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The Zenith 5-R-337 (5R337) from 1939 is a 5-tube AC superhet circuit
radio in a very small chairside cabinet with attached magazine rack. It receives the standard broadcast band
only, and has push-button automatic tuning. This chassis does not
use a Wavemagnet (loop) antenna, and requires an external antenna wire.
The set had seen extensive servicing and a VERY sloppy electrical restoration in the past. I decided to try to reverse any previous repairs and restore the original top and bottom chassis appearance to the extent possible. The schematic for the Zenith 5-R-337 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. The cabinet finish appeared at the time to be original and was in excellent condition. One plastic knob skirt was broken. When I searched for a replacement in the flea market, it was discovered that the broken knob was a reproduction. So it was replaced by a similar reproduction (they are whiter than original knobs). I noticed that the power cord had been replaced, as was the grille cloth and cardboard backer. The usual asbestos sheet (heat shield) had been replaced by a piece of aluminum foil. But there were no other outward signs of restoration, such as new wiring or replaced parts. I always attempt to avoid purchasing radios that have been restored by collectors, and am looking for either all original examples or those which have been "lightly serviced" in the distant past. No statement was made by the seller as to the radio's working or restored status. The 6X5 rectifier tube was originally missing. That was slightly worrying, since the 6X5 tube sometimes fails catastrophically taking out the power transformer.
After some research, the cabinet appears to have been refinished, although very well done (at this writing, I am still not 100% sure!). It is not shiny, and there are no sags, drips or runs in the finish. The restorer even made an attempt at reproducing the faux finish on the sides. It fooled me, but I have never seen an original example.
This radio is quite difficult to work on since the speaker is not mounted on the chassis, but rather is mounted in the chairside cabinet. The speaker lead wires are permanently attached, and no plug is used. So one of the first things I had to do is to disconnect the speaker wiring under the chassis, so as to avoid damaging the wiring, speaker cone or output transformer through handling. Alternatively, the speaker can be bolted to the chassis in the usual manner while the radio is serviced.
Once the chassis was removed from the cabinet and I had my first look at the underside, my heart sank! This was one of the worst examples of a recap (capacitor replacement) I have seen! All the seven replacement bypass and coupling capacitors were Sprague orange drops mostly just hooked to and soldered to the clipped leads of the original components - very messy! All of the original resistors looked to be in place, and none had been replaced. So whoever did this "restoration" did not bother with replacing out-of-tolerance resistors (and most WERE WAY out of tolerance!) When the remnants of the original components were later removed, it was discovered that in several cases, the same component had been replaced twice before (two component wire stubs were found on a single lug)! Two definite examples of this were the two audio coupling capacitors C9. The filter capacitors had also been replaced by modern tubular units. The originals were 8mfd. The replacements were 10mfd and 22mfd 450 volt units. Here is the chassis bottom as received:
I did a quick test of the power transformer by applying about 20 volts from a
fused variac and measuring the high voltage winding. It showed balance
across the center tap, indicating no shorted turns or other serious
problems. So the 6X5 tube was replaced, and the radio powered up slowly on a variac while
monitoring power usage with a wattmeter and B+ voltage with a DVM. The radio actually worked. But I could not live with this mess! I
had most of the original Zenith dud paper capacitors in stock needed to reverse the "restoration".
I collect Zenith and other branded wax capacitors just for this situation.
These are of course restuffed with modern components before use. However I had no clue
as to what the original filter caps looked like - they appeared to be individual
axial lead tubular electrolytics just looking at the parts list and where the
replacements were mounted. And of course, I had no clue how the original wax capacitors were placed and connected. I would have to find an
un-restored (or largely original) photo of a chassis 5528! And I have had almost NO luck finding original
Zenith dud filter capacitors, even from the good folks on the Antique Radio
Forums Classified ads (most collectors apparently just toss them away!)
I did get a few original under-chassis photos of similar radios from the good people at Radio Museum, by asking for help on their Forum. These were VERY helpful for restoring most original component placement, but these photos did not provide photos of the original filter capacitors (they had apparently been replaced). It appeared that all of the original wiring in my radio was still in place.
The power cord been replaced by a new brown vinyl cord with old style acorn plug (the original would have been a molded plug). The cord used was the thicker type - much larger than the original.
The grille cloth and cardboard backer board had been replaced.
The asbestos heat insulation had been replaced by a piece of aluminum foil.
It appeared that all knobs and push buttons were modern reproductions.
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. Since the radio actually worked, there were obviously no serious component problems. Only the resistors and tubes were tested.
Seven resistors were out of tolerance - only one was close enough to its original value to use.
There was some frayed wiring to the speaker, where the leads enter the field coil.
The 6X5G tube was missing and was replaced by a Zenith branded tube. The 6Q7G had been replaced by a 6Q7GT. The remainder of the tubes were the correct G types, and the one tube shield was present. Testing showed that the 6A8G (branded Zenith but likely not original) was weak, and the 6K7G (not original) was very slightly weak but likely usable. The 6K6G (not original) was OK. The 6A8G was replaced, and the 6Q7GT replaced by a 6Q7G.
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 of the 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 such as mica capacitors 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 tuning capacitor and automatic tuning assembly were then removed. The tuning capacitor grommets needed replacement. The cardboard cover around the automatic tuning unit was in several pieces as found. The tuning unit had to be removed to repair it. This is a very difficult and dangerous task! 7-8 wires have to be disconnected in order to remove the tuning unit, many of which go to the manual/auto switch which is part of the tuning unit. Obviously each lead needs to be identified so that it can be restored to the proper lug on the switch. In a couple of cases, the wire colors are not obvious. In these cases, the termination point of these wires was traced and noted.
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. 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 5/16" vinyl grommets.
None of the original Zenith capacitors remained. All the replacement capacitors were removed, noting where they were connected. Their original component wiring stubs were then removed from the terminals to confirm the connection points. The Radio Museum photos provided some hints as to component placement. Using the Zenith parts list, I found correct original Zenith wax capacitors to reinstall. In a couple of cases I did not have the exact Zenith part number in stock, so I used a replacement with the same value. These capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to restore 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 how I restuff Zenith tubular capacitors.
Most resistors in the set were carbon composition types, and most were 1/2 watt 20% units. A few were low-value wire wound types. Seven carbon resistors were not in tolerance and were replaced using the same type resistor, although I was forced to use 10% tolerance resistors in most cases. All the wire wound resistors were in reasonable tolerance.
Replacement filter capacitors were fabricated. I picked out two tubular electrolytics from my dud stock that were the correct length and had axial wire leads. These were cleaned out leaving the empty tube. The tubes were then restuffed using 10mfd 450 volt axial electrolytic capacitors (the originals were both 8 mfd at 350 and 450 volts). These were wrapped in strips cut from paper towels in order to center them and to prevent the contents from falling out. The ends were then sealed using rosin salvaged from servicing RCA Radiola Superhet catacombs, which melts at a low temperature. Labels were then fabricated using MS Word, containing the Zenith logo, part number, and value (plus a few other numbers I had seen on other Zenith parts). These were glued on to the restuffed tubes using service cement.
A hole in the speaker cone was patched using the black paper from an old speaker cone, plus service cement.
The speaker cable had insulation breaks near the speaker field coil shield. But the cable was usable. The cable was repaired by covering the breaks using heat shrink tubing.
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 1939 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, but used the thinner type (which are getting hard to find). I grabbed all of these I could find while at the Charlotte Radio Conference (sold by Mark Oppat).
All other wiring in the set was cloth covered and in excellent condition. For some reason, this radio did not have the usual deteriorating rubber covered wiring typical of Zeniths of this vintage.
The tuning unit cardboard cover was repaired as well as possible. As found, it was in three pieces. It is amazing that these pieces were still present! They were reassembled using pieces of thin cardboard from writing pads, plus service cement. Worn places were painted using flat black hobby paint. The piece of aluminum foil, which I assume was installed to replace the original asbestos heat shield, was removed. I do not understand why collectors remove these asbestos shields, unless of course they are damaged and friable! It is likely more dangerous to remove it than to leave it in place.
Before testing and alignment, the speaker was attached to the chassis and its leads reconnected. This would hopefully prevent damage by handling the chassis and speaker separately. This chassis was designed to be used with either the chairside model or a table top set, in which case the speaker is mounted on the chassis in the conventional manner. Once the radio was reassembled and the tubes installed, power was brought up slowly using a variac. A DVM monitored the B+. The radio came alive immediately and worked. The set was then aligned, and the automatic tuner buttons were set to local stations. The radio worked well and picked up lots of stations using a 20' piece of wire in my basement. The radio as acquired had a loop of wire strung inside the cabinet (not a loop antenna - just a length of wire). I am almost sure this wire is not original, since it was attached with new shiny staples, and the type of wire is unlike anything I have ever seen in a Zenith (it looked like old cloth covered solid telephone wire - the kind used inside the home). It was removed. The tone is nothing to write home about, since the speaker is very small. And even though the push-button tuner contacts were cleaned with DeOxit D5 and lacquer thinner, they were somewhat noisy and one was intermittent in operation.
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Chassis Before Restoration |
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Chassis After Restoration - Capacitors Restuffed |
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