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Bose 901 Series 1 Equalizer Repair (Deux)
A client recently engaged me to repair his Bose 901 Series 1 equalizer. This second one was in a bit worse shape electrically than the previous one I’ve written about, with one channel entirely dead and scratchy switches. I received the equalizer and got right to opening it up to check out the internal components. It shows evidence of having been serviced a couple of times in the past, with several of the electrolytic and output capacitors replaced as well as some of the .015uF signal capacitors (the white rectangles near the center and center-right of the image.)
Right away, one problem is immediately visible – one of the output capacitors (C12, near the top right) has split down the side and shows visible leakage from the positive lead. The originally fitted capacitors are the silver cans; the black cans were fitted some time later. They must have been lower quality replacements, as it’s one of the previously replaced units that’s failed most obviously.
My client requested I replace all resistors outright, so I didn’t spend extra time testing the carbon composition resistors before getting straight down to replacement. In the past example I discussed, many of the resistors had drifted far beyond their printed tolerance markings, and as all carbon resistors are going to behave similarly, it would have been a purely academic exercise to see drift figures on these resistors which were going to be replaced anyway.
The circuit board is very easy to remove. All connections, except one set, are along the far side of the board. I de-soldered and cleaned the terminals of the output connections to make it easy to remove.
The underside of the board. Not visible in the photo, but visible when closely looking, I could see where flux had flowed on the board during previous rework. This is normal during rework, and if you know what you’re looking for it can give away previous service if you’re not sure if a board has been worked on before or not.
I started off with the electrolytic capacitors. The two primary filter capacitors were replaced with Sprague Atoms capacitors. Sprague has been around for many, many years and these were manufactured in the USA. I frequently find Sprague capacitors in the vintage equipment I service, it’s great to see they’re still being produced right here at home. The rail filter capacitors I replaced with high quality USA-made CDE capacitors.
Then on to the resistors and signal capacitors. All resistors were switched with metal film replacements which are temperature and time-stable with precision tolerances of 2% or better, with the overwhelming majority being replaced with 1% or better. My supplier sent some with a different body style, so several are physically larger but specified identically. The signal capacitors were replaced with polymer film models at 400V or 630V tolerances, the up-rating ensuring they’re being run well below their tolerance and so should be stable for longer than the life of the speakers they’re attached to. Finally, the output capacitors were replaced with Japanese production Nichicon capacitors which are frequently found in high-end audio equipment.
Fortunately, all 10 transistors (2N5088) were good and didn’t require replacement, but I do stock new replacements just in case.
Ceramic disc capacitors such as the 10pF model in the center of this shot aren’t subject to failure the same way the other models were, and so the originals were retained.
I used control cleaner followed by repeatedly cycling the switch positions to clean their contacts. The Tape Monitor switch remained a bit touchy, but the others cleaned up nicely. My client preferred to preserve the originality of the appearance, and the switch works fine with just a couple seconds of extra touching to make it connect, and so it was left original.
With these repairs, the equalizer should last nearly forever barring some unfortunate outside circumstances. Only the highest quality parts were used to ensure perfect sound reproduction and long life.
(If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
Bose 901 Series 1 Active Equalizer Repair [Vintage Hi-Fi]
(See other Bose 901 Series 1 Active Equalizers I’ve fixed, and get info about your own repair: Bose 901 Active Equalizer Repair Page)
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 specially 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 really sound pretty bad. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, but the lack of using an equalizer might be where derogatory slogan “Bose: No Highs, No Lows” came from.
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, ten low-noise 2N5088 transistors (five per channel) and an assortment of capacitors and resistors and a few inductors to do the work of changing the frequency curve of the audio signal.
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 – exactly like I saw in the 1951 Farnsworth K-262P last month. This is both good and bad: it means the problem was easy to diagnose, but unfortunately, it also means that even 1960s-70s era metalized film capacitors can be nearing the end of their service life and that doesn’t bode well for a lot of other equipment from similar and slightly later years. 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 30% of them had drifted past their stated tolerance and were also going to need replacement.
The filter capacitors. 2x500uF and 7x100uF @ 25V.
The EQ is easy to service. The board is secured with four screws and lifts up.
The back of the board is the business end. This is a simple, single-sided etched PCB common to the era. Very easy to work on with my Hakko desoldering tool. Since both capacitors and resistors have begun to fail, it’s only inevitable the rest of them won’t be far behind so I elected to replace most everything on the board, filters included.
On to the metal film signal capacitors, I’ve replaced them with identical capacitors with a higher voltage rating, between 400V and 630V. These should be more durable and last longer before requiring service again. Miniaturization means these beefier parts occupy the same footprint as their lower-rated vintage predecessors.
Next up, resistor replacement. I made a checklist to follow as I replaced each channel pair of resistors. I’m using high-precision metal film resistors instead of carbon composition resistors, all rated at 1W (versus 1/2W for the originals) and a maximum of 5% tolerance with many coming in at 1% to ensure long-term precision and stability. The original resistors were rated at Metal film resistors are less subject to drift in the first place, and selecting precise values ensures a long life and stability.
Some of the wiring had become brittle with age and needed to be re-soldered to the board after being flexed a couple of times. No big deal.
This project required about 5 hours of touch-time for the re-work and a couple more hours of research as it was my first 901 equalizer on my bench.
Quite a few replacement parts:
With brand new capacitors and high-precision resistors, this Bose 901 Series 1 Active Equalizer should be good to go for many years.
It’s likely that all the Series 1 and Series 2 equalizers could benefit from a rebuild at this point, and possibly even the Series 3 equalizers. (If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
Identifying Your Bose 901 Series 1 Equalizer [Vintage Hi-Fi] [Production Runs]
I’m working on a Bose 901 Series 1 equalizer for a friend. The 1970s-era film capacitors have failed (like I’ve seen before) and the equalizer has begun to introduce distortion instead of remove it like it’s intended. As expensive, top of the line audiophile speakers they definitely shouldn’t sound like that, but fortunately it’s a pretty easy fix.
The Bose 901 Series 1 equalizer was offered in two production runs, which can be interchanged and either will work fine with a pair of Series 1 speakers, but they have slightly different components. I spent some time with the circuit schematics and identified how to tell the two apart – it’s extremely simple!
Along the back wall you can see the row of silver capacitors used in the power supply filtering. These are all 100uF 25V electrolytic capacitors, which help ensure the preamp transistors have clean power. On the Second Production of the Series 1 equalizer, there are 7 of these 100uF capacitors. On the First Production, there are only 3 of these capacitors. See schematic snips, and click through for full sized images:
Both models of equalizer also use different signal capacitors. The First Production run uses a total of 18: (.1 x 2, .047 x 2, .015 x 6, .01 x 4, .001 x 2) while the Second Production run uses only 12: (.1 x 4, .047 x 2, .015 x 6).
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are specially 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 really sound pretty bad. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, but the lack of using an equalizer might be where derogatory slogan “Bose: No Highs, No Lows” came from.
With these very obvious circuit differences, it should be easy to tell which model you’re servicing and which parts to replace. Keep in mind, though, this article only applies to the Series I Active Equalizer. The Series II equalizer is compatible with the Series I speakers, but has a different circuit schematic.
Found: Sansui SP1500 Speakers
It’s been a little while since I’ve gone on my rounds through the salvage and recycling centers in Seattle, so I went out for an hour this afternoon to see what I could find. There wasn’t much out there today, but I did find a very good condition pair of Sansui SP1500 speakers. I already own a pair of Sansui SP2500s and a pair of Sansui SP7500Xs, maybe I’ll find the whole set one day. These were made between 1960 and 1974, widely regarded as the best years of Sansui speakers.
I found a thumbnail, but no full size, of the original spec sheet.
This particular set is a 3-way design with 5 drivers (the midrange stage uses two different sizes), 8 Ohm impedance accepting 50W of power with an efficiency of 97 dB (1W*1m) and frequency response from 40Hz-20KHz. They’re quite heavy weighing in at 47 lbs. each, give a good “thud” when rapped with their walnut and pressed board construction.
They’re in great shape, for having been found at a recycler. I noticed them in the back of the loading dock in an area that’s normally off-limits, and convinced the worker to bring them out front for me. Normally they won’t sell out of the back room, but it didn’t take much convincing. With rated power handling of 60W into 5 drivers, these should be solid performers in my system.
Refurbishing Vintage Sansui SP2500 Speakers (Crossover Repair)
I’m always keeping my eyes out for interesting vintage hi-fi speakers from the ’60s-’70s, and found a nice set of Sansui SP2500s on Craigslist. Produced between 1960 and 1974, these beautiful walnut speakers are solid and very heavy – definitely high quality. I’ll share some photos below, and then dig into reconditioning them!
Here are the vintage ads for the Sansui speakers. Click on the image for the full-size version! The web site is down apparently, replaced with a placeholder park page redirecting to a spam site. Sorry, no vintage ad anymore! I should’ve hosted it here instead.
Unlike newer speakers (including a pair of Sansui SP7500X that are still waiting to be refurbished), these are very easy to remove. Six screws to remove the crossover panel from the back, disconnect the driver leads from the spade terminals, and pull out of the back. Like all older electronics, speakers also have components that can fail inside. The crossover network, distributing and filtering the amplified audio to the different drivers, contains capacitors which can fail with age just like the capacitors in an antique radio or in your modern electronics. It’s easy to forget speakers have parts that can degrade inside, when they’re almost always treated like a single box unit.
If you’re buying speakers from Craigslist and they have adjustable crossovers, make sure to check them on all settings to help assess their condition.
With the crossover removed, it’s easy to see the components that need replacing. The three blue cylinders are the crossover capacitors, in 2.2uF, 4.7uF and 10uF varieties. These are bipolar electrolytic capacitors, a special type that is slightly more expensive than standard electrolytic models, but they are required because speaker current is AC and must flow both directions across the capacitor. A polarized capacitor would be likely to explode if used in the same application.
Parts Express sold me everything I needed for these speakers, and a few others:
The old capacitors are glued to the board and I didn’t want to damage it, so I snipped the leads as close to the end as possible and bent them into terminals, then attached the new capacitor to the wire and soldered together.
Repeat for the other capacitors, and after trimming the leads, much nicer:
Two speakers means two crossovers:
Reinstalled and ready to rock:
My only complaint now is the diffusers send the high-frequency audio straight into my carpet, when placed on the floor. I suspect these were meant to sit on stands somewhat. I’ll experiment with different positions for the speakers in my living room, but if it ends up not helping, the hole is symmetrical so I can just rotate the top assembly and now the speaker will send its sound up, towards my ears.
I have several more pairs of speakers – Sansui SP7500X, Bose 601 Series 1 and Cerwin-Vega D-5s. I’ll post photos of refurbishing their crossovers when I get to it.
My parts list:
- 2.2uF 100V Non-Polarized Capacitor
- 4.7uF 100V Non-Polarized Capacitor
- 10uF 100V Non-Polarized Capacitor
I used 100V capacitors both because that’s the lowest voltage in this type my supplier offered, but also because they’ll last longer if they’re intentionally de-rated and with any luck, this will be the last time I’ll need to work on them.
Total cost: $1.38 + $6 shipping to restore these speakers to original working condition after 40 years of service.
Project Idea: Verve//Remixed 3′s Cello Boombox
I’ve had this idea for quite a few years but haven’t put much time into it, between everything else I’ve been going on. Once or twice a year for the past five or so years, I’ve posted on Craigslist seeking a broken cello to turn into an art project. Finally, the ad hit and I think I can make it happen.
Verve//Remixed 3 has an incredible album cover. You should buy their music for that reason alone.
I’m hoping to make my own version of the Cello Boombox on the cover…and this will be my starting point, a cello with a broken neck the owner wasn’t interested in repairing. The neck snapped off, the bridge is missing, and there are quite a few cracks (although they aren’t visible unless you look closely) and it needs to be re-glued.
I doubt I’ll do anything with this until next year at least, but now it’s a possibility. I’m just glad I was able to find a broken cello, I wasn’t willing to destroy a functional instrument for this. Not to mention, a working cello costs a lot of money.
There are a few things to consider:
Speaker size and placement. How many drivers will I use, and what types?
Cabinet volume and phase cancelling. If I configure this as a stereo boombox, I’d need to isolate the enclosures internally from each other or the out-of-phase parts of the left and right channels of the audio could cancel or introduce distortion.
Audio source. The Verve Remixed album cover is concept art, not a real product, so the knobs and panel meters and a floppy disk drive aren’t things I could realistically include. Do I mount a small music player in the center – maybe an iPod Touch, or a small Android tablet? Or should I just make this into an artistic speaker without its own audio source?
Power. If this is going to have its own amplifier, how am I going to power it? An outboard power pack? Rechargeable batteries inside?
If anyone has suggestions on those design topics, I’d love to hear them, or from anyone else who has attempted a project like this.
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