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The Zenith 5-S-29 from 1936 is a 5-tube AC superhet circuit
radio in a fairly ornate tombstone (upright) cabinet. It has the smaller
"airplane" type dial with a "split second" hand which preceded
the larger black
dial sets in 1937. It receives the standard broadcast band and
two short wave bands. It features a tuned pre-selector for the
broadcast band (no RF amplifier, but has a three gang tuning capacitor).
The set had seen minimal servicing in the past. One of the filter capacitors and a couple of wax/paper capacitors had been replaced. That being the case, I decided to try to maintain the original top and bottom chassis appearance and to reverse any previous repairs to the extent possible. The schematic for the Zenith 5-S-29 can be found on Nostalgia Air. Any part numbers will refer to numbers on that schematic. |
My antique radio restoration logs
This radio was purchased at the 2012 Antique Wireless Association conference and flea market in Charlotte, NC. It appeared to be in original condition with its original knobs, grille cloth, and cabinet finish. There were signs of prior servicing: one filter capacitor having been replaced with a newer unit, metal tubes installed and two tube shields removed, line cord replaced. The cabinet had some scratches, finish loss and oxidation (foggy finish in places), but looked like it would clean up and display well and not require refinishing. There were no signs of an "electrical restoration" such as new wiring. The set was sold as not working.
One original filter capacitor C13 had been replaced.
A couple of wax/paper capacitors had been replaced (C7, C15).
All resistors appeared to be original.
It appeared that all tubes had been replaced. Metal tubes had been installed for the 6A8, 6K7 and 6F6. The original tube shields for the 6A8 and 6K7 had been removed. The 6B6 had been replaced by a 6Q7G with its original shield (an acceptable substitute). In 1936, metal tubes (RCA) would work OK in Zeniths. Later Zenith (as well as Philco) sets REQUIRED G type glass tubes used with shields, since pin 1 (the metal case of the tube) tube was deliberately not grounded (in many cases, there was no pin 1 present at the socket, preventing a service tech from grounding the pin and installing a metal tube.) FYI, Philco went even further by using shield bases with small holes which prevented using metal or even GT tubes!
The power cord been replaced by a white vinyl cord with old style rectangular plug.
The chassis washers had been removed - likely deteriorated - this meant that the control shafts and dial were not properly centered.
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.
All tubes and shields were removed. Before starting repairs, I took photos of the chassis top and bottom so that routing of wiring and component placement could be restored. Lead dress is often critical in radios.
In Zenith schematics, all resistors and capacitors having the same value have the same part number 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 chassis photo with these unique part numbers with a red felt-tip pen. I then removed all non-original capacitors, documenting their locations and connections. 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. Since this process may leave metal residue, I then went over the chassis with a vacuum cleaner followed up by a small magnet and masking tape. The tuning capacitor speed reduction mechanism was disassembled for cleaning and lubrication. The tuning capacitor was cleaned in an old Heathkit ultrasonic cleaner with dilute ammonia. After drying, the ball bearings were lubed with distributor cam lubricant. The three tuning capacitor mounting grommets were replaced using Philco GSm-Tuner grommets available from Renovated Radios, with the small portion sliced off from the bottom side grommets using a single edge razor blade (the upper grommets were left as is, since the projection sits in a small indentation in the chassis). These grommets are not exactly perfect for the application, since their center hole is slightly large, they are very slightly too thick, and not firm enough. But they will work.
The missing chassis washers were replaced using CW2 washers available from Renovated Radios. This washer left the knob shafts and dial centered properly.
The original output transformer had an open primary winding. The transformer was mounted on the speaker using simple bent-over tabs which fit into slots in the speaker frame. No bolts were used. So the replacement would have to have the same frame and tab width. Also, a bracket was spot welded to the top of the transformer which held the terminal board where the speaker cable was connected. At this point I was not sure it could be removed without mangling it. I maintain a stock of (mostly used) single-ended and push-pull output transformers. I examined my stock of single-ended transformers and found a potential replacement. It was marked "42 output transformer", which means it was originally used with a type 42 tube (standard base version of the 6F6G used in this radio). The ratio of the transformer was measured as 39.7:1. The voice coil of the speaker measured 4.7 ohms DC, or about 5 ohms impedance. The 6F6 output tube wants to see a load impedance of 7000 ohms. So the needed transformer ratio is thus 37.4 (square root of 7000 divided by 5). So the turns ratio, DC current and power ratings of the replacement transformer were acceptable. The core of the potential replacement was VERY close to the size of the original. So I was able to remove the original transformer core from its frame and substitute the core and coil of the replacement. That way the replacement could be reinstalled on the speaker and original terminal board bracket used. Other than minor differences in lead routing, the replacement is not obvious!
The four section Candohm resistor (R8) had one open section initially (the 85 ohm section), but most other sections were close to correct. I have found that these metal clad resistors often respond favorably to clamping pressure near their terminals when a section is unstable or has high resistance. I use a small C-clamp and firm pressure on the metal housing on both sides near each terminal. Thin wood or cardboard protects the chassis side of the clamp. Each terminal is simply a band around the resistance element, which is wound on an insulator. Over time, the band expands or the insulator shrinks, resulting in an unstable, high resistance, or sometimes an open section. In this case, after squeezing, all sections were almost exactly correct and were stable (resistance did not change when the terminal lug was moved)! Compressing the metal case applies additional pressure from the band onto the resistance element. There is some risk to this procedure, since the fish paper insulation may be damaged in some cases, resulting in a short. For this reason, many collectors never re-use a Candohm resistor in a restoration, even when good! Instead, they remove the resistor and install terminal strips and individual wire wound resistors. This of course greatly alters the under-chassis appearance of the radio. I would also replace a Candohm resistor, and especially if part of it was bad, if being PAID for a restoration. But this radio will be a keeper (which will rarely if ever be played).
The tone control measured open. It was removed from the radio for examination. It turned out to be the non-repairable type, consisting of a thin layer of carbon deposited on a cardboard form. A thin flexible metal plate contacted the element. The rotating element applied pressure to the plate through an insulator, so that the rotating arm did not directly contact the resistance element. The element measured open, and cleaning simply removed the carbon coating! So this control had to be replaced. It was replaced by the correct IRC part (D13-123, 50K, audio taper) called for in the IRC manual for this Zenith chassis. The D-type shaft that came with the control fit the original knob. The shaft was cut to the correct length, and the original star washer and nut was reinstalled.
All the original wax/paper capacitors were rebuilt in their original cases using modern 630 volt 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). The set uses a mixture of capacitor types including Zenith, Sprague and Cornel Dubilier. However, even though some capacitors are not branded Zenith, they all had Zenith part numbers. Here is how I restuff the Zenith and Sprague tubular capacitors. Here is how I restuff Dubilier CUB capacitors. Two original wax/paper caps (C7 and C15) had been replaced in prior servicing. I was able to find original Zenith capacitors in my parts stock which had the correct values and part numbers that matched the schematic. I collect Zenith and other branded paper capacitors for this exact situation. These replacement capacitors were also rebuilt using new 630 volt film capacitors.
One of the original filter capacitors C13 and was missing and had been replaced in prior servicing. However, I had a screw base can type capacitor in my parts stock which had the correct diameter and approximate length. It also had a threaded stud for the positive terminal, which would allow use of the existing original wiring (both original lugs were still in place - one being simply soldered to the lugs of the replacement capacitor and not cut off). One capacitor must be insulated from the chassis, and the other is grounded. I moved the original capacitor (which was originally grounded to the chassis) to the hole for the insulated capacitor (closest to the edge of the chassis) since I had the appropriate insulating washers in stock that would fit that capacitor. The replacement capacitor can was installed in the other hole. I had not insulating washers that would fit that capacitor. The insulating washers are not simple flat washers: they have a projection that fits in the chassis hole to prevent the capacitor from contacting the chassis should it move. These are generally obtained from parts chassis.
The original C13 and replacement (8 mfd, 440 volts) were both restuffed using 10mfd 450 volt capacitors. The cans were scored deeply about 1" up from their base on my Unimat lathe, and then the cut was completed using a Exacto hobby razor saw. In the case of the original capacitor, the positive electrode (foil) was removed, retaining the stud. The stud was cut off short and a hole drilled for a screw, nut, and solder lug. A plus lead of a 10mfd 450 electrolytic was connected to the solder lug. The negative lead of the replacement capacitor, extended with bare bus wire and with insulating sleeving, was routed through a small hole drilled in the base near the mounting threads. After the capacitor was mounted, the negative lead was soldered to the a replacement negative terminal (the original had been cut off). The original positive terminal was used. The replacement capacitor was a "dry" type and did not have an internal center stud. Once its contents were removed and it was cleaned up, a small hole was drilled next to the positive terminal threaded stud. The positive lead of the replacement 10mfd capacitor was extended with 22 gauge bus wire and routed through the hole. The existing original inside nut on the positive lug was removed, the bus wire wrapped around the stud, and the nut replaced. The negative lead of the replacement 10mfd capacitor was routed through a small hole drilled in the base near the edge. This lead was clamped between the capacitor body and the chassis when the capacitor was mounted. Just in case, the lead was also clamped under a nearby power transformer mounting nut.
All small resistors in the set were dogbone types, and most were 1/3 or 1/4 watt size. Four were not in tolerance and would have to be replaced. In order to maintain the under chassis appearance, I decided to use the same type resistor to replace them. But of course, any replacement dogbone resistors would also likely have drifted! I collect all the used and NOS dogbone resistors I can find, and retain any out of tolerance dogbone resistors that I remove from radios. I searched my 1/4 watt dogbone resistor stocks for resistors which had the correct lead length and value within 15% or so. I then repaint these resistors with the appropriate body-end-dot color code of the ORIGINAL resistor using enamel hobby paint.
The speaker cable had insulation breaks and had bare wire showing near the chassis. But most of the cable was usable. The cable was repaired by cutting each wire at the bare portion and installing two layers of shrink tubing on each wire, one wire at a time. The same process was used to repair the pilot lamp leads. For some reason, the original pilot lamp leads had been cut, spliced (not soldered!) and insulated with tape. Since the dial drive belt had been replaced, perhaps the front of the dial drive mechanism was removed in order to install a replacement (a few wraps of twine!) But the pilot lamp sockets could have simply been pulled off their mounting brackets in order to do this!
During testing, it was discovered that the antenna lead was shorting out to the antenna coil shield, where it passes through an eyeleted hole. It was repaired by pulling the wire back through the hole, cutting off the bare and damaged part, replacing the knot (strain relief) and reconnecting the lead to the antenna coil.
The original power cord had already been replaced, so I had no idea what the original looked like. But postings on Antique Radio Forums indicated that 1936 Zeniths had standard flat brown rubber (vinyl) cords with integral molded plugs (1935 and earlier had cloth covered cords). So I installed a standard replacement brown vinyl cord with molded plug.
All other wiring in the set was cloth covered and in excellent condition.
The band switch was given a good spraying with GC Big Bath cleaner, which leaves behind no residue. The dial drive speed reducer was disassembled and its parts cleaned in my Heathkit ultrasonic cleaner using dilute ammonia, followed by cleaning with soap, water, and toothbrushes. Gear hubs were cleaned using a pipe cleaner. The mechanism was then reassembled and oiled using Turbine oil. The dial drive belt was replaced by a 2" diameter thin (about 1/16" thick) O-ring. The mechanism then worked perfectly.
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 only problem found was that the pre-selector trimmer did not peak up properly (it was left wide open). The radio worked well and picked up lots of stations on all bands using only a 20' piece of wire in my basement. The tone was very good, so the output transformer replacement was successful.
