Zenith 8S434 (8-S-434) Restoration

The Zenith model 8S434 (8-S-434) from 1940 is a tabletop 8-tube AC superhet circuit radio which has a unique cabinet design with decorative turned columns.  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 its original speaker, but the chassis had been changed out to a 5724, which is used for the model 7S434 and similar models. A correct chassis (5810) was later found and restored.  The Wavemagnet antenna that came with the set was for the 5724 chassis, and not correct for chassis 5810.

The schematic for the Zenith 8S434 Chassis 5810 can be found on Nostalgia Air.  Any part numbers mentioned will refer to numbers on that schematic.

My antique radio restoration logs


I had originally restored the chassis that came with the radio (5724) and the radio worked, even though the speaker (49-301) was designed for chassis 5810.  The 49-301 speaker had a different field coil resistance and likely a different output transformer than the speaker used with chassis 5724.  It always bothered me that the chassis was incorrect.  I eventually found a correct chassis on eBay.  It was complete with escutcheon and tone switches, but had no speaker.  This was OK since I had the correct speaker!  

The Wavemagnet that came with the radio was likely correct for chassis 5724, however it did not have a local/distant switch as shown on the chassis 5724 schematic.  It was also not correct for chassis 5810, since it had a single 5-pin plug.  The 5810 chassis expects a 4-pin plug (only 3 used) and a two pin plug (one large and one small pin, similar to those used in standard base tubes).  But it did fit the back of the radio correctly!  I have not yet determined what radio it came from, but it was NOT the original back on the 8S434.  So my original 8S434 was essentially a Frankenradio! My strategy was to modify the Wavemagnet antenna to work with chassis 5810 (hopefully).

This is a very difficult radio to service.  The chassis is very compact, and the automatic tuning unit is under the chassis.  Some parts cannot be accessed without partial disassembly.  There are also errors in the Riders schematic.  For example, the schematic shows two 68 ohm wire wound resistors (R8) connected in series between the 6K7 cathode and B+!  In actual fact, these two resistors are in parallel and connect to B-.  Some of the bypass capacitors have different values than listed in the schematic.  C24 (.25mfd) was not installed.  The broadcast band padder capacitor is not pictured nor mentioned in the alignment instructions.  Here is what the chassis looked like prior to restoration:

In Zenith schematics, all resistors and capacitors having the same value have the same part number callout.  So for example, there may be multiple R2's or C4's on the schematic.  Before I start work on the chassis I annotate the schematic so that all parts have unique identifiers.  I usually add an alphabetic suffix, so that the part numbers are thus R1A, R1B, etc.  Once chassis photos are taken, I then annotate the photo with these unique part numbers.

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.  The automatic tuning unit (push button assembly) was removed in order to gain access to components underneath.  Fortunately, only two wires had to be disconnected.  I found:


All tubes and shields were removed.  The automatic tuning unit was removed.  The dial scale and pointer were removed. The tuning capacitor was removed for cleaning and replacement of the chassis grommets.  The short wave antenna coil (item 2) had previously been removed for repair (see below).  The RF amplifier sub-chassis was eventually also removed, since there are parts inside which have to be checked and replaced, and access is blocked by other components.

RF Amplifier Sub-chassis
Before Restoration

The chassis top and sides were then cleaned using GoJo white hand cleaner and 00 steel wool.  After cleaning, the chassis was carefully vacuumed and gone over with a small magnet and masking tape to remove any steel fragments that could later cause problems.    

I then took photos of the chassis bottom so that routing of wiring and component placement can 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.  At this point, the chassis appeared as follows:

The original line cord had been cut off.  It was replaced by a new brown vinyl cord.

The dial scale that came with the chassis was not in as good a condition as the one on the 5724 chassis (wear, one significant scratch, fading).  So I swapped the two dial scales as well as the dial pointer, installing the better ones on the 5810 chassis I intended to use with the radio (the scale was not yet installed on the 5810 chassis pending replacement of the dial cord.)

The volume control and band switch were cleaned using GC Big Bath spray contact cleaner/degreaser, available from Antique Electronic Supply.  The automatic tuning unit contacts were cleaned using Caig DeOxit D5 and Q-tips, followed by a cleaning spray with Big Bath.  Contact resistance (closed) was measured prior to re-installation of the tuning unit.

Dial Cord Replacement

The extant dial cord was broken, so a replacement was installed.  This was one of the most difficult tasks in this restoration!  The tuning shaft is hidden beneath the tuning unit compensation coil, which has four hair-thin leads that are easy to damage.  It cannot easily be removed, even if the leads are disconnected, since access to the screw and nut that retains it are not easily accessible.  The tuning capacitor and its pulley had by this time been installed, along with new vinyl grommets.  The pulley MUST be installed before the tuning capacitor is mounted, since it sits in a depression in the chassis top.  I did not have any of the original type dial cord in stock, so I was forced to use a thinner type.  Here is the method I used to replace the dial cord.  

  1. One end of the cord was first routed through the right side hole in the chassis (view front), which is under the tuning capacitor pulley.  
  2. The chassis was then flipped upside down.
  3. The end of the cord was retrieved.
  4. The cord was wrapped around the tuning shaft for 2.5 turns.  This was not easy, since the automatic tuning compensation coil is in the way and must not be damaged!  I fabricated a hook out of bare buss wire which was used to grab the end of the cord as the turns were wound - assisted by needle nose pliers.
  5. The free end of the cord was then routed through the LEFT side hole in the chassis, which is also under the tuning capacitor pulley.  Before proceeding further, you must make sure that the cord winding on the shaft is not tangled or crossed.
  6. The tuning capacitor pulley was positioned so that the tuning capacitor was at minimum capacity (maximum clockwise) and the hole in the edge of the pulley was at about the 5:00 position.
  7. The end of the cord from the left side hole in the chassis was then wound in a clockwise direction over the top of the pulley, passed through the hole in the edge of the pulley, and attached to one end the of the original spring.  The other end of the spring was attached to the tab on the pulley designed to retain it.
  8. The remainder of the dial cord (projecting from the RIGHT side hole in the chassis) was wound onto the pulley in a counter-clockwise direction, under the pulley, and back over the top of the pulley.  About an extra foot of cord was left slack, since it must pass over a small pulley on the left side of the dial scale, over the top of the dial (retains the dial pointer) and then onto the tuning capacitor pulley and through the hole in the edge.  I used a series of test clips as well as masking tape to keep the dial cord in place during this process, to prevent tangling.
  9. The dial scale was then installed.
  10. The excess cord was then cut, leaving about 6" slack.
  11. The cord was then routed correctly and tensioned.  The loose end was routed through the hole in the pulley (along with the other end) and attached to the same end of the spring as the other end of the cord.  A knot was then tied.
  12. The dial pointer was then mounted and attached to the dial cord.
  13. The mechanism was then tested for correct operation.  Adjustments were made to the dial pointer position and tuning capacitor pulley.  Once satisfied, I secured the knots in the dial cord using Super Glue.

Short Wave Antenna Coil

The short wave antenna coil, item 2, had been partially eaten by mice!  At least they did not leave any deposits on the radio chassis. Part of the coil form as well as the topmost windings had been destroyed.  At first this looked like a fatal injury, perhaps relegating the chassis to a parts set.  The coil had three windings: a primary and two secondary windings - one for short wave (Short Wave 2), and one for police band (Short Wave 1)  Before starting repairs, or even doing my survey, I removed the coil from the chassis - not an easy task!  It is located under the small sub-chassis holding the 1232 RF amplifier tube and filament transformer.  The sub-chassis had to be unbolted and raised a slight amount in order to gain clearance to remove the coil.  Of course, all 5 connections to the coil had to be unsoldered.  

Once the coil was removed, it was discovered that the primary and short wave windings were still intact!  The only coil damaged was the Police band secondary coil, which was on the top of the form.  I first melted off the wax from that portion of the coil using a heat gun, taking care not to damage the rest of the coil. I then carefully unwound the coil, counting turns (difficult, since about 1/3 of the coil had broken turns).  The coil was in two sections: a 4-turn section at the top, then a 0.08" gap, then a 17-turn section below.  I also measured the position of the coil sections as well as the winding direction.  I measured the size of wire used using a micrometer (6.5 mils) and matched it up with my stocks of magnet wire, getting as close as possible to the original wire size used.  I used 34 gauge enamel magnet wire to rewind the coil.

The gap in the coil form was first patched using a section of a donor coil.  The patch was attached using epoxy cement after that part of the form was first cleaned using lacquer thinner to remove any remaining wax.  The coil form was then mounted in my small Unimat lathe using an improvised jig (a screwdriver handle).  The drive belt was removed from the lathe and the belt guard opened so that the lathe could be turned by hand.  The magnet wire supply spool was held in an improvised holder fabricated from a piece of metal strapping screwed down to the bench, and a wooden dowel.  The coil was then rewound with the correct number of turns and spacing, keeping the winding as straight as possible (the result was far from perfect).  Once the winding was complete, it was coated with rosin salvaged from servicing RCA Radiola Superheterodyne catacombs in order to stabilize the winding. 

Of course at this point I had no idea if the coil would work properly, and if the Police band would properly track.  There is no trimmer for the police band.

T2 Damage

Wire Removed

Form Patched

Coil Rewound

Wavemagnet Antenna

The Wavemagnet antenna was modified per the schematic diagram.  Only minor changes were required.  The original Wavemagnet had a single 5-pin plug.  The 5810 chassis had a 4-pin socket (no center pin) and a two pin socket with pins similar to those used in standard base tubes (such as a 6D6) - one pin was larger than the other.  I found that the pin size and spacing was the same as those on a standard tube base.  So I salvaged the fiber base plug of an old automobile vibrator (I never throw anything away) and cut away all except the two pins needed and the fiber web between them.  It was them cleaned up using a Dremel Mototool sander and files.  The un-needed center pin of the small plug was removed.  The wiring of the Wavemagnet was then modified to match the 5810 chassis.  Only minor changes were needed.  One wire had to be added, which shorts out the broadcast band antenna coil primary winding through the band switch in Short Wave mode.  Of course, at this point I had no idea if this Wavemagnet antenna was electrically the same as the one I needed.  It was later determined during testing the it worked perfectly!  Here are the modified and re-created plugs for chassis 5810:

Resistors and Capacitors

All the original Zenith paper capacitors 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 servicing RCA Radiola Superheterodyne catacombs (it melts at a low temperature and will not damage the replacement capacitors).  Here is my restuffing process for Zenith capacitors.

In order to remove and restuff the capacitors on the right side of the chassis, major surgery is required!  The ground lead to the three trimmers must be removed and the trimmer capacitors unbolted and moved out of the way.  The oscillator coil must also be unbolted and carefully moved out of the way.  Some of the bypass capacitors are partially hidden by the band switch and one lead is soldered to the chassis.  Originally these capacitors were likely installed BEFORE the band switch was installed!  There is hardly any room for a large enough soldering iron to replace these capacitors.  In a couple of cases, the original capacitor leads going to chassis ground were cut, and the restuffed capacitor lead spliced to the grounded stub.

One original Zenith filter capacitor C19 had been replaced.  I purchased a dud twist-lock capacitor on Antique Radio Forums Classifieds which had the correct diameter and approximate length, as well as a black cardboard cover.  It was an original Zenith capacitor, but not the correct part number.  This capacitor was opened, its contents removed, and was rebuilt using a modern 33mfd/450 volt electrolytic cap.  The dud capacitor I used had two lugs.  One was removed.  The cardboard cover for this capacitor is needed since the metal case is connected directly to the AC line! The original main filter capacitor C20/C21/C22 was 30/20/10mfd at 350/250/250 volts.  It was rebuilt as 33/22/10mfd at 350/250/350 volts using new modern radial lead electrolytics.

The two dogbone resistors that were out of tolerance were replaced by a NOS dogbone resistor (that was in tolerance) and another that had drifted to near the correct value.  That one was repainted using hobby paint to the correct color codes.  While this resistor may continue to drift, so will the others in the set.  I wished to maintain the original above and below chassis appearance.  The two metal end carbon composition resistors were also replaced using NOS and repainted dogbone type resistors (1/4 watt) since I did not have any good original parts.

The defective R7 (22 ohms wire wound) was replaced by a 17.2 ohm 1/2 watt flexible wire-wound resistor originally salvaged from a Philco farm radio.


Correct G type tubes were installed except for the 25AC5G (I have not yet found a suitable replacement).  I purchased one Zenith and one Raytheon 25Z6G on eBay.  I had a good Zenith 6AF5G in stock.

Testing and Alignment

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 the audio section worked.  There was some reception on the two short wave bands, but nothing on the broadcast band.  It was later discovered that I had made a wiring error, and the lead from the broadcast band padder capacitor was swapped with another lead originating in the RF amplifier sub-chassis.  The wire colors are easily confused due to fading and dirt.  Once the wiring was corrected, the radio worked on all bands.  The automatic tuning unit was then installed, and it also worked.  

The B+ voltage was slightly low: 215 volts vs. 225 volts (at 122 volts AC input vs. the 120 volt specification).  I never figured out why this was the case.  The new filter capacitors were tested for correct capacity before installation, and the two 25Z6G tubes were also tested (as well as swapped).  The 6AF5G and 25AC5G tubes were also substituted - no change.  The two R7 wire wound resistors were close in value to the specifications.  They were also temporarily shorted out with no B+ voltage change noted. The wiring for the voltage doubler circuit did not match the schematic, but I left it as it was, since it was obviously original, and there were other errors discovered in the schematic (so I did not trust it).  In any case, the radio worked well, and the wattage draw was very close to published values in Riders.  Zenith voltages in Riders seldom match the radio in my experience.  The police band (short wave #1) also worked, so the repairs to the mouse-eaten short wave antenna coil were obviously successful.

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!  There were several problems during alignment.  If the short wave band #2 antenna coil trimmer capacitor M was peaked for maximum output, the radio would break into oscillation in the middle of the band.  I diddled with the wiring in the 6A8 tube area with an insulated probe - moving wires around and changing the lead positions with respect to each other and various components.  Only minor changes in pitch of the noise resulted.  So I had to back off the adjustment of the RF trimmer M for short wave band #2 somewhat to prevent this.  This likely affected the performance on the high short wave band.  The police band worked well after the repair to the antenna coil.  There is no trimmer for the police band.

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

Restoration Results

Chassis Before Restoration

Chassis After Restoration