The Emerson Model 32 (chassis U5S) is a five tube AC/DC Superhet that receives the broadcast band and one short wave band (550-1500kHz and 1.5-3mHz). The radio had been serviced several times in its past, but many of the original parts were still in place. I decided to try to reverse all prior servicing and to restore the original chassis appearance to the extent possible. No recent restoration work had been done based on the vintage of replacement components used.
The schematic for the Emerson model 32 can be found on-line on Nostalgia Air.
My antique radio restoration logs
The radio was purchased on eBay and was sold as not working. The cord had been cut. The radio had been identified by the eBay seller as an Emerson model 32, and number 32 was stamped on the rear of the cabinet. The chassis serial number plate indicated that the chassis number was U5S. But the Riders schematic for the Emerson 32 did not match my radio! There were some significant differences found. I manually searched through all of my set of Riders manuals for Emerson radios having the matching tubes, but did not find a matching schematic. The circuit used by my radio is quite unusual. It is a 5-tube AC/DC superhet with AVC using standard base tubes (6A7, 6D6, 75, 43, and 25Z5). But the method of volume control is quite unusual. In most superhet radios of this vintage, volume is controlled by varying the audio voltage from the 2nd detector to the grid of the 1st audio amplifier. But in my radio, the 1st audio amplifier receives the full audio signal level. The volume is controlled by a dual taper potentiometer (wiper grounded) which shunts and antenna coil primary on one end of the control and controls the bias on the IF amplifier tube on the opposite end. All examples of radios using this method of volume control that I have seen have been either non-AVC superhets or TRF sets.
The radio is very compact and difficult to service. Many components are buried under other parts that must be removed for access. The radio appears to have been built in layers!
I always attempt to avoid purchasing radios that have been "restored" by radio collectors or flippers, and am looking for either all original examples or those which have been "lightly serviced" in the distant past by radio service shops. A service shop would typically replace only enough parts to restore operation, and no more! This radio had been serviced several times. There was one service tag on the cabinet bottom dated 1944, when one "condenser" and the antenna hank had been replaced (Babcock Radio Service - city unknown - 4 digit phone number). Other servicing found::
None of these repairs were recent, based on the vintage of the repair parts used. The replacement paper capacitors were Sprague "600 Line".
The chassis was very dirty with some rust. All tubes were removed. The dust was blown off, top and bottom, using an air compressor. Major parts were removed in order to gain access to capacitors and resistors for replacement, and for cleaning. These included:
The tuning capacitor and its shield (the attached IF transformer was not removed, but rather left hanging by its leads).
The oscillator coil.
The chassis top and sides were then cleaned using GoJo (white) hand cleaner, 000 steel wool, old tooth brushes and other small brushes. The steel wool was kept well away from tube sockets and the bottom of the chassis.
The reduction gearing and anti-backlash mechanism was removed from the tuning capacitor. The capacitor was cleaned using my old Heathkit ultrasonic cleaner using dilute ammonia. This required multiple steps, since the capacitor would not fit in the cleaner's small cleaning tank. Alter rinsing, the capacitor was further cleaned using soap, water, and old toothbrushes. Before cleaning, the mica trimmer insulators were removed to prevent damage. Before removal, each trimmer was tightened and the number of 1/2 turns to fully tight noted. This allowed the trimmers to be returned to their approximate original positions after cleaning, drying, and reassembly. The capacitor assembly was lubricated using light turbine oil.
The first thing I did was to draw a schematic of the radio as found using several available Emerson schematics for reference. Neither of the two available Riders schematics using the same tubes matched my radio, but when combined came close. I then assigned part number callouts to my schematic. Next I took "before" photos of my chassis, top and bottom. These photos were then annotated using the part callouts from the hand drawn schematic. I then checked all parts possible before starting repairs:
The speaker field coil was OK (it was the high resistance B+ shunt type).
The output transformer was OK.
The filter choke was OK.
All RF and IF transformers were OK.
The volume control was OK. The switch was initially bad, but responded to a shot of GC Big Bath cleaner through one of the rivet holes. This is fortunate, since this type of control (combination antenna shunt and grid bias control) is almost impossible to replace.
The metal clad combination filament dropping and pilot lamp shunt resistor was OK
The flexible resistor (43 tube cathode bias resistor) was open.
Five small resistors were out of tolerance by more than 20% (some as high as 50%).
Two chassis bolts were missing.
One pilot lamp socket center terminal had pulled loose from the socket.
The IF amplifier (6K7G) grid cap lead insulation was worn and the conductor was shorting to the transformer shield can.
I assume that all paper capacitors are leaky and thus should be replaced. I always "restuff" the original components if possible. I do not replace mica capacitors, but may test them in place if possible (usually this requires disconnecting one lead of the capacitor). Since many of the original parts were still in place I decided to try to maintain the original chassis appearance to the extent possible. Normally I would rebuild all original wax-paper capacitors in their original cases (restuff them). In cases where an original wax-paper capacitor has been replaced, I attempt to find an original type capacitor in order to restore the radio to its original appearance under chassis. I maintain a collection of dud capacitor just for this situation. In this case, all of the existing original wax-paper capacitors were Micamold brand. So I assumed that the three capacitors that had been replaced were also originally Micamold branded. I found quite a few Micamold duds in my dud capacitor stock with the required values. But I had to guess the voltage ratings, since the voltage ratings were not on the schematic. Since this was an AC/DC set, I used 400 volt ratings for the missing 0.1mfd line bypass capacitor and the 0.01mfd audio coupling capacitor. I used .05/200 volts for the missing 1st audio plate B+ decoupling capacitor.
When I replace a component, I always remove the original part completely from a terminal. Other components connected at the terminal are protected from heat using old medical clamps (hemostats). I frequently find and remove wire stubs from previous component replacement. Remaining wire stubs often provide clues as to the types and connections of components replaced in servicing. Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.
All original paper/wax capacitors as well as the three Micamold duds used to replace original parts removed in servicing were restuffed with new 630 volt film capacitors. Here is my restuffing process:
All of the filter capacitors had been replaced - no original parts remained. The original filter capacitor block was likely a single cardboard case unit mounted under the chassis. The schematic lists a "combination by-pass and filter condenser". There would have been room for such a capacitor on the bottom front chassis between the filter choke and a terminal strip on the opposite end of the chassis - both mounted with screws and nuts. The original filter block had been replaced by four individual tubular electrolytics. The schematic indicated that the filter capacitors were 16mfd (input) and 8mfd (output), likely at 150 volts. Another similar schematic uses two 20mfd filter capacitors. The 1st and 2nd audio amplifier cathode bypass capacitors were listed as 5mfd. My restoration strategy was to fabricate a cardboard case capacitor block that would fit under the chassis between two suspected mounting points. Of course I had no clue what the original may have looked like. There was a maximum size known, and the original surely had wire leads, since remnants of these leads were found on other components and terminals.
I have kept a pattern used to build a replacement rectangular cardboard box type capacitors. The pattern was constructed by disassembling a cardboard case capacitor by melting the glue joints, then removing the contents. The various sides and flaps of this pattern can then be adjusted to the actual size needed and a pattern for the required capacitor drawn on a piece of thin cardboard from the back of a writing tablet. The box is then glued up and clamped (rubber bands), leaving one end open. The replacement parts are then placed inside the box, with wire leads protruding from a hole punched in one end. I have also built replacement block capacitors that used terminals vs. wire leads. I used two 22mfd/160 volt capacitors for the filters, and two 4.7mfd/50 volt capacitors for the bypass capacitors.
Five small resistors were out of tolerance by more than 20% (some as high as 50%). All existing resistors were the small black rectangular Micamold type. I had a few Micamold resistors in stock, but none were in tolerance. So I was forced to use 1/2 watt carbon composition resistors to replace them. The 2nd audio cathode bias resistor originally was a flexible type resistor, and was open. Again, I did not have a suitable flexible replacement in stock. But I did have a Micamold type 430 ohm resistor in stock - the original resistor was listed as 450 ohms. So I decided to use the Micamold resistor, since it matched the original types used.
All of the supplied tubes tested strong. They were all reused. Three were Sylvania brand, two were other brands. I have no clue what brand originally would have been supplied.
The center terminal lug of one of the pilot lamp sockets had pulled loose due to failure of the rivet. The socket was disassembled by removing the remnants of the rivet, the internal and two external fiber shoulder washers, and the copper center contact spring. The rivet was replaced using a 4-40 brass screw and nut. The head of the brass screw was ground down flat and to size so that it could be used to retain the copper inner contact spring. The socket was then reassembled and the original terminal lug reattached using a 4-40 nut.
The cabinet was vacuumed then cleaned using GoJo (white) hand cleaner and paper towels, then waxed. Nothing else was done to it.
After completion of the electronic restoration, power was applied through my combination fused Variac and isolation transformer. A DVM monitored the B+ voltage. Power was slowly increased while B+ was monitored. The radio came alive and worked on both bands. The radio was then aligned. The IF frequency is documented on the schematic (456kHz). No other alignment instructions were found. The radio cannot be aligned in its cabinet, since the trimmers cannot be accessed. The dial scale is attached to the cabinet and thus cannot be used to set the dial to scale. So I did not change the oscillator trimmer and only peaked the 1st detector trimmer (on the gang capacitor) at 1400kHz. The short wave antenna trimmer was peaked at 2.7mHz.
The radio performs quite well on both bands using my basement ceiling mounted wire antenna, which is about 40' long.
My original restoration objective was to return the radio to its un-serviced condition - to reverse any prior servicing - and to restore operation. Ideally no repairs would be visible. Not all restoration objectives were met, as some of the original replacement parts were not available or were unknown. Some of my "misses" were:
Non-Original Filters Removed - Oscillator Coil Moved
Chassis After Restoration