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The U.S. Radio & Television Model 24 is an 4-tube AC superhet circuit
radio in a small cathedral cabinet. As purchased, the radio was
stated as powering up with weak reception. It receives only the broadcast band.
The radio had been serviced in the distant past, as well as some recent restoration judging from the age of the parts used. I decided to attempt to reverse all previous repairs to the extent possible and restore the original above and below chassis appearance. The schematic for this radio can be found on-line at Nostalgia Air. |
My antique radio restoration logs
This radio was purchased on eBay. The cabinet was in good original condition, as were the knobs and grille cloth - just the usual wear, dings and scratches. The radio was sold as powering up with weak reception. The only evidence of prior restoration visible in the eBay listing was the antenna and ground leads, which had been replaced by modern plastic covered wiring. The line cord and plug were original, and in good shape. I always avoid knowingly purchasing a radio that has been restored, as many collectors take shortcuts such as removing the original capacitors and filters. Since the seller did not mention hum, it was likely that at least the filters had been replaced. There was no sign of an original above chassis filter capacitor having been removed.
The Model 24 is a 4-tube superhet circuit radio that receives the broadcast band only - typical in 1931/32. The circuit is quite unusual. It consists of a standard first detector and oscillator tube followed by a regenerative detector! There is no IF amplifier stage and only one IF transformer. The regeneration is not user adjustable - it is set during alignment, and would greatly increase the gain of the detector stage. Of course, if the regeneration setting is advanced too far, the set would break into oscillation. The Model 24 is a non-AVC set, which means that the volume control must be manipulated while tuning to prevent blasting, overloading, or completely missing weak stations.
The radio had been serviced perhaps several times. This was a "well loved" radio, and someone had paid a lot of money to keep it running! A few newer components indicated that more recent repairs had also been done, perhaps by a collector.
Three of the four tubes (two 57's and 80) were National Union brand with date codes of H or L. These tube are likely the originals. The 47 tube was a Raytheon and likely a replacement. There is no way to know for sure what brand of tube was originally installed, but usually all tubes are the same brand.
Only one original wax/paper capacitor remained. Two capacitors had been replaced (C-8 and C-11): one with an older vintage capacitor and one with a modern film capacitor. The remaining original capacitor was a Dubilier CUB type, which is typical of early 30's radios. It is likely that the other capacitors originally were the same type.
The original filter capacitor had been removed and replaced by three modern tubular electrolytics. The original filter capacitor was likely a rectangular cardboard case affair consisting of two 8mfd filters and a 4mfd low voltage filter. There were empty screw holes in the chassis which indicated the original position of the capacitor under the chassis, as well as its mounting centers. Wiring location clues also indicated where the original terminals were located, and part of an original solder lug still remained.
Two resistors had been replaced with older 1 watt carbon composition type resistors (R-5 and R-7). All the remaining resistors were old-style "dogbone" types (two were cast-end types).
The volume control and switch had been replaced. The original was 20K ohms, and likely had a reverse taper since it shunted the antenna coil primary (rapid resistance change from minimum volume position - exactly opposite of a standard volume control). It had been replaced by a 10K linear taper type control. The IRC reference manual indicates that the correct replacement control is a D16-119 20K with dual taper. I just happened to have one in stock if needed.
My usual restoration procedure is to first make a complete survey of the condition of all components. The survey results guide my restoration strategy. If major and unique components are defective or missing and cannot be restored or replaced, I may elect to sell the radio for parts rather than restore it. I always assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original containers if possible). Any mica capacitors are assumed OK until testing proves otherwise.
Before starting repairs I made BEFORE photos of the chassis bottom. I use these photos to ensure that replacement parts and wiring are placed as close as possible to their original positions. Some radios are subject to problems such as oscillation or motor boating if wiring is re-routed or lead dress is not the same as the original. Since this radio had been heavily serviced or restored, I could not count on any of the wiring or component placement being original.
All tubes and the tube shield were removed. The tuning capacitor and dial assembly was removed for cleaning.
The top and sides of the chassis were cleaned using GoJo hand cleaner and 00 steel wool. Since this may leave behind metal fragments, I follow up this cleaning with a good vacuuming, a small magnet and masking tape. The tuning capacitor was cleaned in my old Heathkit ultrasonic clean using dilute ammonia. After drying with a heat gun, the bearings were lubricated using automotive distributor cam lubricant, which is similar to the original grease used.
Two original dogbone resistors were out of tolerance by more than 20% and two others had been replaced with older type 1 watt carbon composition type resistors. Three original resistors were likely 1/4 watt size and one was definitely 1 watt. I collect NOS as well as used dogbone resistors, and buy all I can find on eBay and at swap meets, at reasonable prices. I did find one 500K 1/4 watt dogbone that was in tolerance, but not the other three needed. In these cases I attempt to find a replacement that is the correct size and has the correct measured value (within 20% tolerance) but not the correct markings! I then repaint the resistor with the value required using hobby enamel paint. In this case I did find suitable replacements. Examples below - the 1 meg resistor was originally 820K, now measures 1.1 meg.
Only one original paper tubular capacitor was still in place. It was restuffed using a 0.1mfd 630 volt film capacitor. It was a Dubilier Cub type, which are difficult to restuff since they have a wooden dowel down the center! Here is my method of restuffing Dubilier Cub capacitors. For the two missing capacitors (0.02mfd, likely 600 volts and 0.1mfd 200 volts) also assumed to be a Cub type, I found a 0.1mfd 200 volt and 0.02mfd 600 volt Dubilier Cub and in my junk capacitor stocks. These were restuffed with 0.1mfd and 0.022mfd 630 volt film capacitors. I collect branded dud capacitors (Zenith, Philco, RCA/GE etc.) and well as common generic brands (Solar, Sprague, Dubilier Cub) just for the purpose of restoring capacitors replaced in servicing or "restoration" by collectors. One Zenith radio I restored had all its original capacitors replaced by modern film capacitors. I was able to replace all of these with the original Zenith parts that were then restuffed with new capacitors. See Zenith 5-R-337 (5R337) Chairside Restoration. Here are the restuffed Dubilier Cub capacitors:
The original dual filter capacitor had been removed and had been replaced by three tubular units under the chassis.
I formed a cardboard case using thin cardboard from the back of a writing tablet. Two 10mfd 450 volt electrolytics and a 22mfd 50 volt capacitor were mounted inside, and their wire leads connected to ground lug terminals mounted on one end. The ground connection (C-16 positive end) was mounted on the opposite end of the capacitor box.
The cardboard case was assembled using carpenters wood glue and various clamps. The finished case was then painted using gray spray enamel. A label was fabricated using the correct original values. The Cornell Dubilier brand and logo was used, since several original capacitors were CD brand Here is the result:
The original line cord and plug were reused. The antenna and ground leads were replaced using cloth covered stranded wire.
All of the original tubes were reinstalled, since three of the four may have been original. The grid cap on one of the 57 tubes was properly secured using epoxy, followed by re-soldering the cap connection to the tube.
The cabinet only needed a good vacuuming inside and then cleaning on the outside with GoJo and 00 steel wool. It turned out definitely presentable without refinishing.
Once the radio chassis was reassembled and the tubes and tube shields installed, power was brought up slowly using a variac. AC power consumption was monitored using a watt meter, and a DVM monitored the B+. The radio powered up and worked immediately. The radio was then aligned. The regeneration control was left in the maximum gain position, since the radio was stable when tuned across the dial. The replacement volume control worked well and provided smooth control of volume, even though it had the wrong taper.
I was able to successfully reverse most previous repairs and restore the likely original appearance of the radio under the chassis. Of course, I did not know the exact appearance of some of the original parts. The reproduction filter capacitor was the correct size, but coloring, labeling and terminal type and configuration was just a guess. Exact placement of components and wiring was also unknown, but there were some clues such as some original component lead lengths. The existing volume control (replaced) was left in place.
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