Zenith 6S439 (6-S-439) Restoration

The Zenith model 6S439 (6-S-439) from 1940 is a small tabletop 6-tube AC superhet circuit radio.  It receives the standard broadcast band and two short wave bands, has "automatic" or push-button tuning, and the Zenith RadiOrgan tone control (but has only two controls rather than the usual 5 or 6)

The radio had seen minimal servicing  in the past.  I decided to reverse previous repairs to the extent possible.

The schematic for the Zenith 6S439 Chassis 5678 can be found on Nostalgia Air.  Any part numbers mentioned will refer to numbers on that schematic.

My antique radio restoration logs

Overview and Preparation for Service

This is a very difficult radio to service.  The chassis is very compact, and the automatic tuning unit is under the chassis, covering many other parts.  Some parts cannot be accessed without partial disassembly.  

Previous Repairs


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.  I found:


Before starting repairs, the dust was blown off the chassis using an air compressor.  The top of the chassis was then cleaned with GoJo hand cleaner and 00 steel wool.  After cleaning, the chassis is carefully inspected for steel wool fragments.  It is important to keep steel wool away from the tuning capacitor (it had already been removed).  The tuning capacitor was cleaned in my old Heathkit ultrasonic cleaner, followed by soap, water, and toothbrushes.  The unit was then dried and bearings lubricated using distributor cam lubricant (similar to the original grease used).

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.  Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.  

The automatic tuning contact springs were cleaned with lacquer thinner on a pipe cleaner followed by DeOxit and then more lacquer thinner and a pipe cleaner.  

The dial drive cable was originally OK, but it broke during alignment and had to be replaced.  It is VERY difficult to route the cable 2.5 turns around the tuning shaft, since access is blocked by the compensating coil (Part 4) and bandswitch.  During the replacement one of the hair thin leads on the compensating coil was broken!  I then had to remove this coil from the set.  I made careful notes as to where the 4 leads were connected.  It is VERY difficult to remove the coil.  I had to hold one end of the screw using a right angle screwdriver and remove the nut and lock washer from the other end. There was very little room to maneuver the wrench, so small movements were necessary.  The coil could then be removed from the set, leaving the screw in place.  There was enough of the broken lead visible to attach a piece of 40 gauge cloth covered magnet wire.  The coil was then reinstalled and the leads reconnected.  

Several rubber covered wires had failed insulation.  These were replaced using new cloth covered hookup wire having a similar color.  The 6K7G grid lead from the first IF transformer was rubber covered and missing most of its insulation.  In order to repair this I removed the transformer from the chassis (very difficult!) and replaced ALL the wiring with new cloth covered hookup wire of the correct color.  Some collectors simply cover any bad wiring with shrink tubing!  I do NOT like the looks of this type of repair.  A new replacement vinyl power cord was installed.

I ordered a replacement reproduction tuning knob from Alan Jesperson.  He provided an ORIGINAL knob instead!  This is great because the reproductions are not exactly the same color as the originals.

Resistors and Capacitors

The non-original capacitor C10 (part 22-229, .005, 600 volts) was replaced using an original dud from my stocks.  It was restuffed using a .0047mfd 630 volt film capacitor.  I have an ever growing collection of original branded (Zenith, Philco, RCA/GE, Atwater Kent etc.) wax-paper capacitors for just this situation.  All original Zenith paper capacitors remaining were rebuilt in their original cases using modern 630 volt axial film capacitors in order to maintain the original under-chassis appearance.  I reseal the cardboard tubes using rosin salvaged from RCA catacombs.  This material is a mixture of rosin, beeswax, and other material which melts at a low temperature and will not damage the replacement capacitors.

One original Zenith filter capacitor C12 (22-975, 20mfd, 450 volts) had been replaced.  I found a dud Zenith 22-1036 14mfd 450 volt clamp mount electrolytic capacitor in my junk capacitor box which had the correct diameter and length.  This capacitor was restuffed using a new 22mfd 450 volt electrolytic.  Although the value marking was incorrect, at least it was a Zenith part.  

The original filter capacitor C13-C14 can was removed from the chassis, restuffed with new electrolytics, and reinstalled.  The original was 10mfd/450 volts plus 15mfd/350 volts.  It was restuffed using 10mfd and 15mfd 450 volt radial electrolytic capacitors.  My procedure for restuffing FP type can capacitors is as follows (there are many discussions and examples with photos on Antique Radio Forums):

The resistor R19 that was out of tolerance by 50% was replaced using a 10K 1/2 watt carbon composition resistor, which was similar to the original.

The Local-Distant slide switch on the Wavemagnet antenna had been replaced with a toggle switch, thus damaging the label plate.  I decided I could NOT live with this kludge.  I took a close up photograph of the plate on another similar radio in my collection.  This photo was then pasted into a Word document, sized correctly (it was exactly 1" x 2") and printed.  The photo was then trimmed and glued on top of the original metal plate.  The hole for the slide switch was carefully cut out using a sharp Exacto knife.  The original slide switch was white.  I could not find a replacement white switch, so a normal black switch was painted using enamel hobby paint.  The original switch and plate were attached using rivets.  Fortunately, most slide switches seem to have the same mounting centers.  I turned down the heads of some 6-32 screws to simulate rivets and used these to mount the switch and name plate.  I plan on painting the heads bronze, but have no bronze paint at the moment.  Here is the original kludge and restored switch assembly:


Correct G type tubes were installed.  The original tube shields were reinstalled, plus a replacement Goat shield for the 6Q7G and a Zenith copper colored shield for the 6K7G IF amplifier.  I always insist on using original 6X5G rectifiers in these sets.  There is extensive discussions on Antique Radio Forums about the dangers of using this tube, and many examples of shorts causing catastrophic failures of the power transformer.  Many collectors use 1N4007 diodes instead.  I personally have NEVER seen a situation where this has happened.  I have seen a few examples of failed power transformers, but these had leaking tar or other evidence of long term overload - likely due to leaky or shorted capacitors.

Testing and Alignment

I do not install the automatic tuning unit until the radio is working and aligned.  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 came alive immediately and worked.  

The set was then aligned.  The radio is difficult to align because the loop antenna and speaker must be connected while it is aligned.  One trimmer is under the chassis!  The alignment instructions in Riders (listed under model 7S432) are incorrect for this radio, in that the trimmers specified are not correct.  For example, in step 5, trimmer G is specified for the Police band oscillator.  Trimmer G (on the Wavemagnet) is obviously the broadcast band antenna trimmer.  I had to analyze the oscillator coil and trimmers to come up with the correct trimmers:

Step 3, 18mHz, Oscillator:    Trimmer K (not F)
Step 5, 4.5mHz, Oscillator:   Trimmer N (not G)
Step 6, 1500kHz, Oscillator: Trimmer F (not H - there IS no H)
Step 7, 1400kHz, Antenna:   Trimmer G (not K)

My signal generator only goes up to 15mHz, so I was forced to align the high shortwave band using 9mHz and looking for the harmonic.  Although I found the signal at 9mHz, as well as its image at 8.1mHz, I was unable to set the oscillator to scale at 18mHz.  So I just aligned the set a 9mHz and peaked trimmer L at 16mHz (8mHz).  Reception on the high short wave band was fine.

The push buttons were adjusted to local stations.  The radio performs well on all bands and has very good tone.  

Restoration Results

Chassis Before Restoration

Chassis After Restoration