|The Silvertone model 1938 from about 1936 is a 6-tube small
tombstone or upright style radio. It receives the standard broadcast
band and two short wave bands.
This example had seen minimal servicing in the past. I decided to try to reverse any previous repairs and to retain the existing above and below chassis appearance to the extent possible, and yet get it working again.
The schematic for the Silvertone 1938 can be found on Radiomuseum.org. Any part numbers will refer to numbers on that schematic (there is also a schematic on Nostalgia Air, but the Radiomuseum schematic is more accurate).
My antique radio restoration logs
This radio was purchased on eBay. The cabinet was in fair original condition, as were the knobs and grille cloth - lots or wear, dings, scratches and finish loss. The finish was cloudy or oxidized in areas. The top of the radio may have been recoated with a dark stain or varnish. The radio was very dirty and dusty when received - the dirt hid a lot of defects not visible in the eBay photos! There were a few nuts and nut fragments inside, which was a bit worrying - this is often an indication of rodent activity. However, there was no signs of any nesting, urine stains or damage found. There was a fair sized veneer chip on the top front. The radio was sold as not tested. There was no external evidence that the radio had been restored - even the power cord and plug were original. I always avoid knowingly purchasing a radio that has been restored, as many collectors take shortcuts such as removing the original capacitors and filters. The original final inspection tag was still attached to the speaker cable.
The Silvertone model 1938 is a 6-tube AC superhet circuit radio, and the circuit is very unusual. This is one reason that I purchased it. It was manufactured for Sears by Colonial radio, and the build quality was quite high. The radio has no RF amplifier, but does have a double-tuned pre-selector (3-gang tuning capacitor) to reduce image response due to the low IF frequency used (175kHz). The volume control system is very unusual: it varies the coupling between the primary and secondary of the second IF transformer! I have seen this variable IF coupling used in other Silvertone radios for variable selectivity or for tone control, but never before for volume control. Here is the mechanism:
A sixth tube is used as a separate AVC stage - the second detector and AVC functions are separate tubes. The signal level to the second detector is varied by the volume control, but the AVC rectifier is driven from the primary of the second IF transformer and thus is not affected by the volume control. If this were not done, then reducing the volume would reduce the AVC voltage and allow the radio to overload on strong signals. So in reality, the weird and complex volume control method costs an additional tube. The tube complement is also unusual: a mix of standard base (big pin) tubes (84/6Z4, 75, 6A7) and octal metal-glass tubes (6C5MG, 6K7MG, and 6F6MG).
The 6C5MG AVC tube had been replaced with a metal 6C5. The 6F6MG had been replaced by a 6F6G. The Silvertone 6K7MG tested weak but may have been original to the radio. The remainder of the tubes were likely replacements, since they were not branded Silvertone.
All paper capacitors were original (Sprague branded).
All resistors were original (all were dogbone style)
Two tacked in tubular electrolytic capacitors had been installed under the chassis and were connected to the original filter capacitors, which were still in place.
There was a small piece of metal (a shield) covering part of the fixed secondary coil of the second IF transformer (which is part of the volume control mechanism). Perhaps this was to reduce the coupling?
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 or unique components are defective or missing and cannot be restored or replaced, I may elect to sell the radio 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. I did not test the radio prior to restoration.
All tubes and shields were removed. I removed the tacked in filter capacitors. I then took a photo of the chassis bottom so that routing of wiring and component placement could be restored. Lead dress is often critical in radios. Since no parts had been replaced, this photo should represent the original condition of the chassis, and thus the desired appearance after restoration:
The dust was removed from the chassis using an air compressor. The dial assembly and tuning capacitor were removed. The top of the chassis was cleaned with GoJo hand cleaner and 00 steel wool. This process often leaves behind metal fragments which can cause shorts, so I then went over the chassis with a small magnetic screwdriver to retrieve them. The tuning capacitor (previously removed) was cleaned in an old Heathkit ultrasonic cleaner with dilute ammonia. After drying, the bearings were lubed with automotive distributor cam lubricant.
All of the original Sprague paper capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance. I reseal the cardboard tubes using rosin salvaged from servicing RCA Radiola Superhet catacombs (it melts at a low temperature and will not damage the replacement capacitors. When I replace a component, I always remove the original part (or wiring stubs left over from past component replacement) completely from a terminal. Other components connected at the terminal are protected from heat using old medical clamps. Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.
The original filter capacitors C9 and C20 were rebuilt in their original cases. Both were originally 8mfd wet electrolytics. Both were now completely dried out, which is why they could be left in place (and paralleled with new capacitors) with no problems. C9 had a cardboard insulating cover, since its case is not grounded. The cover seemed firmly attached to the aluminum can - likely glued. In a previous similar situation, I split the cover and was able to remove it with minimal damage, although the split was very visible. This one had a top cover in addition, which also did not want to come loose and would prevent removal of the bottom part of the cover. If the covers were firmly glued in place, they would be destroyed by any attempt to remove them. So I decided to leave the covers in place. Both capacitors were chucked into my small Unimat lathe using the original mounting nuts from both units to hold them in the chuck (the nuts were installed back to back). The capacitor terminals would otherwise have prevented mounting them in the lathe. A live center was used to steady the other end of the capacitor. The cases were scored deeply about 1" up from the bottom of the cases - almost all the way through - and then the cuts were completed using a hobby fine tooth razor saw. The positive electrodes were removed down flush to the insulator. The cases were then cleaned inside. The existing positive electrode stubs were drilled and tapped for a 6-32 screw and a ground lug attached inside the capacitor. The positive lead of the replacement 10mfd 450 volt axial electrolytic capacitor was attached to the ground lug. A small hole was drilled into the base adjacent to the threaded stud for the negative lead of the replacement capacitor, which was extended and insulated using spaghetti tubing.
I was able to find replacement Metal-Glass tubes for the 6K7MG (which was weak) and the 6C5MG (which had been replaced with a normal metal 6C5). The only available 6C5MG I could find had about 30% of the paint missing! I later found a 6F6MG and installed that tube.
The tone control and switch was cleaned with GC Big Bath cleaner. The switch then worked OK. The band switch was also cleaned using Big Bath. The chassis washers were replaced using Silvertone chassis mounts (GS-1954) purchased from Renovated Radios.
Once the radio was reassembled and the tubes 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 the audio worked, but there was no reception. It was discovered that the replacement 6K7MG tube had died - the filament was open, even though the tube tested good when received from the vendor. I reinstalled the original 6K7MG that came with the set, which tested weak, but it worked OK. Later, I resoldered the pins of the replacement 6K7MG, and it then tested OK.
The set was then aligned. There are NO oscillator adjustments on this radio other than a broadcast band padder. The two short wave bands were very close to correct dial calibration. But the broadcast band was about 50kHz high. There was not much I could do about this.
The variable IF coupling volume control moving coil was returned to its as found position. At minimum volume, there was still some sound heard. I temporarily reinstalled the aluminum shield that was found originally installed on the radio. It did reduce the minimum volume considerably, but was very likely NOT original. There were instructions for improving minimum volume performance found in Riders. This procedure was followed and the result was satisfactory (but there was still some sound output at minimum volume!)
There was severe modulation hum and resulting distortion on stations at the high end of the broadcast band. There was no problem on the short wave bands or on the low frequency end of the broadcast band. This is a common problem in my shop. My antenna is indoors (strung from the basement ceiling), and there are lots of sources of interference, such as electrical wiring, fluorescent lighting fixtures, wall wart power supplies, computers, and other devices. Some radios have this problem, while others do not. Zenith radios with shielded Wavemagnet (loop) antennas never seem to have the problem. In the past, I have been able to eliminate or reduce the problem by grounding the radio, moving it away from my work bench, reversing the AC plug, installing a line bypass capacitor, or simply rotating the radio 90 degrees. But nothing seemed to work with this radio! The best cure seemed to be connecting a 0.1mfd capacitor between the hot AC lead and ground (center screw of a grounded electrical outlet). I was getting inconsistent test results until I realized that the original line cord and plug were NOT polarized, and thus reduction of modulation hum depended on which way the radio was plugged in. I permanently installed a 0.1mfd 630 volt capacitor between one side of the AC line and the chassis, and always ground the radio when in use. This resulted in acceptable performance if the radio is plugged in a particular way. Ideally there would be a line bypass capacitor from both sides of the AC line to ground (chassis). However in this case, one end of the AC line is directly connected to one lead of the power transformer using a taped joint, and is thus not accessible without wrecking the originality. The taped joint was also directly underneath one of the RF coils. If I ever sell this radio, I will have to remove the capacitor, since a user could get quite a tingle if he/she touches the chassis that is not grounded!
Non-original line bypass capacitor is not shown. The 6F6G tube has been replaced by a 6F6MG in the next to last photo (back view).
Chassis Before Restoration
Chassis After Restoration