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Drive-In Speaker Restoration
So you've found a couple of nice drive-in speakers at the flea market or swap meet and you want to restore them to use with your stereo or PC sound card?
Here's what I do to return them to "like-new" working condition.
They will probably be full of paint, dirt and crud from being exposed to the weather for fifty or more years. How do we get that off? I've found that the best way is to disassemble them and bead blast them in a glass bead cabinet under air pressure. This removes all dirt and rust and leaves the metal nice and clean.Don't be surprised when you take the speaker apart to see a mess like the picture below.
As you can see, there's really not much salvageable inside this one. The speaker is completely rotted away and the housing was full of dried mud. Fortunately the castings were good and it cleaned up nicely in the glass bead cabinet.
Here's that same pair of RCA speakers above after I bead blasted them and replaced all of the internal parts.
If you don't have access to a bead blast cabinet and air compressor, you can also disassemble them and just wash them with common dish detergent and scouring powder. If they were painted, you will also need to use some of that nasty stripper to get the paint off. Or try soaking them for a day or two in the one gallon buckets of carb cleaner found at many auto parts stores. That will remove the hardest of paints. It's a lot of work but it can leave them looking fairly nice. A Scotch-Brite pad works wonders in polishing up the outer surface to a semi-glossy finish. You can buff the surface to a nice shine using automotive buffing compound.
Don't be at all surprised if you find that it takes you two or three hours per speaker to get them looking decent and working properly again. I often spend an entire evening and only get two or three restored. Especially if they were painted or left outside and all of the screws needed drilling out. This is why I ask nearly the same price for refurbished speakers as for new ones. With the new ones, all I have to do is box them and ship them but the originals may require hours of labor before they are ready to ship. And in the long run, if I have to replace the speakers, potentiometers and cords, I end up with more money invested in the originals than in the new ones.
Some drive-ins painted their speakers for identification or just to have a different look. If you want to paint your speakers, try some of the "Hammerite" brand hammertone pebbled finishes available in aerosol cans at many large hardware and home centers. I've seen quite a few RCA speakers finished in a brownish-copper hammertone finish so I think this was possibly a factory option, just as many drive-ins had their names stamped in the housings.
Also, you often see speakers on eBay that are painted with silver hammertone paint. This is most likely because the castings were in poor shape. Many of the castings I've found have lots of pits and small holes in them once they are glass bead cleaned. This is caused by environmental issues.
The way to salvage those is to fill the pits and holes with something like JB-Weld or a Bondo type product, sand smooth then paint them with the Hammerite paint. They will often come out looking quite nice and it's a way to salvage an otherwise very ugly original speker.
By the way, most drive-in speakers were made of aluminum. There were some speakers made during the war years out of bakelite, a kind of molded plastic. Mostly these were made by Motiograph although I have seen some Eprad speakers partly made of bakelite.
Getting the speaker apart may be a challenge. Many speakers were assembled using various styles of "tamper-proof" screws. And even if they used a regular straight slot or phillips head screw, they may be rusted in place.
If I can't get the screws out using one of my tamper-proof screwdriver bits, then I have to drill the head off. Just center punch the head and use a 3/16ths inch bit to carefully drill through only the head.
If you are really lucky, the speakers will come apart after you drill the heads off and you should be able to remove the remainder of the screw by using a pair of locking pliers.
Soak the screw with some penetrating oil for a day before trying to remove it. If you break it off in the casting, your job just got an hour longer as you will now carefully have to drill down the center of the screw and try to use a tap and die set to rethread the hole so another screw will fit. I sometimes have to heat the casting near the screws with a propane torch to break the screws free. If you do this, be careful as you can get it so hot that the aluminum melts.
If you are taking apart Eprad brand speakers that use the Gullimite knurled head security screw, you can often take a sharp punch and place it against the side of the head and tap it with a hammer. Once the screw has backed up by one turn, you can use needle nose pliers to grip the screw head and finish unscrewing it. These screws are number 10 sheet metal screws, 5/8ths inches long. Replace them with a common phillips head screw from the hardware store.
You will probably find the speaker in sad shape. The cone may be warped or even rotted completely away. That's what gave many drive-ins that "lo-fi" sound. The speaker is normally a 4-inch square, 8-ohm speaker that was commonly used in many table radios of the era. You probably won't be able to find an exact replacement but most large electronic stores carry something that will work. Also, check out the automotive section of your local department store like Wal-Mart or K-Mart.
Here's the speakers out of the pair you saw above. Any wonder that there was only some rattling noises and no sound?
Not all speakers will look this bad. I've actually found some that were still usable. Those usually have a little label stuck on them that say "Reed Speaker Company - 1979" or "Remanufactured by the Drive-In Theater Manufacturing Company" or something similar.
Originally there were companies like Eprad, Koropp, Projected Sound, Simplex, or the Reed Speaker company that manufactured and rebuilt speakers for drive-ins in the '50's and '60's for as little as $3 and that included a new speaker, potentiometer and cord.
I believe that 1979 was the last year that the Reed Speaker company of Golden Colorado was in business. All of the other companies also seem to be gone now.
If you can find a speaker small enough to fit in the case, buy the one with the largest magnet. These generally sound better than the originals anyway.
The speaker may be held in place by pressure, a small spring or short screws. Soak the screws in penetrating oil before trying to unscrew them. Use the largest screwdriver that will tightly fit the slots. Again, you want to avoid breaking a screw off as it only complicates things.
For those that are clamped or held in by a spring, you may have to use a spacer like a small block of wood or metal to space the magnet out on the new speaker so it will clamp snugly. Be careful not to put too much pressure on the back of the speaker or you could warp it. For the replacements I carry, a 1/2 inch thick block of wood brings them to the original two inch thickness. Just add a dab or two of wood glue to hold it in place.
The volume control is a wirewound potentiometer of 50 ohms resistance rated at 5 watts. Again, I carry these or they may be available at larger electronic suppliers but tend to cost around $8 or more nowadays. You might try carefully disassembling the existing potentiometer and cleaning the insides with something like WD-40 or electrical contact cleaner before looking for a replacement.
I clean them out and pack them with white lithium grease from an auto parts store. This will permanently lubricate them and they will last forever without getting scratchy or intermittent.
The cord is nothing more than 2 conductor, 18 gauge SJO or SJOW jacketed electrical wire. Again, some electronic stores or home improvement centers may carry this. If not, I carry these cut to length with the proper spade lugs crimped on one end.
One variation on the cord had a third wire, which was actually steel aircraft cable. This was a security measure to keep you from cutting the cord and taking the speaker. If you accidentally drove off with one of these speakers, it either yanked the window glass right out of the car or pulled the pole out of the ground.
Other drive-ins sometimes drilled holes in their speakers and cabled them seperately to the junction box with the same aircraft cable. These were awkward to use as they had not only the regular line cord but a stiff steel cable to deal with and potential problems of scratching the patrons automobile paint.
Many drive-ins were plagued by complaints from neighbors about sound from the speakers after patrons had left.
One cure for this was the "Patented Automatic Sound Cut-Off Speaker" , patent No. 3484552. Basically these were wired with an extra wire in the speakers and in the junction box so that when the speaker was placed back on the hanger or in the basket, the speaker was grounded and the sound muted. Some versions used a special potentiometer to prevent grounding out the other speaker in case a patron was still using it. Later versions just added a 3.9 ohm resistor in series with the regular potentiometer.
You can wire the speaker directly to the 8-ohm output on your amplifier or sound card. You can also install a plug to plug it directly in the headphone jack. Many stereos limit the volume at the headphone jack so you won't damage your hearing so they may not be very loud. The preferred hookup would be directly to the speaker terminals on the stereo.
Originally, the speakers hung on a junction box that was mounted on a pole. The junction box had a little line-matching transformer in it to match the 8-ohm speakers to the 70-volt field wiring from the projection booth sound system. The junction boxes also sometimes had a little light bulb in them to illuminate the pole.
Very early boxes sometimes used a 7-watt, 110-volt lamp. It is very dangerous to use these as a bare wire or short can make the whole pole and speakers an electrocution hazard.
Later boxes replaced the bulb with a miniature 24 - 30 volt bulb like a 6S30 that ran off a low voltage transformer at the projection booth. These were much safer. If your junction box has a socket for a bulb, try to find a suitable 12 or 24 volt bulb that will fit. Then use one of the little "wall-wart" plug in 12 or 24 volt transformers to light it.A good miniature bayonet 12 volt bulb is part # 1893 and a 24 volt miniature bayonet bulb is part # 1864. The transformers should be rated at the appropriate voltage and at least 1/2 amp. These should be available at any electronics store or special order through Radio Shack.
If your junction box does have the line matching transformer and you are hooking the speakers to a stereo, just remove and tape off the wires from the transformer to prevent them from grounding anything out. The transformer is not used in a stereo configuration. This simple wiring diagram shows how to do it.
Enjoy your speakers and keep the drive-in memories alive. If you still have a drive-in in your neighborhood, attend them often and enjoy movies under the stars on that giant screen.
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Revised Feb 20, 2012 GWC