Zenith 10S531 (10-S-531) Restoration


The Zenith model 10S531 (10-S-531) from 1941 is a large tabletop 10-tube AC superhet circuit radio with push-pull 6V6 output tubes.  It is in reality a console size chassis (Zenith deep chassis) in a very large table top cabinet.  It receives the standard broadcast band and two short wave bands, has "automatic" or push-button tuning, and the Zenith RadiOrgan tone control system with six controls.

The radio had seen minimal servicing in the past - most of the original parts were still in place.  I decided to try to reverse all previous repairs to the extent possible.  I did have a 7S342 parts set, just in case a major part was bad!

The schematic for the Zenith 10S531 can be found on Nostalgia Air.  Any part numbers mentioned will refer to numbers on that schematic.

My antique radio restoration logs

Condition As Found

This radio was purchased at the 2011 Charlotte AWA Radio Conference in the auction.  It was obvious that the cabinet would need to be refinished.  Two holes had been drilled into the side of the cabinet, and one hole had a toggle switch installed with leads going to the speaker voice coil (apparently for a switched extension speaker - the other hole was likely for a jack).  I do not normally purchase radios that have been refinished or that NEED to be refinished.  But since this would be a keeper, and since we have a very capable craftsman in town that can refinish cabinets, I decided to bid on it, even though the cost of restoration and purchase would likely exceed its value (and if sold, it would be VERY expensive to pack and ship)!  The radio had its back cover, so I did not get an opportunity to check out the chassis before the auction.  Later I discovered that the chassis had quite a bit of surface rust on the top.  There was considerable corrosion on the dial bezel, and one automatic tuning tab was broken.  The tuning knob was warped, as was the dial pointer center plastic disc.

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 back cover was removed, and then the radio was removed from its cabinet.  Since the RadiOrgan tone control panel will not pass through the hole in the dial bezel, the switch mechanism had to be removed from its frame first (one screw plus a ground lug) and then passed through the hole in the bezel.  I did not want to cut the cable, even though it would have to be replaced (I wanted to keep the cable intact until its leads could be measured for replacements). The dial bezel was then removed, which provided access to the screws holding the automatic tuning unit to the cabinet.  The automatic tuning unit was then unplugged and removed, followed by the chassis and speaker.

At this point I made BEFORE photos of the chassis top and bottom, as well as the cabinet.  I use these photos to ensure that replacement parts and wiring are placed as close as possible to their original positions.  Some radios are subject to problems (such as oscillation) if wiring is re-routed or lead dress is not the same as the original..

All tubes and tube shields were removed.  The dial pointer was removed. The dial scale and attached band change mechanism, volume control shaft, tuning shaft, flywheel and tuning intermediate pulley were removed for access to components below and for cleaning and lubrication.  The tuning capacitor was removed for cleaning and replacement of chassis grommets.  All non-original parts were then removed.  

The top and sides of the chassis was cleaned with GoJo hand cleaner and 00 steel wool.  Since this process may leave small steel wool fragments that can cause problems later, I follow up with a thorough vacuuming and go over everything with a small magnet and masking tape to pick up any stray fragments.

Almost all of the failed rubber covered wiring in the radio was then replaced using non-rated cloth covered wire (both solid and stranded).  The same color was used as the original, except that there is no gray color available.  I use the smaller diameter wire available from Radio Daze.  Their larger diameter 600 volt rated wire (and the only type now available at Antique Electronic Supply) is totally inappropriate for radio restoration.  I feel the non-rated wire is at least as safe as the original wire in the radio!  Only a few buried wires in the RF section were left, for fear of damaging a coil attempting to replace them.  Most of these were very short and there was minimal risk of a short.  As the wiring was replaced, any terminals that also held components that would eventually have to be replaced (resistors or capacitors) were left unsoldered at this point.  I continue to this day to use the old Heathkit convention for soldering (or unsoldering!) a terminal: for example, (S-2) means Solder 2 leads, (NS) means No Solder (at this time).

I as able to purchase a replacement wave trap (L10), automatic tuning tab assembly, the missing chassis suspension spring and antenna plug from Tom Myers on Antique Radio Forums.  The tuning shaft had apparently broken at the outer slot (knob end) at the groove where the retaining C-clip would be attached.  The shaft had been soldered together.  Unfortunately, this solder blob prevented removal of the shaft for cleaning and lubrication.  The solder blob was removed.  I decided to try to adapt the tuning shaft from my 7S342 parts chassis.  It was in good condition, but slightly SHORTER than the one in the 10S531.  After a trial fit, it was determined that there was enough length to hold the tuning knob.  But I had to cut a new slot for the outer C-clip.  That was done using my Unimat lathe and squared up using a small file. I also used the 7S342 drive pulley rather than the original pulley, which was designed for a flat belt rather than the round O-ring I planned to use.  All the volume, band switching, and tuning parts were cleaned and lubricated prior to re-installation.  The tuning drive belt was replaced using a large O-ring which was cut at 45 degrees and joined using super glue.  The tuning worked smoothly and correctly.

The dial pointer plastic disc was warped badly.  This was repaired by first removing the metal dial pointer, which is held to the plastic disc by bent-over tabs.  The plastic disc was then placed on a flat piece of MDF with a hole to clear the hub on the back.  A few seconds with a heat gun then pressing evenly while it cooled greatly improved the situation.  The metal dial pointer was then reattached.

The tuning capacitor grommets were replaced by modern 5/16" rubber (vinyl) grommets.  The tuning capacitor was cleaned in an ultrasonic cleaner followed by soap, water, and toothbrushes.  The bearings were then lubricated using distributor cam lubricant (which is similar to the original grease used).  Before cleaning, the trimmer micas were removed in order to prevent damage.  In order to get the trimmers back to approximately where they were originally set, I first note the position of the trimmer screw on the clock, then count the half turns (and fractions) from that position to fully tight.  The micas and trimmer hardware were also cleaned in the ultrasonic cleaner and then dried before reassembly.  After reassembly, each trimmer is again turned to fully tight position, then backed off the appropriate number of half turns and fractions.  This process will return the trimmers to close to their original positions (later fixed by a full alignment).

The output transformer on the speaker was replaced using a Stancor A-3852 universal push-pull transformer rated at 18 watts.  The voice coil was measured as 3.4 ohms DC resistance, and thus I estimated the impedance as 4 ohms.  The appropriate transformer secondary taps were then selected to match as closely as possible push-pull 6V6 tubes to the 4 ohm voice coil (using the instruction sheet that came with the transformer).  The closest available ratio was 10000 ohms to 3.38 ohms.  The mounting holes in the transformer bracket were slightly enlarged to accommodate the replacement transformer.  A suitable speaker plug was attached to replace the broken plug - it was found in my junk connector stock.  The replacement looked similar to the original (black bakelite), but had 5 pins instead of 4 (only 4 are used - the socket on the chassis has 5 pins so that the 5-pin plug could be used without having to cut off the extra pin).  Once the restoration was complete and during installation of the chassis into the cabinet, I found out that the larger output transformer interfered with the loop antenna!  So I had to rotate the speaker clockwise one hole in order to provide clearance.

The power switch on the volume control was flooded with Big Bath cleaner/degreaser through a small opening in the control.  After the switch was cycled many times, it worked well.

Resistors and Capacitors

All 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 original cardboard tubes using rosin salvaged from RCA catacombs (it melts at a low temperature and will not damage the replacement capacitors).  I collect original Zenith (as well as Philco and other branded type) wax/paper capacitors for use when the originals are missing or have been replaced.  Zenith schematics in Riders Manuals indicate the Zenith part numbers of the capacitors used.  I was able to find the correct part numbers for the two missing original Zenith capacitors in my stocks or the 7S432 parts chassis.  They were then restuffed and sealed.

The line bypass capacitor C19 was a Micamold 0.005mfd/600 volt capacitor.  These capacitors may look like mica capacitors, but are instead normal wax/paper capacitors.  This one was very leaky - not good!  I was able to slice it open using a hobby razor saw, remove the contents, enlarge the inside using a Dremel tool, insert a 0.0047mfd/630 volt film capacitor, and cement the halves back together using epoxy.

The original Zenith filter capacitor C17-C18 (20+20mfd at 450+350 volts) had been replaced by a newer clamp mounted unit with the same values.  I restuffed the replacement capacitor using two 22mfd 450 volt units, attached new wire leads, and sealed the case using melted rosin.  I created a reproduction label for this capacitor using Microsoft Word.  The label verbiage used on the label was copied from an original Zenith filter in my parts bin (which was also a dual unit but not clamp mounted).  Remnants of the rivet that held the original capacitor were found on the chassis, and the replacement attached at that point using a screw (the remnants of the rivet was drilled out).  While I do not know exactly what the original Zenith filter actually looks like, my copy is likely closer than a generic replacement.

Both out-of-tolerance resistors were replaced by the same type dogbone resistors.  One was R22, a 22K 2 watt dogbone resistor.  I collect NOS and used dogbone resistors just for this purpose.  I found a NOS 20K 2 watt dogbone in my stock that measured almost exactly 22K.  It was repainted using hobby paint as a 22K 10% resistor, just like the original.  The other bad resistor was R2, a 100K 1/4 watt dogbone.  I found a NOS dogbone resistor in my stocks that was within 10% tolerance.  All I had to do was to paint the end silver to indicate 10% tolerance.


Correct G type tubes were installed for the two 6X5G's, two 6V6G's, 6K7G and the 6J5G phase inverter.  The original weak 6J5G phase inverter was installed as the detector.  An original Zenith gold 1941 tube shield was installed for the 6K7G IF amplifier.


The original automatic tuning unit had one broken tab and was in poor condition.  I was able to obtain a replacement panel as well as other parts needed from Tom Myers on Antique Radio Forums.  Here is the original unit (left) and the replacement.

The station selector tabs are retained by a long shaft that is held by soft metal projections on the back.  Usually these projections will break if disturbed (the casting is likely some sort of pot metal).  The latch push button assembly was first removed - it is held to the casting by a screw.  The shaft holding the tabs was then freed from the retaining metal projections and carefully routed out of the casting through the hole in the side of the casting made vacant by removing the latch push button mechanism.  The plastic tabs could then be removed.  The tabs were cleaned in an ultrasonic cleaner.  The casting was stripped, wire brushed, and then painted using Rustoleum Aged Brass spray paint.  The tabs, shaft, and latch button were then reinstalled.  A dab of epoxy was used to secure the shaft, since some of the original retainers were broken during removal.  This same process was used to refinish the RadiOrgan tone control panel.

I purchased a replacement tuning knob for the radio while at the Charlotte AWA Radio Conference.  The original Zenith volume knob was warped also, but I found a better example in my knob stock.  The dial glass rubber extrusion (U Channel) was hardened and falling apart.  It was replaced by a reproduction available from Alan Jesperson and Radio Daze.  The dial bezel was stripped and repainted using Rustoleum Aged Brass spray paint.  The cabinet was refinished and the two holes plugged by a local craftsman.

Repainted Bezel and Control Panels

Testing and Alignment

Once the radio chassis 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 section worked, but there was no reception.  Recall that the Wavemagnet antenna plug was originally missing, and the pins were simply plugged into the antenna socket.  I was not sure if the connections were correct.  I decided to determine the correct connections before attaching the replacement plug.  After some investigation, I swapped two leads and was able to received a station strongly.  However, the same station was received all across the dial!  It was later determined that the radio was GROSSLY out of alignment, and especially the IF transformers.  Once the IFs were adjusted correctly at 455kHz, the radio worked well.  At this point, I went through a complete alignment procedure. 

Once the radio was working correctly and aligned, the replacement Wavemagnet antenna plug was attached.  When I picked up the replacement plug, I did a WELL DUH!  The replacement plug had remnants of the Wavemagnet wires still attached!  That would have told me which wire went where, and saved a lot of time.  The automatic tuning unit was cleaned up and lubricated, and the push buttons were adjusted to local stations.  

The radio performs well, and has very good tone, but has excessive bass if the LO BASS tone button is pulled.  The speaker cone excursion was excessive to the point of distortion and possible damage on stations with lots of bass (classic rock or classis country stations - my only choice locally other than talk or sports).  I originally suspected the replacement output transformer, since it was much larger than the original (more iron, thus more bass response).  After consulting the good people on Antique Radio Forums, a common opinion was that this is normal in many cases, since today's stations have a lot more bass than when the radio was new.  I did experiment some with the tone control capacitors, but any change I made to capacitor values made matters worse.  So I decided to leave the radio as is and to not use the LO BASS boost button.

Restoration Results
Chassis Bottom Before and After Restoration

Top Of Chassis Before Restoration

Top Of Chassis After Restoration

Completed Restoration - Cabinet Refinished