The RCA UZ-1325 Horn Speaker: A Brief History and How to Restore It

Jason Rogers


A Brief History:

General Electric designed the RCA UZ-1325 horn speaker, like the other three known production horns sold by RCA in the 1920s.  It was first sold in 1924 for $25—about $500 in today’s dollars.  It was designed to replace the previous model, the UZ-1320, which had sold for $36.50 a year earlier.  In February 1925 RCA dropped the price of the 1325 to $18, where it stayed until the end of 1926 when RCA dropped it from its official price list.


Two versions of the speaker were made: an early model with a painted pot metal cast base and the later, and more common version, with a stamped steel base. These versions can be easily told apart without even picking up the horn: the stamped steel base had the model identification plate attached to the base with two hollow-center rivets. The cast pot metal base had its plate attached with either straight-slotted, round head screws or round-headed pins.


The Speaker:

The speaker was a marvel of modern manufacturing, in that RCA/GE managed to make a good quality horn speaker as efficiently as possible and in such a way that they were able to drive down the cost of making the horn over time.  This was made possible by forgoing the more expensive pot metal base for the cheaper stamped steel base.  The other element, the horn, was made from hard rubber, which is vulcanized rubber heated in a mold similar to a modern-day plastic.  This made the horn durable and probably less expensive to manufacture than the usual stamped tin horns.  This was in the time before modern plastics, such as High Density Polyethylene (HDPE), which would later lead to extremely inexpensive to manufacture and very durable items, like we have today.  I think the takeaway from this is that RCA had the right ideas as far as mass-producing durable, yet less expensive items, but that the materials science had yet to catch up with the manufacturing technology.  This situation is further illustrated by the fact that unlike HDPE, hard rubber has the unfortunate propensity of reacting with UV light and moisture in the air to oxidize, resulting in a brown to greyish-green oxidation layer on its surface.  Such a layer is a headache to remove and a detriment to the items that use hard rubber because the oxidation is essentially slowly destroying the material.


Restoring the UZ-1325:

One of the things that makes the 1325 a great restoration project is the hard rubber horn and RCA’s simple, yet elegant design.  The horns were popular, so today they are relatively common and easy to find, which means they are not too expensive.  The stamped steel base and hard rubber horn have proved to be quite durable over 90+ years, which only helps their availability and ease of restoration.


When restoring these horns, which I have personally done with six of them, it’s best to separate the various major components and treat them as mini-projects to be done one at a time or while the other parts are drying.


To get started, I first unscrew the horn from the base and start on it.  If it does happen to have any damage, it makes restoring the rest a bit pointless. Cleaning the horn is best done with OZ Polish from Mohawk.  I put the polish on a super-soft toothbrush and start cleaning the oxidation and dirt off the horn, and periodically wipe the gunk off the surface with a terrycloth rag or towel.


I then work my way to the steel nut on the bottom of the horn.  It is molded on the horn and won’t come off, so don’t try to remove it.  I use a green 3M Scotchbrite scrub pad to polish it up. Sometimes the nuts rust, sometimes they are just dirty and slightly oxidized.  I clean the nut so it looks nice, but not brand new.  Remember: true restoration skill is to make it not look like you restored it, but more like it was well cared for and untouched.


Once the horn and nut are cleaned and polished, I move onto the base, which consists of two pieces, a metal bottom “cup” piece and the base to which the horn nut screws. Frequently, the bottom cup is missing, if not, it is or was covered with wool felt.  The felt is most likely chewed up and filthy. If so, remove the felt and purchase either an acrylic fiber, sticky-backed felt from Rockler Woodworking online, or, for the purists, go to Joann’s Fabric & Craft and get a small piece of black wool felt.  The sticky felt sticks to the cup by itself, but the wool felt will need a few small dabs of hide glue applied to the bottom of the cup to hold it in place.  The trick with the felt is that it must go up the side of the cup piece and slightly into the inside of the base.  The felt is what holds the cup to the base; without it, the cup falls off, which is probably why many of the cups are missing from the bases.


After you have the felt in place on the cup, proceed to the base.  The base consists of the stamped steel (or pot metal) base, the pot metal diaphragm assembly inside, the sensitivity lever, and the speaker cord.  The good news is that, in my experience, RCA used well-formulated pot metal, so the diaphragm assembly should work and not be blown apart like so many pot metal radio parts of that era.  Replacement replica speaker cords can be purchased online, so it is one of the easier parts to fix.  I have not found a lot of the diaphragms not working, but folks in online radio forums say they found them frequently not working.  Fixing them requires unscrewing the diaphragm from the base and opening it. If it is stuck, use a rubber oil filter wrench to remove and open it.  I am not going to go into detail on fixing the two coils inside, but my experience with other horn speakers is that the fine #30 magnet wire frequently breaks within the first inch or two of where it is wound on the coil.  You can unwind an inch of wire and re-solder it to the speaker contact terminals inside the assembly.  Sometimes the wire breaks on the very inside of the coil too, and in that case the only option is to rewind the coil, which I have yet to succeed in doing myself.


The lever that protrudes from the arc shaped slot stamped into the base is frequently misunderstood. Most folks erroneously believe it to be a volume control but it is not. I would characterize it as a sensitivity control because it adjusts the space between the coils and metal diaphragm.  This lever was intended to allow the user to make the adjustment based on the strength of the signal of the radio station to which the receiver was tuned.  As far as cleaning the lever, all I use is the green Scotchbrite pad to remove rust and clean it up.  Sometimes the levers are stuck in position.  The only fix I know for this issue is to disassemble the diaphragm assembly and figure out why.  Usually, it is a matter of cleaning, whereupon the lever will move again.


At this point I prefer to put the diaphragm assembly back into the base, assuming I have removed it.  It is better to do it now and mask it from over-spray, than to do it later and risk banging up fresh coats of lacquer.


The base, because it is stamped steel, is likely to be slightly to horribly rusty.  Again, I use a green Scotchbrite pad to remove the rust.  If it is really rusty you might have to resort to something stronger than the green pad.  Once I have removed the rust, I mask off the threads for the horn nut, the sensitivity lever, and the diaphragm assembly, and prepare to lacquer the base. I use a Mohawk Gloss or Satin Black opaque toner spray to paint the base.  It was what was used originally and looks better than regular acrylic spray paint.  I think spray paint goes on too thick and looks wrong on the base.  The lacquer is a thinner coating and it is easier to control the thickness of the amount being applied.  I apply the lacquer in multiple thin coats and sand with a 400-grit sandpaper between each coat.  The best thing about lacquer is that its drying time is much faster than that of paint and allows you to apply more in a shorter amount of time.  Once you have coated the base and the product identification plate, let it dry.


Now, you might notice that you have sprayed over the ID plate and it is now unreadable. Do not fret!  I have a trick to fix that, based on the fact that the raised lettering on the original was bare metal and the area between the lettering was lacquered black.  Once you have allowed the lacquer to dry, take a straight razor blade and carefully run it across the top of the raised lettering. If you do this correctly, the lacquer on top of the lettering will be gently scraped off without damaging the lacquer in the areas between the lettering.  The other reason to be gentle here is that the ID plate is nickel plated brass and if you abrade the top of the letters too hard, plating will come off and the lettering will be yellow-brass-colored rather than nickel-colored.  If you need to get to hard-to-reach spots, use a #11 Exacto knife blade for those areas and to clean the lacquer off the two rivets.  In the end, and if done correctly, the ID plate should look like it did originally, but freshly lacquered.


Once the base and ID plate are all done and look good, I use one of two products: either Mohawk Blender Flo Out, which will smooth the lacquer to a nicely smooth finish, or (when not needed because the lacquer coats came out nice and smooth) a few coats of their clear lacquer in my chosen sheen.  I prefer satin because it has the right amount of shine to make it look unrestored and original, yet not like you just painted it. Gloss is obsessively shiny and looks over-done to me. Again, the horn not looking obviously restored to the average viewer is a testament to your skill and the ultimate compliment.


Once everything is done, you can install your new speaker cord, which is just a matter of screwing it onto the two terminals in the base.  A small nut driver is all that is needed for this.  Remove the masking from the parts you masked-off, attach the cup to the bottom and screw the horn back on.  Now you have a freshly restored UZ-1325!


Materials will run you $50 or less, and good news is that you now have enough of everything left over that you can restore additional 1325s.  The materials can be purchased from Woodcrafters in Portland over the phone and shipped to you, and the two local Rockler Woodworking and Hardware stores (Northgate and Southcenter, and online at should stock the other items. Joann’s Fabric & Craft stores are located throughout the U.S.  Everything else can be purchased on Amazon or at a local big-box hardware store or Ace Hardware.  As far as skill level, I would call it a good project for a beginner up to a more advanced radio restorer.  Enjoy!