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teknofossil

Restoration - replication of 2/3 a set of a early 901 system

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Ok, I am a fan of long gone original New England audio manufacturers. After all, I'm a New Englander so there is some "home team pride" that has it's hooks into me .(FYI  I am a techno geek of sorts and have little interest in Red Sox, Patriots, Celtics, the Big E and so on...only audio / historical cool stuff like Springfield made Indian motorcycles, Gee Bee race planes,  Rolls Royce cars, one of the first commercial radio stations, WBZ in the US, all those computer and electronic companies).  I have been a fan of the Bose 901 (along with 3 way AR's and Allisons) for a long time. No, I don't believe it is "the best" as some early adverts from Bose would suggest but they were a cool concept, still are actually and despite the nay sayers when set up as recommended, when operated within the power capabilities of the amplifier and the equalizer is operating properly and in spec they sound impressively good.  A few years back while poking about on Ebay I spied an original Bose 901 equalizer. Bid low and won it, thinking I'd maybe do something with it and a pile of series 2 drivers I had siting in my storage room. But all of it ended up collecting dust as other projects came up. Moving on a few years this past October I saw a single original 901 with a few dead drivers for 50 bucks.  So, I have decided to copy the cabinet,  utilize the series 2 drivers, restore the equalizer and then perhaps give the completed system to my brothers son who is somewhat of a retro tech guy.  Below is the original system with the drivers removed. With a set of verniers and gauge blocks I determined the original 901 cabinet side walls were 1/2" plywood. Angled rear panels are also 1/2" plywood. Front panel is 3/4" ply. Top and bottom panels are 3/4" total thickness, veneered particle board, both sides of each panel. Linen covers the front and rear of the fiberglass filling inside the cabinet. Turns out the equalizer is very early production. Some indications of this are Philips head screws to hold the RCA jack assemblies to the chassis, date codes on some of the caps appear to show 1967 mfg dates and the transistors are the early 2N3393 types (later were 2N5088). There were 2 versions of the original 901 circuit board according to the Rainy City Audio website. Below is the initial production board per their info. More later... 

 

 

 

 

 

 

 

 

Original cabinetrear.jpg

Cap date.jpg

PC board close up transistors.jpg

PC board bottom.jpg

eq up close.jpg

PC board top.jpg

Damaged veneer top.jpg

Original cabinet front.jpg

20181103_065017.jpg

Clone sides and rear panels.jpg

Clone of cabinet partially completed.jpg

Original and duplicated cabinet in process.jpg

Clone cabinet front upper quarter  view.jpg

Driver test fit 3.jpg

Driver test fit 1.jpg

Driver test fit 2.jpg

 

Gussets in place.jpg

Original  and slightly altered clone.jpg

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Nice work on cabinets and first person I have run across that are/have built 901 cabs. Lots of measuring and work! Can I ask why not save the old cabs? They didn't look that bad. Are you going to use walnut veneer?

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On 12/4/2018 at 1:41 PM, lakecat said:

Nice work on cabinets and first person I have run across that are/have built 901 cabs. Lots of measuring and work! Can I ask why not save the old cabs? They didn't look that bad. Are you going to use walnut veneer?

On 11/28/2018 at 1:54 PM, teknofossil said:

Only was able to land one 901 loudspeaker. No idea what happened to the other.  I suppose I could have poked around on Ebay for another but I thought it would be interesting build a duplicate. Yes I am using walnut veneer (fiber backed).  To the left of the bottom photo  you can just see the 48" by 96" roll that I picked up from Woodcraft (love that place.... GOD help them if I win the Powerball). I veneered the inside surface of the upper and lower panels prior to attaching them to the mid section assembly. With the enclosure assembled I am at the point of veneering the outer surfaces of the panels. Since I have a ill spouse lately I can only work on these for short amounts of time so the veneer won't happen until the weekend hopefully. I did layout the driver wiring harness so I can fabricate another. Bose used 22 AWG solid strand wire, using a wire wrap tool to make the connections. My wire wrap gun is I believe for 30 gauge wire for prototyping circuit boards. I will likely rework the ends and carefully solder them back in place.

Also, one point of interest. I have a complete set of the blue cone Bose monogramed cone CTS drivers from EBAY, bought years ago.  I tested all 18 with my Dayton DATS v2. I am very surprised and how close these measure. These drivers appear to have a 1974 date code. I would have thought the spiders and/or coated fabric surrounds would have changed their characteristics over 4 +decades. I am please and impressed. I also measured the original dark grey coned CTS drivers that were in the one "real" 901 cabinet. One is dead,  two are "crunchy" with the remaining 6 testing fine. I noticed overall the older CTS units Fs is lower by about 15hz. Curious... If anyone is interested in my driver data for comparison to the units in their early 901's (or 800's) I'd be happy to post it.

 

 

 

 

 

 

 

 

driver wiring harness.jpg

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Does anyone have any idea who supplied the grille cloth for the original 901 systems. I know this is a "stretch" for someone to recall something trivial like this. I have the original grilles but to help match something but I'm looking to narrow my search.

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Nice work!

We first saw/heard the 901's at, I think, Sam Goody in White Plains NY back around 1968.  

The sales guy showed us the "amazing" full range driver, had one on the table in the room

and used it as an ash tray.  It had a round magnet but otherwise looked exactly like the CTS

XS-510 that had been used in projects in Popular Electronics.

Do your drivers have a number on them?  They look like the XS-510 from what I can see.

Just a month or two ago I saw a used pair of what I thought were Bose 901's and they said

they're home made.  They looked too good to be a simple copy and I've read that Bose 

encouraged/helped his students to build their own copies and I'd guess that's what these are.

I also heard that the plans made it to other colleges so there were many other copies made also.

You might want to try AudioKarma for more info on the grille cloth.

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I have three pair of the Continental series II Bose 901's and have refinished about five other pair of different series over the years...so watching with interest. To me...the Continental was the best looking 901 made and still had the cloth surrounds....and the blue nose drivers.

I have looked in the past for the grill material with no success. Quite the secret I guess. EBay has an occasional sale of grills only for different series.

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On 12/9/2018 at 8:06 PM, Pete B said:

Nice work!

We first saw/heard the 901's at, I think, Sam Goody in White Plains NY back around 1968.  

The sales guy showed us the "amazing" full range driver, had one on the table in the room

and used it as an ash tray.  It had a round magnet but otherwise looked exactly like the CTS

XS-510 that had been used in projects in Popular Electronics.

Do your drivers have a number on them?  They look like the XS-510 from what I can see.

Just a month or two ago I saw a used pair of what I thought were Bose 901's and they said

they're home made.  They looked too good to be a simple copy and I've read that Bose 

encouraged/helped his students to build their own copies and I'd guess that's what these are.

I also heard that the plans made it to other colleges so there were many other copies made also.

You might want to try AudioKarma for more info on the grille cloth.

Below is an image of two of the 901 drivers I have. On the left is from the one "authentic" 901 (so called series 1)  and the series 2 is on the right. I have recently seen some images of different series 2 drivers. Cabinets were marked series 2.  Drivers had round magnets but the diaphragms were black (like the series 1). As you can see my drivers have the Bose logo printed on the cones. I have also seen the same CTS looking drivers sporting blue cones with out the Bose logo printing. These drivers had dust caps embossed with Bose in the dust caps. No idea if this was decorative or there were performance enhancements.

901 drivers.jpg

20181124_113609_resized.jpg

Late model series 2.jpg

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Update on the 901 project. Cabinet veneer work is complete except for finish sanding, staining of the walnut and whatever I decide upon for a finish. I read somewhere Watco Danish oil was used. Can anyone confirm that? All of the 901 original and series 2 seemed to have a dark stain applied. Almost a mahogany like color to the walnut. Later 901s seemed to have only a dull clear finish resembling the natural unfinished color of the walnut veneer I purchased from Woodcraft. Also, the equalizer, which has a walnut veneered cabinet had deep scaring and water stains on it so it gets the Full Monty treatment as well.

In case anyone hasn't noticed, I took artistic license here and mirror imaged the front baffle of the clone speaker cabinet.

Grill cloth. No luck finding any "official" grill cloth. Closest thing is beige burlap. Bose put a fine black mesh between the (outer) grill cloth and the Masonite frames. Not sure what the logic there was.

 

-T

Cabinets veneered.jpg

Cabinets veneered2.jpg

Cabinets veneered3.jpg

Cabinets veneered4.jpg

Original grill and beige burlap.jpg

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Beautiful work. Keep the pictures coming!

KLH used a black scrim behind the grille cloth before Bose. I believe it was for appearance, so you wouldn't see the drivers through the cloth. Whenever I replace KLH cloth I omit the scrim. Which brings me to the second point. I have experimented with burlap on some speakers but my concern is the burlap is not acoustically transparent. I'd go with sound over authentic appearance myself but YMMV.

-Kent

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I understand your point about the burlap. I am keeping an eye on the web for something more suitable. I bought a couple yards of the burlap since it is dirt cheap. I am thinking about doing a test on one of the 901 drivers with my Omnimic, without any grill, with the original grill and then trying different types of grill material.  Just curious. about the result.  

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Happy New Year all!

                                        I finally got to start to apply the finishes. After trying various stains I have around here I settled on a blend of Watco oil stain. I used about  2/3 black walnut and 1/3 mahogany. Watco suggests flooding the surface with the stain using a brush, letting it soak the wood for a while and then rub off the stain until the wood appears dry. Then apply another coat and doing the same. Below is the "real" 901 cabinet with "wet" Watco. Next is both cabinets and finally the equalizer cover. Not sure what the final finish will be. Either wiping poly or oil. Both have their pluses and minuses.

Coat of Watco.jpg

Coat of Watco 2.jpg

EQ cover with Watco.jpg

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Ambitious project with terrific documentation and descriptions. Thx for sharing, will be watching with interest as it comes to completion. Great work!

 

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11 hours ago, teknofossil said:

...about  2/3 black walnut and 1/3 mahogany....

Yeah, I like to mix the flavors, too, to give a bit of the red tint to the walnut. 

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12 hours ago, ra.ra said:

Ambitious project with terrific documentation and descriptions. Thx for sharing, will be watching with interest as it comes to completion. Great work!

 

 

12 hours ago, ra.ra said:

Ambitious project with terrific documentation and descriptions. Thx for sharing, will be watching with interest as it comes to completion. Great work!

 

After the Watco stain is applied the manufacturer recommends a 72 hr drying period before applying a poly type finish. I am probably next going to replicate the grill cloth frames. I have all of the parts including new transistors for the equalizer. I am hoping to not replace to funky water tower shaped transistors, but if I have to I have 20 fresh stock, all from from Mouser. I am going to try polishing the RCA's and treating them with Cramolin contact cleaner and preservative.

-Tom

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The speaker on the left looks exactly like what we could buy as the CTS part number XS-510.

I'd guess that the round magnet, blue cone version has the same specs, but just a guess.

It has been noted that there are some response issues that can be helped out by treating the

outer diameter of the cone and you can see that this was done in the last driver you show.

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Amazing work and pictures by the way!

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