Zenith 5S642BT (5-S-642BT) Restoration

The Zenith 5S642BT (chassis 5B10BT) from 1942  is a 5-tube AC Superhet circuit radio that receives the broadcast band and three short wave bands. The 5S642BT is a Zenith export model built in the USA but capable to operating on either 115 or 225 volts, 50-60 Hz. The radio appeared to be all original, with all original parts still in place - even the original Zenith OEM tubes!  Only the power cord with attached plug had been replaced so the radio could be used (it was used in Chile).  Since the radio was original, I decided to attempt to restore the radio and maintain the original chassis appearance if possible.  I was unable to find a schematic or any service information on this radio on-line. However the circuit is virtually identical to the Zenith model 5S042BT, chassis 5C63 except that my radio does NOT have a phonograph connection and some tube types are different (6K6G vs GT etc.) The schematic for the Zenith 5S042BT can be found in Riders Volume 15 Zenith page 21. There is an entry for 5-S-42BT on Nostalgia Air, but the actual schematic has not been loaded - only the clarified schematics, chassis layout, and service information.  Any parts references in this document will refer to the Riders/RadioMuseum schematic.  That schematic can be viewed at Radio Museum, but membership is required for access.

My antique radio restoration logs

Overview

The radio was purchased on eBay and was sold as "not tested". The power cord with attached plug was identified as the type used in Italy, Chile and Uruguay using an excellent article found in Wikipedia on mains plugs and sockets used worldwide . The plug was marked as made in Chile. The eBay seller later confirmed that indeed this radio had been used in Chile. It had been inherited and had been in storage for 30 years. The fact that the plug could not be used in the USA likely saved the radio from damage due to plugging it in "to see if it worked".

Previous Servicing

It is indeed rare to find a radio that is all original. The only part that I could find that had been changed was the power cord and plug. The dial drive cord had been repaired, but was original - a break had been repaired by splicing in another piece of cord. Even the tubes were original Zenith OEM! Date codes were 2W, 1M, Y1 and Y2 (1941 or 1942?). None of the tubes had an "R" indicating a Zenith replacement.  All tubes except the 6K6G were weak or bad. It is likely that the radio would have been used until it quit working, since parts would have been difficult to obtain during WWII and especially in Chile!

Cleaning

The chassis was very dusty, but not rusty. All tubes, were removed. The dial mechanism was removed for cleaning access and to replace the dial cord. The dust was vacuumed and blown off with an air compressor, top and bottom. The top and sides of the chassis was cleaned using using old tooth brushes and a vacuum to removed dust from the crevices, followed by GoJo (white) hand cleaner and 000 steel wool. The steel wool was kept well away from the tube sockets and tuning capacitor. The chassis was again vacuumed in order to catch any steel wool fragments.

Survey

In Zenith schematics, all resistors and capacitors having the same part number have the same schematic reference call out.  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. I then annotate the schematic and under-chassis photo with these unique part numbers using a red pen. 

• The cabinet was in poor condition with extensive finish loss. The knobs were original, and even the knob felts were present. The back cover was present and in good condition.

• The dial lamps were OK.

• The line cord was OK but not the factory original. 

• The tuning capacitor mounting grommets were OK, which is fortunate, since the screws retaining them are underneath the complex band switch wiring.

• The dial drive cord had been repaired, but the knot sometimes caused the tuning shaft to bind up.

• The speaker field, cone, and output transformer were OK.

• The power transformer was OK. The high voltage winding was balanced across the center tap with 20 volts AC applied through a Variac, and the wattage was less than 10 watts at full line voltage (no tubes).

• All coils and the IF transformers were OK.

• The radio used some rubber covered wiring, all of which was bad. If a wire were moved even a small amount, the insulation would fall off. Most of this wiring would have to be replaced. Fortunately all of the signal level RF and band switch wiring was cloth covered and OK.

• Only one resistor (R5, 10K 2 watt) was out of tolerance. 

• The AC switch was bad, but did respond to a shot of GC Big Bath spray cleaner followed by repeated operation.

• The 6K6G tube was OK. The 6X5GT/G indicated a short. The 6SQ7GT and 7Q7 were very weak. The 7A7 was slightly weak.  

Restoration Strategy

Since the radio was all original and the chassis was in excellent condition, I debated about restoring it at all!  Replacing all of the rubber covered wiring is difficult and is prone to error.  But I feared that if the capacitors and wiring were not replaced, future capacitor failures or shorts could easily destroy the radio.  I have found that the majority of eBay radio sellers will plug in the radio "to test it" if physically possible.  Experienced radio sellers often cut off the power cord if the radio is being sold as needing restoration, untested, or for parts, since it may pass through many hands over many years before someone attempts to test or restore it.  In this case the Chilean power plug likely saved it, plus the eBay seller inherited the radio directly from his/her grandfather and no collectors or dealers were involved. The cord would be replaced with a new brown vinyl cord with molded plug, somewhat similar to the likely original type.

I assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original components if possible). I do not replace mica capacitors, but may test them in place if possible (usually this requires disconnecting one end of the capacitor).

Since all of the original parts were still in place I decided to maintain the original chassis appearance to the extent possible. Normally I would rebuild all original wax-paper capacitors as well as the filter capacitors in their original cases (restuff them). The out of tolerance resistor R5 (a 10K ohm 2 watt 20% old style carbon composition type) would have to be replaced. Unfortunately any original replacement would likely also be out of tolerance, so another type of replacement would have to be used..

When I replace a component, I always remove the original part completely from a terminal.  Other good 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.  

Repairs

Rubber Covered Wiring

All of the crumbling rubber covered wiring would have to be replaced.   The insulation would fall off if the wire was even slightly moved. Fortunately the RF section signal level wiring was all cloth covered and was OK. Most of the existing wiring was about #24 solid in various colors.  To replace it, I used #20 cloth covered hookup wire (unrated voltage), available from Radio Daze and possibly other suppliers. Their 600 volt rated wire is not suitable for radio restoration - it is MUCH too large. One particular Zenith color (slate), is not available. In this case I used black. Another color (a grayish green) was replaced using green. One difficulty in using the #20 wire is that the conductor diameter is much larger than the original wire used.  This makes it difficult, or in some cases impossible, to thread multiple leads through some component lug holes - and especially loctal tube socket lugs (which I hate). I did manage to replace all filament and B+ leads, as well as most signal level leads.  I have not found a source for #24 solid wire with cloth covering - only plastic/PVC etc.

The wiring to the second IF transformer was also rubber covered and had to be replaced.  This required removing the transformer from the chassis. The wire colors on the schematic did not match the colors used on the transformer. So I decided to use the colors specified on the 5S042BT schematic (for slate, I used black).

Wax/Paper Capacitors

All of the original Zenith cardboard tube type capacitors were restuffed using modern 630 volt axial film capacitors. Unfortunately two of the original  capacitors were branded Zenith but made by Solar (Seald-Tite - part number suffix "AM"). These are a solid body of wax with a thin paper wrapper and cannot be restuffed!  So I decided to replace these capacitors with the more usual Zenith parts made with cardboard tubes sealed on each end with wax. These can be restuffed and then resealed.  I collect branded (Zenith, Philco, Sprague, RCA/GE etc.) dud capacitors just for this case, and for cases where an original part has been replaced by a modern part. The part number suffix (not shown in the schematic) varies with the supplier. For example, a Solar branded part may be coded 22-819AM, vs. a Sprague supplied part with the same value may be coded 22-819E.  

In cases where I did not have the correct Zenith part number which corresponded to the Solar provided part, I used a Zenith part with the same capacitance and voltage rating.  For C5 (22-818AM, .05/400v) I used a 22-828C (.05/400v). For C23 (22-1063AM .001/600v) I used a 22-887F (.001/600v).  For C25, the output tube plate capacitor (0.004mfd/1000v), I used two 0.01mfd/630 volt capacitors in series.  These would just fit inside the original case when placed end to end.

My re-stuffing process is as follows:

• The original capacitor is removed from the radio, and the required lead length and any insulating sleeving use noted.
• The low melting point wax from each end of the original (or replacement) capacitor is melted and removed using an old 25 watt soldering iron.
• The original wire leads are removed, as well as any remaining wax.
• While the internal wax is still molten, a small screwdriver is used to push out the original paper-foil roll.  In some cases, the contents came out when the leads were pulled out.
• The original cases are then cleaned out, and any wax and dirt on the outside removed by gently heating the body with a heat gun and wiping with a paper towel while still molten.  
• If the required lead length is longer than that of the replacement capacitor, a piece of buss wire is attached before restuffing. The splice is made close to the capacitor body so that the splice is hidden inside the cardboard tube after re-sealing.
• The replacement capacitor is wrapped in a narrow strip of paper towel in order to keep the new capacitor centered and to keep it from falling out.
• The finished capacitor is then sealed with melted rosin (salvaged from early RCA Superhet catacombs, and donated by or purchased from members on Antique Radio Forums).  
• I do NOT recoat the outside of the rebuilt capacitors with wax (I'm not sure what was originally used - probably beeswax).  The original capacitors had a VERY thick covering of this wax, plus embedded dirt.  Once removed, the capacitors appeared yellow.  Most Zenith capacitors of this vintage appear orange colored. I suppose it depends on the parts supplier.

Filter Capacitor

The original filter capacitor C28/C29 was removed from the chassis for restuffing.  It was a twist lock type capacitor rated at 10mfd/450 and 10mfd/350 volts with a cardboard cover. But it was not the usual type of twist lock capacitor, in that both sections had individual positive and negative connections, rather than a common negative connection. The positive lead of C29 (10/450) went to the bottom terminal marked with a square.  The negative lead went to the metal can. The positive lead of C28 (10/350) went to the bottom terminal marked with a triangle.  The negative lead went to the unmarked bottom terminal. The capacitor was restuffed using two 10mfd 450 volt radial lead capacitors.  My procedure for restuffing twist lock type can capacitors is as follows (there are many discussions and examples with photos using slightly different techniques on Antique Radio Forums):

• Remove the cardboard cover.  In this case, I had to use a heat gun to soften the tar holding it to the can. Sometimes the cover will pull off easily without heating. It is important not to pull using the capacitor's terminals, but rather the ground tabs, while the heated cover is held using an old wash towel or rag.
• Clean the outside of the metal can using lacquer thinner.  
• Remove any excess tar from the inside of the cardboard cover using mechanical means (small screwdriver).
• Uncrimp the bottom of the metal can using a small screwdriver and small diagonal cutters (try to minimize damage to the outside of the can)
• Remove the mounting ring, the top terminal board, the gasket material and another circular fiber board, cutting the aluminum leads to the capacitor body to free up the terminal assembly. I had to peel back a small area of the can in order to be able to insert a small screwdriver under the terminal board and start the removal process. One must be VERY careful and not break the terminal board, and to keep the terminals attached to both fiber boards (do not allow the terminals to pull free of either board).
• Remove the sticky sealing material between the two terminal boards using needle nose pliers, screwdrivers, or whatever is needed.  
• Clean both parts of the terminal board assembly using lacquer thinner and old toothbrushes.  This is done so that the two parts can be glued together.
• Glue the two parts of the terminal board assembly using epoxy secured by clamps. 
• Remove the can contents using a heat gun to release the tar, followed by a thorough cleaning.  I first remove as much tar and other crud using mechanical means.  Next the can is filled with mineral spirits and allowed to soak overnight. The dirty solvent is discarded and the can cleaned using clean mineral spirits and an old tooth brush. 
• Connect the replacement capacitor leads to the original terminals by drilling small holes through the terminal board close to the terminals, passing a lead through the holes, and soldering them to the terminals on the inner side of the lug. I use small numbered drill bits and a Dremel tool - the drill bit used is only slightly larger than the component leads to be used. 
• The common ground lead of C29 is passed through a small hole drilled near one of the capacitor ground/mounting tabs.  This lead is not soldered to the ground lug until after the capacitor is secured in the chassis mounting wafer (otherwise the solder would prevent the lug easily passing through the hole in the mounting wafer or could damage the wafer - ASK ME HOW I KNOW THAT).
• Reinstall the terminal board and mounting ring and restore the crimp around the base (I use a plastic faced hammer).
• Reinstall the cardboard cover (it was not glued - just held by friction).  The cover hides any signs of the bottom crimp being disturbed. 

Resistor R5

Resistor R5, a 10K 10% 2 watt old style carbon composition resistor was 30% out of tolerance.  I attempted to reproduce it by drilling a hole in another similar resistor large enough to insert two 4.7K 1 watt resistors in series.  The similar resistor was held in my Unimat lathe.  But this did not work - I as unable to drill the hole without damaging the resistor.  Another alternative would be to install a modern 10K 2 watt resistor.  But this would really be out of place.  So instead I used a 2 (or 3) watt old style dogbone resistor I had in stock that was in tolerance for 10K ohms.  It was originally marked as 6.8K but now measured 9.5K.  So I repainted it as a 10K 10% resistor using hobby enamel paints.  I collect old style dogbone resistors just for this purpose.  Many Zenith radios, even post war models, used old style dogbone resistors.  So this type of resistor would not be out of place in 1942.

Tubes

The bad 6X5GT/G was replaced by a 6X5G, which originally would have been used (the tube socket is so labeled). The weak 7Q7 and 6SQ7GT tubes were replaced. The 7A7 (slightly weak) and 6K6G (good) were left in place. Unfortunately I could not find Zenith branded tubes. However, the original bad Zenith OEM tubes will be kept with the radio and not discarded.

Other Repairs

The line cord was replaced using a new brown vinyl cord and plug. The dial drive cord was replaced using a similar type material. This was a difficult procedure since the cord goes through two holes in the chassis and twice around the tuning knob shaft, access to which is partially blocked by band switch and RF wiring. The cord was replaced before the dial assembly was reinstalled. The original spring was re-used.

Testing and Alignment

Once the radio had been reassembled, the radio was powered up using a fused Variac.  The radio came alive and actually worked on all bands. The set was then aligned.  I did not touch the short wave oscillator trimmers - only the antenna coil trimmers (if used).  The short wave bands worked very well as is, and my signal generator does not go higher than 10mHz. The radio is quite sensitive and has very good tone.  The only tone control is a treble cut switch on the rear chassis.

Cabinet

The cabinet was vacuumed then carefully cleaned using GoJo (white) and cleaner and 000 steel wool. The finish was very fragile and had heavy wear and finish loss. I then went over the cabinet using Howard's followed by a coat of Johnsons Wax.

Restoration Results

Chassis Before Restoration

Chassis After Restoration