Introduction: How to Refurbish a Big Pair of Stereo Speakers
This instructable covers the rebuilding of a big pair of commercial speakers. This rebuild is not about fine craftsmanship, it's about taking a ratted out set of once epic speakers and making them rock once again. Good sounding speakers don't always look good and good looking speakers don't always sound good. My goal was to repair cabinet damage, reinforce them, and get a proper set of drivers into them.
As a long time audio hobbyist, I have been coveting a pair of big Altec theater speakers for a very long time. Due to a stroke of luck, I went from wanting A PAIR to owning (4) pair in one weeks time! Not all were in good shape but ugly Altecs are better than no Altecs. How big are these? Take your average double door house fridge, chop it in half and there you have a pair of these big boys. This rebuild covers the work I did to a pair I kept for rocking parties and general neighborhood mayhem.
Big speakers like this are built a bit differently than your average bookshelf speakers. Many rely on some form of horn shape to project their sound and achieve insane amounts of efficiency which is why audio folks desire them. Big plywood boxes, lots of wood bracing, and big woofers are par for the course. Can modern speakers achieve great sound in a small box? Sure it can but nothing is free. Small speakers achieve big sound through lots of cone movement and lots of power. These big Altecs remain popular to this day because of their signature sound and the fact that 50wpc into a pair can produce enough audio to easily fill a theater. All this while the cones barely move.
Altec wasn't the only company to build big horn loaded cabinets, there are many others all having similar attributes. Big box, big sound, little drive power needed. The fact that as little as 5wpc can produce an amazing acoustical experience is why tube amp guys look for these big speakers.
Here we will be stripping, sanding, bracing, gluing, replacing, filling, sanding some more, painting, and then some. You will need basic carpentry tools, basic hand tools, and the skills to use them.
The speakers we will be working on are Altec Voice of The Theater A7's. Altec fans just call them VOTT's. These were industry standard in theaters for decades. There are other speakers of similar design using similar materials so what you learn here, you can apply elsewhere.
Step 1: Some of the Damage You May Have to Contend With
It's rare you'll find a pair of Altecs, or any other big commercial speakers like these that haven't been working for a living. Theaters, concert halls, and auditoriums were just some of many venues that used large cabinet systems. Since they are there to fulfill a business need it's not uncommon to find them dinged, scratched, or have holes drilled in them for mounting.
Back in the day, there were bands that would tour with these behemoths, just look at the Grateful Dead's Wall of Sound! Being on the road is particularly hard on gear. Speakers that lived on the road got banged up, beat up, rained on, drank on, and who knows what else. Expect to see bashed corners like in the first pic, holes drilled for various mounting configurations like in the second pic, or replacement parts like in the third pic.
One other harsh reality of speakers that have been on the road is that they got played hard. If the original drivers (actual woofers and tweeters) are still there, you'll be lucky if they are still good. Chances are you'll have blown drivers to deal with or incorrect replacements.
Big cabs installed in theaters and other venues may fare out better as far as abuse but they still have age. Over time, speaker cones develop weird sags, magnets may loose strength, capacitors inside crossovers change values. These are all things we'll address.
Step 2: Let's Dig In!
Go ahead... satisfy your curiosity and hook them up.. let's get it over with..
I highly recommend you use a known working stereo amp you dont care too much about for this. The speakers my play fine, they may play terribly, they may have such severe electrical damage that they are shorted out. This is why you should use a sacrificial stereo amp to first test them. If you already tested them prior to purchase then you know what you got.
What are we looking for before pulling them apart? A manufacturers ID plate with brand and model. This info may prove very useful. Look over the cabs and note external damage. Don't forget to turn them upside down. Nobody looks at the bottom and that's the part that sits in a puddle during a rained out concert.
*keep in mind these instructions are geared towards the VOTT A7's but can serve as a guide to getting into other cabinets*
In the case of these Altecs, the inside of the cabinet is accessed via a big wooden plank that takes up almost the entire top half of the backside. Remove all the screws on that plank and use a flathead screwdriver to carefully pry the panel off. Be careful as the factory crossover is sometimes mounted to the panel! If you drop it, you risk ripping wires off the crossover, speakers, or both. Save the screws, you'll reuse them.
Inside the back of the cabinet you should be able to get a clear shot of the woofer. One some VOTT's they mounted the high frequency horn inside the cabinet on the bottom, on some it's perched up on top. Either way, all the drivers should be removed.
The woofer on a VOTT *SHOULD* be held on by 4 large flathead machine screws. Speakers that have been on the road may have all kinds of mayhem going on in this department from drywall screws to nuts and bolts. The horns will be mounted to an L shaped piece of wood with their back end supported by a bracket assembly. The horns and woofers are heavy! This is made in USA old school stuff. Be careful you dont drop any of it. Make sure to draw yourself a diagram of what color wires went where. It could help later.
Inside the cabinet you should also find a crossover. This is an electronic device that splits the sound up sending the mids and highs to the horn and the lows to the woofer. In an ideal situation the original crossover may still be there and will be branded same as the speaker manufacturer.
Speakers that have seen lots of abuse and endured low budget or emergency repairs may yield many surprises like car woofers, mismatched drivers, hacked crossover repairs, no crossover, etc. Expect the worst, hope for the best. A good sense of humor helps here! I once found a cannabis pipe inside a speaker.. nothing is sacred!
Once your drivers and crossover are all out, set them aside in a safe place and you can start cabinet work.
Step 3: Strippin'
Depending on what life your speakers led, they may have the original coat of paint they left the factory with or they may be covered in half a dozen coats of whatever the band could get. Since we're doing up a set of party speakers here let's assume you got yourself a hold of someone's old band speakers.
You're going to be stripping old paint and following that up with sanding. You'll need a good commercial paint stripper, some metal spatulas, and a sander with lots of sand paper in various grit sizes. I went with a citrus based paint stripper to get my work started. Read the directions! It's usually something like clean surface of debris, apply stripper with a brush, wait, and scrape away old paint that has now turned into a weird jelly. I suggest you wear gloves that can deal with the chemicals in the paint stripper. It will burn skin so it's best to not get it on you. You'll more than likely end up doing a few rounds with the chemical paint stripper.
Chemical paint strippers release fumes. Work in a well ventilated area!
The paint stripper will turn the old paint into a jelly like consistency for a short while. In pics 2-4 you'll see what the stripper does to the paint as it works. If you just let it sit, it will re-harden into an uglier mess. You'll have to figure out how much to work with at a time. Once again, the directions on the product are your friend.
The chemical stripper will get much of it off but not all. Once you have gotten as much old paint as you can it will be time to resort to the sander.
some popular questions..
Do you need to strip the entire cabinet of all it's old paint? That's up to you. Ideally you want to at least strip down everything that's easily visible and areas that have wood damage. Paint clogs sand paper. The more you chemically strip, the better.
Will paint stripper mess with the glue holding things together? It may. We'll deal with that in the next few pages.
Step 4: Sandin'
EVERYBODY LOVES SANDING RIGHT? Nothing like being surrounded by a big cloud of pulverized paint and the constant drone of an electric motor right? Probably not.. Get yourself a fan and do this outside. Don't forget to wear a dust mask.
Sanding by hand is crafty but you'll get much more done with a power sander. Leave the hand sanding for delicate areas or details. Here's one area you can get by with budget priced tools from Harbor Freight or a similar place. If you don't already own an orbital sander, theirs is just fine for this kind of work. You will also need an assortment of sandpaper to fit your sander. Sandpaper is rated by grit number. The higher the number, the finer the particles on the sandpaper, the finer the finish. The lower the number, the coarser the particles, the faster the material removal but rougher the finish.
If you are not familiar with using an orbital sander or how different grits remove material, start on the back of your cabinet where it wont be as obvious till you get the hang of it. You're supposed to start with coarse grit first for heavy material removal and then use a finer (higher number) grit to leave a smooth finish. In my experience, 180-200 is about as fine as I have needed and 80 is about as coarse as i have been comfortable using without gouging things up.
What if my cabinets are covered in (insert insane number here) coats of barn paint that wont come off?
Then it may be time for heavy artillery so to speak. You delicate craftsman types may cringe at this but an angle grinder might be the solution. I had to do this on the pair of cabinets I kept for parties. An angle grinder is a powered tool that can be fitted with abrasive discs (either sanding or grinding types) for heavy material removal. Once again, if you don't own one, the cheap one at Harbor Freight is just fine for this.
In pic 2 you see an angle grinder and a device called a variac. It's a variable voltage transformer. I used this to control the speed of my angle grinder as these tools have no speed control, it's on or off. This just made my life easier. In pic 3 you see the effect of the angle grinder. It's a bulk material removal tool. It's like the sledgehammer of the sanding world. be careful with it. If you have never used one practice on scrap wood till you get a feel for how the tool reacts as it catches onto material.
The reason I used a variac was that old paint will go from being hard and crusty to melted and sticky if you grind on it with a fast grinder. This was my solution but by all means a crude one. The grinder can be used to get rid of the thick crusted on stuff and then follow up with the orbital sander until you get an acceptable feel to the wood surface.
What about curved sections? Those are best done by hand with a sanding block and some patience.
You dont need to get the finish down to bare wood since we are going to repaint them. Our goal is to get rid of old crusty paint and achieve as smooth a texture as needed. The only places that need to be bare wood are those that will require repair. Wood glue needs to stick to wood, not old paint.
Step 5: Repairing and Strengthening
If you have done woodwork before, you'll surely know of ways to fill voids and fix cracks. If you haven't, here's what worked for me.
Unnecessary holes in speaker cabinets are bad. Woofers move in and out pumping air as they work. Holes in places they shouldn't be are air leaks. Air leaks in speaker cabinets can affect sound and can impart their own sound in the way of whistling or puffing noises at high volume.
Small holes like those left by screws and bolts from previous mounting hardware can be filled with wood putty. You can buy commercial wood putty or make your own by mixing wood glue with sawdust to a consistency similar to mashed potatoes. This mixture is then forced into the holes with your finger tips or a small spatula and allowed to dry. One trick to keeping the stuff from oozing out the other side of the hole is to use a small piece of adhesive tape on the backside of the hole you're filling. The fix should dry in about an hour and be ready for sanding in a few hours.
Cabinets made of plywood can suffer from delamination if they have been wet. This can be fixed. Carefully sand area without breaking off loose wood. Squirt some wood glue into a disposable cup and carefully add water till its about the consistency of milk. Position cabinet so delaminated area is horizontal and facing up. With a small brush, work watered down glue into separated layers of wood and wipe off excess. Place a piece of wax paper on area your repairing and find a way to apply even, flat pressure to it. An old book and something like an old car battery are great for this. Yes, it requires a bit of weight. Let it dry a few hours.
Small corner dings like the one shown earlier in this instructable can be fixed with wood putty and hand sanding. What has worked for me is to first sand all the area around the damage so I have clean dry wood to work with. Don't remove a bunch of material, it's not necessary. Take wood putty (or your sawdust and wood glue homemade wood putty) and work it into corner. Let it dry. Sand it and repeat as needed until you have built up the corner once again and it can be sanded into proper shape. This will take a few rounds so it's best to work all your dings at the same time so one is drying while your fixing another.
Wood glue and wood putty shrink as they dry. You will have to touch up repairs to get them right.
Loose panels are a bad thing on a speaker cabinet. Nobody wants a rattle and flexing panels cause sound cancellation. Loose panels can be fixed with wood glue and clamping, nails or screws. Wood glue creates a bond that is stronger than wood itself if properly done. If you have access to a brad nail gun it will make cabinet repairs soo much easier. Apply glue to mating surfaces and use the smallest nails you can get away with to hold the parts together as they dry. DO NOT move cabinets around while the glue dries. This will result in lousy joints. The glue should be applied so that mating surfaces make full contact but you dont need it running everywhere.
Internal bracing can sometimes be beneficial. Even cabinets made by top notch companies all had compromises instilled upon by accountants. Any noticeable flexing of cabinet panels can be easily fixed by gluing strips of 1x2 or similar wood along the inside of the offending panel. It doesn't take much to fix a flexing panel. The secret to a tight repair is clean mating surfaces, even glue coverage, and some way to clamp the material while it dries. Remember speaker cabinets that flex will cause sound cancellations at high volume.
Don't forget to inspect the bottom of the cabinets. Since you are currently doing all this work, it makes sense to add wheels on big cabinets. A super cheap way to get good casters for big speaker cabinets is to buy the wooden furniture dollies at Harbor Freight and rob the casters off them. They're often on sale for a bit less than it costs to buy the casters alone.
One more area to look at on horn loaded cabinets, the throat. Some horn loaded cabinets may use a curved wood surface that runs from the woofer cone to the front of the speaker. This curved wood surface can be accessed from the inside of the cabinet. A bead of wood glue run along where the curved section meets the interior supports of the cabinet will help tame any rattles caused by dried out wood glue that has given up. Do this on the inside where its out of sight, not the front of the cabinet where it looks awful.
Some paint strippers can attack wood glue. Pound around the cabinet with your fist and check for rattles.
When applying wood glue always check for drips. Wood glue can be easily cleaned up while wet by using a water damped rag. Once a drip is dry, all you can do is sand it.
Step 6: Drivers, What Makes the Speaker Sing!
Finally we get to the drivers, this is the term for the individual speaker units in the wooden box that actually produce the sound.
A great many professional grade speakers will use a compression horn (pic 1) and a woofer. The horn produces highs and mids, the woofer takes over on the low end producing lower mids and onto the bass end. The electrical signal to these drivers is split up by a crossover. this is a device that splits the sound up sending each driver a signal it's capable of reproducing properly. Without a crossover the woofer would be sent signals it can't reproduce properly and the horn (tweeter and midrange coverage) would be sent bass which will destroy it.
Here you have a plethora of choices!
Ideally you want the correct drivers in your cabinet. The engineers that designed the cabinet, did so knowing a bunch of complicated operating parameters about the drivers and matched them as best they could with the cabinet. Can you successfully replace the factory drivers with other brands or models of drivers? Yes. Is it easy? No.
In the speaker world we have specs such as sensitivity, power handling, required cabinet volume, and resonant frequency just to name a few. These things are all important in order to achieve a balanced sound output from your speaker system (the box and the associated drivers). If you mismatch drivers you can end up with colorful issues such as dull highs, squawky mids, or farty bass.
Finding exact replacements for any damaged drivers is the best way to assure proper sound from your vintage speakers but sometimes this isn't financially possible. Some vintage drivers can easily fetch hundreds of dollars. A few will creep into the thousands. While it's the correct way to fix things, we are after all saving a pair of epic ratty speakers here and setting them up for fun things such as parties or curing neighbors of incessant reggaetton playing so onward towards financially feasible repairs!
Now, there are many many opinions of what drivers to replace other drivers with. The audio world is filled with experts and opinions. Here's what worked for me!
My cabs came with a hodgepodge of blown drivers and mismatched woofers. I saved what I could for the other better looking cabinets and decked my party speakers out with the best a little money could buy. The first thing I did is find fullrange woofer options I could afford. Car subs won't work here. They must be fullrange type woofers meant for big boxes. Next up was find compression driver options I could afford. On many pro grade speakers the actual compression driver (pic 2, the part that reproduces the music) can be removed from the horn assembly. This was done to make servicing more affordable.
I first matched up my options based on sensitivity. This is how much audio a driver produces with 1 watt of signal at a specified distance. This is important so all the drivers produce an equal amount of sound and you dont have one overpowering the other. The higher this number, the more sound you get per watt.
Next I matched up my drivers based on frequency response. This is a measurement of how well a driver can cover a range of frequencies and do it as evenly as possible. You want the woofer and the horn to both be able to crossover in their ability to reproduce audio. The woofer needs to reproduce some of the mids the horn reproduces and the horn needs to reproduce some of the mids the woofer reproduces. This assures a smooth audio transition from one driver to another as music is playing. Not having this results in a "hole" in the sound where it sounds like somebody cut out part of the music. Your choice should match the factory drivers as close as possible. A bit more highs and a bit more lows are an added bonus but make sure both drivers can reach into the same midrange coverage as the original drivers.
Finally I looked at power handling. Here the woofers on the market will easily exceed the compression drivers so your choice will likely be limited by the horn driver. Keep in mind 50w blasting out of a horn is ungodly loud, don't be fooled.
On my party Altecs I went with Peavey Black Widow High Efficiency woofers and Selenium compression drivers on my original Altec metal horns. Is it audiophile approved? Nope. Is it fun to listen to and better than anything my buddies with self powered speakers have? Yep! On the woofers I opted to add 1" spacer rings. This sets the woofer back one additional inch allowing it to swing without crashing into the cabinet on high volume. Modern woofers can swing further than vintage ones.
Your drivers will need a crossover. If the factory crossover is still there and it works, good, roll with it! If your crossover is toast or missing then you need a replacement. Your replacement needs to be able to handle the power you plan on feeding your system and it needs to crossover at a frequency that is within the operating range of both your horn and woofer. On the VOTT's the magic number is 1khz. Anything above 1khz goes to the horn, anything below goes to the woofer. There are plenty of options on crossovers from a simple capacitor to an actual network of capacitors and other parts. Don't go for bottom dollar. Get something that has caps and coils meaning it keeps the lows out of the horn and the highs out of the woofer. Woofers trying to reproduce highs results in a weird buzziness at high volume.
Can the original drivers be fixed? Yes and No. Woofers can be reconed, compression drivers if repairable (the Altecs can) require a replacement diaphragm. The quality of the replacement parts varies. If you opt to replace vintage drivers dont just toss the old ones even if they are bad. There are folks looking for vintage speakers all the time on the auction sites and some bring considerable money even if damaged.
On some systems like the VOTT's, the horn can be mounted on top or on the bottom. The bottom mount was traditionally for road speakers as it protected the horn. Up top is where they belong for best sound. mounting instructions can be found on the web just by looking for Altec VOTT A7 instructions.
Connections to the drivers should be made from the crossover to the drivers using solid connections. I soldered mine but you can use conventional crimp type connections where applicable. 14 gauge cable is plenty fine for internal cabinet wiring. If you're mounting the horn on top keep in mind the wire that feeds it needs to exit the cabinet, Ideally at the top near the bracket via a hole in the wood. Make that hole only as big as needed.
Step 7: Paint, Load Drivers, Enjoy!
Now that your drivers have been selected comes the worst part. This part is not bad in the sense of work, it's bad because you'll need to have patience so things dont get messed up.
So your cabinets should be fixed up as needed and are sanded to your satisfaction. Now we paint. I used rustoleum black enamel in the quart can and rustoleum enamel spray cans. The stuff in the quart cans was rolled on using a proper roller for enamel paints. The spray can stuff was for the throats and other curved or hard to get to places. The trick to getting a nice finish with the roller is paint on a warm day and roll the right amount of paint. Practice on scrap wood or the cabinet backs if needed. A smooth coat can be rolled. You roll a coat and move on. Don't roll a second or third coat until the last coat you applied is dry. Same with the rattle can. Spray a coat, let it dry.
I rattle canned the throats and hard to reach places first, then finished the rolled sections. This assured me no overspray weirdness.
Additionally if you want to go for a really smooth finish like I did on one of the sets, paint a coat, allow it 2 days to dry, light sand it, wipe off dust with clean rag, lay down another coat of paint, repeat. I got an amazingly smooth coat like this.
This is where patience will get the best of you. If you try to do it all at once you may get away with it but the chances of smudges are very real. If you're looking to just get it done and a smudge here or there isn't an issue then have at it and knock her out.
Once you're done painting, however you choose it's really important you let it dry for a minimum of two days. In order to get the drivers mounted you will be leaning onto or into the cabinets. Paint that isn't fully cured is easily messed up.
So the cab is painted and dry, now what? Get your drivers, crossover, and wiring done.
Finally get your access panel back in place. You did sand and paint that right? If you have a problem with blown out screw holes (holes in which a screw will no longer thread) an old school fix is to stick one or two toothpicks cut to a length that fits the hole but doesn't stick out in the blown out hole. This gives the wood screw something to bite into and tighten up nicely.
Above you see my completed party set. Rock On!