|The Zenith model 6G038R (6-G-038R), 1946 model year, is a
large tabletop 6-tube superhet circuit radio that operates from AC, DC or dry batteries. This
is the second example of this radio that I have restored. This example had seen
minimal servicing in the past. The cabinet in this example was not as good
as the first one I restored, but had its original finish (the first example had been
refinished). The chassis in this example was almost original.
The schematic for the Zenith 6-G-038R can be found on Nostalgia Air. Any part numbers in this log will refer to numbers on that schematic.
My antique radio restoration logs
This radio uses 1.5 volt battery tubes although it can operate on 117 volts AC or DC. The radio's large cabinet accommodates the battery, which supplies 9 volts for the tube filaments and 90 volts for B+. It receives the standard broadcast band and two short wave bands and has the Zenith Radiorgan tone control system. The circuit is similar to Zenith Transoceanic and suitcase style portable radios, however instead of a Wavemagnet (loop antenna) this radio uses an extendable whip antenna (Waverod) for local stations and can accommodate a long wire outdoor antenna. The circuit is quite complex due to the fact that all the filament type tubes are in series, which complicates the proper biasing and AVC operation. Also, lots of bypass capacitors are needed to prevent tube interaction and feedback. The small compact chassis and high component count makes servicing difficult. This type of radio was often purchased by rural customers who currently had no electrical service. They could use the radio on (expensive) batteries and not have to purchase a new radio when electrical service arrives.
Only one of the tubes (3Q5GT) was branded Zenith, and thus most were likely replacements.
The dial cord had been replaced (spliced) - the tension spring looked original. The dial cord had been wound in the wrong direction on the tuning shaft, which must have confused the operator since the dial pointer would rotate in the opposite direction as the tuning knob!
All resistors and capacitors appeared to be original.
The 1A7G tube had been replaced by a 1A7GT and the required tall tube shield was missing (in my first restoration of this same model, the tall shield was still in place but a 1A7GT tube was installed).
The original chassis bolts had been replaced by self-tapping screws and washers. The chassis must have been removed in order to replace the dial cord, and perhaps the original chassis bolts had not been replaced.
The radio had been purchased on eBay. It was sold as not working. The cabinet finish was original, but had some rather serious scratches, dings, finish wear and oxidation (it was quite dark). The grille cloth was original and was in excellent condition The knobs were original and in good condition with minimal warping. The dial glass was intact, and the dial glass rubber extrusion was still intact, although I'm sure it would crumble if disturbed! Several cabinet glue blocks had broken loose, likely due to rough handling in shipping. Some of the rubber covered wiring was still supple and in great condition - including the leads to the Radiorgan tone control switch panels - very unusual. The battery cable had some leads that would have to be replaced. The chassis was quite dirty, but not rusty. The insulation on the rubber covered wiring under the chassis would fall off, exposing bare wire, if moved even the slightest amount. Every effort was made to retain the original wiring where possible.
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 or keep it 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.
It was interesting that a handful of paper capacitors in this radio were branded Aerovox or Solar without Zenith part numbers. This was also true in my first example of this radio. All other Zenith radios dated after about 1932 that I have restored had either Zenith branded capacitors, or else other brands which had Zenith part numbers (22-xxx). The non-Zenith capacitors appeared to be original based on examination of the associated solder joints. Inquiries on Antique Radio Forums yielded opinions that in 1946, right after WWII when this radio was made, manufacturers were scrambling for parts, and might have used whatever was available at the time, even if not branded Zenith. This may also have applied to the tubes, since only one was branded Zenith in this radio (but could have been original for the same reason).
Before starting restoration I had to deal with the potential showstopper issue with the antenna coil, which had an open primary winding (the one used with an outside antenna). The coil was covered with dirt and dust which was imbedded in the wax coating. So it was not obvious which of the multiple coils was open. So I decided to remove the coil from the chassis for closer inspection and possible repairs or rewinding. This was a very difficult task, since the terminals of the coil under the chassis, as well as the mounting studs, are under the band switch! I started disconnecting wiring and other components from the lugs, while making careful notes about wire colors and where each wire was routed. Some thicker bus wiring as well as one capacitor lead was cut rather than unsoldered in order to prevent possible damage to the coil, or if there was no room for a soldering iron! The capacitor would have to be removed and restuffed anyway. The bus wiring and capacitor would be replaced (reattached to the coil terminal) before the coil was re-installed. It is easier to reattach some leads or components if they are attached before the coil is fully fastened to the chassis (while hanging in mid air).
Once removed from the chassis, the coil was heated using a heat gun to remove most of the wax and dirt. I simply let the wax drip into a puddle on a piece of newspaper. The primary winding that was open was located. It was the very top winding, and was bank wound. The break was found at the point where the inside (start) of the winding was routed over the top of the coil form and then down to a terminal on the bottom of the coil form. This left about 3/8" if wire available for a splice - very little room for error! A piece of #28 solid bus wire was routed from the bottom terminal up inside the coil form to a small hole drilled near the primary winding stub. The stub was carefully scraped clean of enamel and tinned, then attached to the solid wire. Measurements of all windings now indicated the coil was OK. I then melted the puddle of original wax left and poured it onto the coil, and then smoothed it out as best I could using my heat gun. The coverage was not as thick or uniform as it was originally, since I did not have enough wax to dip the coil.
Before starting any repairs I took photos of the chassis top and bottom so that routing of wiring and component placement could be restored. Lead dress is often critical in radios. 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). Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.
The tuning capacitor and the two filter capacitors were removed for cleaning access to the chassis. The filters later would be restuffed. The top of the chassis was cleaned with GoJo hand cleaner and 00 steel wool. The tuning capacitor was cleaned in my old Heathkit Ultrasonic cleaner after removing the trimmer hardware and mica insulators. The capacitor was then cleaned using soap, water, and toothbrushes. In order to return the trimmers to near their original positions, before removing the screws I noted the original position of the screws (on the clock) and how many half-turns (and fractions) to completely tight. Once the cleaning and drying was completed, the trimmer hardware was reinstalled and the trimmer screws returned to where they were by fully tightening the screws then backing off the documented number of half-turns and fractions. I like to get them close to where they were originally before alignment. After drying, the bearings were lubed with automotive distributor cam grease.
The Radiorgan tone control contacts were cleaned in my old Heathkit ultrasonic cleaner, using dilute ammonia. Only the fixed and moving switch contacts were cleaned, and were carefully rinsed and dried afterwards.
The dial drive cord was replaced with a suitable black dial cord material. The original spring was re-used.
The power cord was replaced using a modern brown vinyl cord with molded polarized plug (the original was brown rubber with a molded plug). Replacement tubes were installed for the four defective tubes. The blue and black leads in the battery cable were replaced. The remaining two were OK.
In most Zenith schematics, all parts which have the same value or type have the same reference number. For example, there may be multiple occurrences of R1 or C2. Before starting restoration, I normally add a suffix forming unique identifiers: R1A, R1B, etc. That way I can make the correct reference in my notes. I usually also annotate the under chassis photograph with the parts callout.
All the original Zenith branded paper capacitors were rebuilt in their original cases using modern 630 volt axial film capacitors in order to maintain the original under-chassis appearance. Here is the process I use. 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). The paper capacitors that were not Zenith branded were replaced with Zenith duds having the correct Zenith part number. These were also restuffed with new 630 volt film capacitors. I collect Zenith and other branded capacitors (Philco, RCA, GE etc.) just for this situation, and also to reverse any prior servicing where original parts have been replaced. Since all capacitors listed in the parts list had Zenith part numbers, I assumed that later examples of this radio would have all Zenith capacitors, once the post-war parts shortage was alleviated.
Both original filter capacitors (C36-C37-C38 and C27-C28) were restuffed to preserve the chassis appearance. The original cans were restuffed using the following technique:
R12 (140 ohm, 2.5 watts, 10%, Zipohm) was replaced by a 130 ohm 4 watt wire-wound resistor that measured 142 ohms.
Two 2.2meg dogbone resistors were replaced with 1/4 watt dogbone resistors that had drifted within tolerance of the needed values. These were repainted as 2.2meg resistors using hobby paint. A 470K 1/4 watt dogbone resistors was also replaced by a repainted dogbone resistor that now measured within tolerance. All the other resistors needed were either 1/4 or 1/2 watt carbon composition types (new stock is available, but differs in appearance from the originals).
Once the radio was reassembled and the tubes installed, power was brought up slowly using a variac. Two DVMs monitored the B+ and the critical filament voltage. The radio worked immediately on both broadcast and both short wave bands. The Radiorgan controls worked correctly. With 120 volts input, the B+ was correct, but the filament voltage was slightly low (8.4 volts vs. 8.7 volts).
Most restoration objectives were met, but there were a few misses:
Chassis Before Restoration
Chassis After Restoration