 |
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 example had seen extensive servicing in the past. I decided to reverse previous repairs to the extent possible. 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.
None of the tubes were branded Zenith, and thus were likely all replacements. The 1A7G tube had been replaced by a 1A7GT, with the taller tube shield left in place.
Several pieces of wiring had been replaced.
The dial cord had been replaced - the tension spring looked original.
The dial drive shaft friction spring had been replaced.
The black fiber washer that belongs between the dial pointer and the dial scale was found on the bandswitch shaft!
A filter capacitor had been tacked in across the first filter capacitor section C36. The two original filter capacitor cans were still in place.
One paper capacitor had been replaced.
For some reason, a 0.25mfd/600 volt bypass capacitor had been tacked in across the existing .05mfd AVC bypass capacitor C8 connected to lug 5 of RF coil L1. C8 had NOT been removed. This would surely have screwed up AVC action.
Power resistor R12 had been replaced by a 150 ohm 10 watt unit (the original was 2.5 watts - I'm not sure what it might have looked like).
There were signs that the volume control and associated tuning hardware had been removed and replaced - I'm not sure why! Perhaps to replace the dial cord? The volume control was original. But some of the wiring to the control had been spliced.
The 1LD5 first audio tube plate load resistor R15 had been replaced with a 5 megohm dogbone type resistor (should be 1 meg) - the radio could not have worked very well after that! All the other resistors looked to be original.
The power cord and plug had been replaced
A replacement plastic attachment had been fabricated to hold one of the Radiorgan panels to the escutcheon. Perhaps the original had broken off. This attachment was not visible from the front.
The radio had been purchased on eBay. It was said to be working. I did not test it, since all the capacitors were likely original and thus leaky, and the tube filaments are very fragile. The cabinet had been refinished and the escutcheon and speaker grille painted gold. The grille cloth looked original and was in excellent condition (it looked like the example found at Radio Museum). The knobs were original and in excellent condition with minimal warping. The dial glass was intact, and the dial glass rubber extrusion was still in good condition (it may have been replaced). Most, if not all the rubber covered wiring was still supple and in great condition - including most of the leads to the Radiorgan tone control switch panels - again, very unusual. The chassis was quite dirty, but not rusty. All tube shields were still in place.
I NEVER apply power to a radio "to see if it works", even with a Variac or "dim bulb tester". 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 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 Aerovox and Solar branded without Zenith part numbers. All Zenith radios that I have worked on 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 and the difficulty of access for servicing. 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, none of which were branded Zenith in this radio (but could have been original for the same reason).
I first removed the tacked-in filter capacitor, the non-original paper capacitors, and the non-original R12. All tubes and shields were removed.
The tuning capacitor was removed for cleaning access to the chassis. I then took photos of the chassis 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 top of the chassis was cleaned with GoJo hand cleaner and 00 steel wool. The tuning capacitor was cleaned in an old Heathkit ultrasonic cleaner with dilute ammonia. 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 switch contacts were cleaned, and were carefully rinsed and dried afterwards.
The dial drive cord was replaced with a more suitable black dial cord material. The original spring was re-used.
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 paper capacitors were rebuilt in their original cases using modern 630 volt 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 RCA catacombs (it melts at a low temperature and will not damage the replacement capacitors. The non-Zenith bypass capacitors were replaced with Zenith duds having the correct Zenith part number, which were then restuffed with new 630 volt film capacitors.
In preparation for restuffing the filter capacitors, I removed the cardboard cover from a Zenith 22-1047C (10+20+30mfd @ 150 volts). Underneath, the metal can was labeled PHILCO 61-0089, which was a 15+10@350, 20@25! I can only assume that Zenith's capacitor supplier (who also supplied Philco) had a surplus of these aluminum cans and used them to make the needed Zenith part. The can would not be visible since there was a cardboard cover. Just guessing that this and other things went on during the restart of manufacturing in 1946 after the war, when supplies and parts were likely scarce.
Both filter capacitors were removed from the chassis and restuffed to preserve the chassis appearance. The original cans were restuffed using the following technique:
The non-original R15 was replaced using a 1 megohm 1/4 watt 20% carbon composition resistor I found in my resistor stock. It measured very close to1 megohm, and was likely the same type as the original used. The non-original R12 was replaced by a new 140 ohm 3 watt wire-wound resistor. It was very small and hardly visible unlike the 10 watt unit replacement originally installed (the original was a 2.5 watt "Zipohm", whatever that is!) R19, originally a 870 ohm 1 watt wire wound Micamold type, was replaced by a new 860 ohm 1 watt wire-wound resistor that was epoxied into a depression cut into the back of the original resistor. The original leads were cut off. The resulting repair is not visible from the top.
 |
 |
Two 2.2meg dogbone resistors were replaced with 1/4 watt dogbone resistors that had drifted close to the needed values. These were repainted as 2.2meg using hobby paint. All the other resistors needed were 1/4 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. There was a slight rattle in the speaker at high volumes or high bass levels. With 120 volts input, the B+ was correct, but the filament voltage was low (8.4 volts vs. 9 volts). This may be due to the fact that Candohm resistor R13-R14 was about 10% high in value, which would decrease the filament voltage. It also may be normal, since the filament voltage is not indicated in the Riders documentation. Since the radio works OK, I decided to leave well enough alone!
The set was then aligned. I initially set the dial pointer so that the position at each end was about equal. However, at this position, the broadcast band could not be set to scale with the broadcast band trimmer as loose as possible. So I had to move the pointer backwards a small amount to get the radio to track on the high end of the broadcast band. The high short wave band tracked OK, but the police band could not be set to scale (it was close with the trimmer all the way out). The radio was not that sensitive using only the Waverod antenna. However, we are in a rural area. Performance with a short wire antenna was excellent on all bands. The radio was not tested in battery mode.
|
Chassis Before Restoration |
Chassis After Restoration |
 |
 |
