbon composition resistors which will further improve performance.
After service, this unit was still giving me some trouble with the channels being slightly different volumes, so I pulled the transistors and selected new units for very careful gain matching.
Here’s the results of the matching:
Nice and level! With that replacement, the channels were perfectly balanced. Not all equalizers need their transistors replaced, but it’s easy enough if they do. And with it all cleaned up, the performance curve looks great!
With brand new capacitors, transistors, precision resistors, and a new neon power lamp this Bose 901 Series 1 Active Equalizer should be good to go for many years.
Active Equalizer Repair by Rain City Audio
Do I really need the Bose® Active Equalizer with my 901 speakers?
is it possible to replace the active equalizer for the 901 series1 by another equalizer of one of those equalizer of Bose?
So, if a early production series 1 eq requires replacement of some of it’s original 2n3393 transistors is it a better plan to replace them all with the later 2N5088 devices? I’m assuming Bose went to those from the originals for a good reason (performance, noise, reliability). It interesting to notice the quality of manufacture declined as they moved manufacturing out of MA.
2N5088 is slightly lower noise, I think, but in general you could use pretty much any NPN at all in this application and it’d work fine. I have a test platform Series I with sockets for the transistors rather than solder-in, and think I tested ~30 different types of vintage and modern NPN BJTs. Except for some which were very, very low gain they all produced the same effect curve, and all sounded just fine.
I bought a few thousand 2N5088s a few years ago and expect to use the last of them in the next few months, I’ll be switching to KSC945YTA after that.
Podemos usar o crossover da bose 901 v na Bose 901 II ?
Yes, you can use Series I equalizer with Series II speakers.
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Hello. Pls provide current ordering and pricing information to repair my Series 1 AEQ. Thank you.
Great write up! I have a 901-e (series II) and I’d like you to quote me on a rebuild. Thanks!
I also have a series 1, now without a left channel. Would like a rebuild. Please provide me with details for such. Thanks.
I have a Series 1 EQ that needs reconditioning. If you are still doing this please email me instructions to get this done and pricing info. Thank you.
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Are you still doing this? I have a series 4 in need of help
I sure am. I’ll send you an e-mail at the address you provided over the weekend and we can talk about your Series 4 EQ’s needs!
I have Bose 901’s. I have no active equalizer. where to get one?
Do you know where I can get a 901 series 1 equalizer?
Good project. I have a Series III EQ that I started replacing the caps. All good until I decided to replace the resistors. Perhaps I can PM you as I have a couple of questions on that regard. Callsign KP4N.
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Hi, I;m interested in having my Bose series ii 901 eq repaired by you, pls. email me thanks so much! email@example.com
I have a Series I Equalizer that needs a rebuild. I can jiggle the connections and the audio cuts in and out. Speakers play as expected without the equalizer.
Please send me the details of sending details to facilitate the rebuild.
Thanks for providing the service.
I have this same equilizer bought in 1972.
One problem is also the switches which have bad contacts. This explain the loss of one channel. Moving the Monitor_Tape switch up and down may fix the problem. I was not able to source new switches (Nor the Bose repair Shop in Paris France). The Bose shop changed the transformer for a 220V one but could not change the switch.
Greetings from Southern California,
This is not about the EQ but you clearly know Bose and technology and I’m hoping for a little counsel. I’m remodeling and turning a room into a music room and bringing out my vintage SAE system and vinyl. I’ve had someone go thru and tune up my SAE gear. I’m considering new Bose 901’s for the room and prefer hanging them from ceiling. Specifics:
Room is 20x20x8.75; hardwood floor; beed paneling ceiling.
Front wall has 3′ wall on each end, then 12′ picture window. I want to hang speakers here facing back wall (back of speaker facing picture window wall).
Side walls will have grass cloth wall paper.
Furniture consists of two puffy chairs in center or room and one odomen.
Two desks directly below picture window
Two 6×9 thin antique oriental rugs on ground
Back wall has stereo cabinet about 12′ length and 7′ tall housing equipment and vinyl
Power Source: SAE 2400 Power Amp, SAE 2100 Preamp, SAE 2800 Parametric EQ
Currently have BIC Venturi 6 speakers from 1975ish. They are big and bulky.
Question: do you think the 901s are the type of speaker that would fit this room? It seems like they from what I’ve read as the room is “bright” in sound and “lively”. I wanted them years ago but got the BICs on a real deal back then. Also, any advice on placement for hanging. I understand that placement is essential is optimizing sound.
I certainly appreciate your thoughts. Please feel free to email me directly.
I have a Series I Active Equalizer that apparently needs a complete rebuild. First I lost one channel on the toggle switch, and now the radio plays in the background, along with the open reel tape deck.
Damm the cost! I’ll make this a Christmas present to myself.
How can I get the unit to you?
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Thanks for the post! It really helped me get my friends EQ working well again.
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I’d have to charge a bit more than the $95 flat fee for Canada due to higher shipping charges. I’ve sent you an e-mail with more details. Thanks for your interest!
i have a series one equalizer thet needsyour attention. i live near vacouver bc. what would you charge?
Hal, if it’s an exact copy (so I can use the same schematic), sure thing! If I ever ran into an equalizer for purchase that needed refurbished, I’d definitely snap it up and do that, but I haven’t run into any to buy and resell just yet. I’m keeping my eyes open, though.
I’ll send you a follow-up e-mail a bit later, and we can discuss getting yours working again.
Back around 1971, a friend sold me a professionally designed and electronically exact copy of a Bose 901 Series 1 Equalizer. Finally, after more than 40 years of service, I lost the right channel like several folks have. Components on the printed circuit board look ok, but I’m sure some have failed. Would you consider repairing it for your $95 flat fee?
Do you ever run into equalizers that you refurbish and sell to others?
Grettings. Nice article/project. Mine is a series I with 1 channnel missing. What would you charge to repair it? Also, I once saw someone selling Bose EQ reburb kits. I can’t seem to find them again. I would like to do the work myself and a kit would be better for me than sourcing all the parts myself. Have you ever heard of a kit or would you make one up yourself to sell? Thanks, Patric in Austin, TX
Hi Patric, thanks for your note. I’ve replied to you via e-mail as well. I charge a flat fee to repair the Series 1 equalizer, which includes all needed common parts (resistors, capacitors, transistors) and return shipping to you. Unfortunately I don’t offer refurbishment kits right now, but that’s a great idea – I think I’ll look into getting those together for sale, one of the most annoying parts of repair is finding all the parts in one easy location.
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I have a BOSE 901 Series II Active Equalizer that seems to have lost the left channel. Would you be able to rebuild it for me and around how much. please contact me