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The Zenith 6D2615 from 1942 is a 6-tube AC/DC Superhet radio that receives only the broadcast band. The radio had been serviced in the past but most of the original parts were still in place. I decided to try and reverse all prior servicing and restore the original chassis appearance if possible. The schematic and a parts list for the radio can be found on Nostalgia Air. Any part number references in the text below reference that schematic. |
My antique radio restoration logs
The radio was purchased on eBay. Externally it appeared to be all original, complete and in good condition. There was a 1/2" hole in the grille cloth. The knobs were slightly warped (typical for this radio). The back cover was present but not firmly attached to the cabinet. The radio features a tuned RF amplifier stage (3-gang tuning capacitor) so it should perform well. It also features a cut down version (4 buttons) of the Zenith Radiorgan tone control system. The circuit is quite complex, with a high parts count. The chassis is small, and access to parts is limited and difficult. Zenith radios of this vintage use rubber covered wiring which by now has deteriorated and the insulation will fall off if the wiring is disturbed.
I always attempt to avoid purchasing radios that have been "restored" by collectors or flippers, and am looking for either all original examples or those which have been "lightly serviced" in the distant past by radio service shops. Once I began removing capacitors for restuffing, it was discovered that many parts had one lead cut near a terminal then soldered back to the terminal! It is as if someone had disconnected one lead of many capacitors for the purpose of testing the part, then reconnecting the lead. In most such cases, the stub of the cut component lead was still attached to the terminal and the original lead reattached with a solder blob without securing the lead to the terminal.
One of the pilot lamps had been replaced. The originals were the unusual type 1490, which are 3.2 volts at 160ma. One lamp had been replaced with a #47, which did not work well (the original lamp was quite dim and only one side of the dial was lighted).
Two paper-wax capacitors had been replaced: C3 and C12. The filter capacitor was original.
All resistors appeared to be original.
The 35Z5G and 35L6G tubes had been replaced with GT types. All of the remaining loctal tube types were branded Zenith and likely original. Date codes were W2 and Y1 (1942 and 1941?) These loctal tubes also had a "7Z" code following the date code (perhaps the tube's manufacturer for Zenith).
The two dial cords had been replaced with cord that was likely much heavier than the original cord (remnants of which were found in the eyelets on each end of the replacement cord).
The chassis was very dusty, but not rusty. The dust was blown off with an air compressor. No further cleaning was necessary.
The radio was briefly powered up through a fused variac in order to verify the eBay seller's statement of condition. I normally do not apply power until the radio has been restored. But in this case it was known that the seller had plugged in the radio. It powered up OK, and there was no reception as claimed (a lot of noise when the tuning capacitor was moved).
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.
I normally measure the values of all resistors as part of the survey process. But in the case of this radio, there are three cases where loops exist which prevent accurate measurement of all affected resistors unless the loop is broken. Otherwise all resistors in the loop will measure lower than true resistance due to the shunting effect of parallel resistors. The three loop paths are shown below (I added a suffix "a" or "b" to indicate two parts with the same value and part number which are unique parts in the radio):
R16a to R7 to R16b to R5 back to R16a. I broke the loop by disconnecting R16b (2.2 meg) from the oscillator coil left lug (with lead to B-).
R16a to R7 to R3 to R4 back to R16a (a loop within a loop!) I broke this loop by disconnecting R3 22K from the 14Q7 tube socket pin 5.
R9 to R13 to R14a to R14b to R15 back to R9. I broke this loop by disconnecting R13 2.7K from the 14A7 tube socket pin 4.
The speaker field coil and cone were OK.
The output transformer was OK
The antenna, RF and oscillator coils were OK (tested for resistance and/or continuity).
The IF transformers were OK.
The only original dogbone type resistor (R10) was out of tolerance. All the remaining resistors were carbon composition types and all were in tolerance!
All the supplied tubes were good but two were GT types vs. the original G types.
The volume control and switch were OK.
One of the dial lamps was not the correct type (#47 vs. the original #1490)
The tuning capacitor was slanted to the right and the drive pulley (which is at AVC potential) was rubbing against the speaker frame (at B- potential). The capacitor is only attached to the chassis with bent-over tabs through shoulder washers in rubber grommets, and was being pulled in that direction by the dial cord. I suspect that this was due to deterioration of the grommets and/or replacement of the dial cord which was very thick and likely too tight. The plates of the tuning capacitor were also rubbing and making contact during parts of its rotation. The scratching noises during rotation could be from the rubbing plates or from the pulley rubbing against the speaker frame.
Since almost all of the original parts were still in place I decided to try and maintain the original chassis appearance to the extent possible. All original capacitors would be rebuilt in their original cases (restuffed), including the original filter capacitor. Any parts replaced in servicing would be replaced with original parts if available. Any out of tolerance resistors would be replaced with the same types if available. 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.
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).
This is what the tuning capacitor looked like as found. It was tilted to the right and the pulley is rubbing against the speaker frame. The angle prevented full movement of the dial pointer. In addition, some of the plates are rubbing and causing shorts as the pulley is rotated. These problems were a showstopper to restoration if they could not be repaired. Removing the capacitor was a complex and difficult task, involving first removing the dial cord (the stringing is quite complex) and dial assembly.
The capacitor was not attached to the chassis with the usual bolts and nuts. There were three studs welded to the capacitor frame which ended in tabs which were passed through a flat washer, the chassis hole grommet, and a shoulder washer on the other side. These tabs were then simply bent over flat, and in one case, soldered together. In order to gain access to these tabs, several parts had to be removed (paper capacitors). The oscillator coil had to be moved out of the way to gain access to the tabs for unsoldering and straightening and to prevent damage to the coil. This coil is riveted to the chassis, but the coil form can be gently pulled off its base and moved out of the way - only one part had to be disconnected (a 220 ohm resistor). One tuning capacitor tab had several components attached. In addition, wires from the capacitor's stator sections had to be disconnected from the oscillator and RF coils and pulled back through the chassis.
Once the capacitor was removed, it was discovered that many plates were touching as it was rotated, and there was a relatively low resistance from each stator to the frame. I first thought this was due to dirt or debris. The capacitor was first cleaned in my Heathkit ultrasonic cleaner and dried using a heat gun. This cured most of the problems, including the leakage from the stators to the frame, but two sections of the capacitor still had shorts when rotated (measured by brief flickers on my DVM).
In order to locate the shorts I connected a 9 volt battery and a pair or antique radio earphones in series between each stator section and the frame. This allowed me to locate where in the rotation the plates were touching. Several plates were bent where touching in order to eliminate the shorts. I was eventually able to eliminate all shorts, but noticed that one of the stator sections was not firmly attached to the capacitor frame. And if moved slightly, shorts would result. There was not much I could do about this short of replacing the capacitor! And that would mean finding a parts chassis - no one would go to the trouble of parting out this radio! I will setup a search in eBay for a junker set using this same chassis. I may get lucky at some time in the future. There was a Zenith 6R631 radio for sale on eBay (same style boomerang dial) for which the seller had posted a Youtube video of the radio in operation. It had the same problem as mine - noise and scratching when the tuner was rotated. It very likely had the same problem as mine! I wound up buying this radio also, and sure enough, its tuning capacitor was also slanted to the right and the drive pulley was rubbing against the speaker frame, although in this case the dial cord was original. The plates of the tuning capacitor used in these radio are VERY close together - the three gang capacitor is very compact, and is the same size as a typical two-gang capacitor. So all radios in this family may have these problems.
The capacitor was then reinstalled using new 5/16" vinyl grommets. All the parts that were moved or removed were then reinstalled (the paper capacitors removed were restuffed with new axial film capacitors before they were reinstalled). The dial cord was replaced with thinner cord, and I made sure that excessive tension was not used for the cord driving the dial pointer. Excessive tension in this cord likely caused the tuning capacitor to tilt right as found.
The radio used one older style "dogbone" type resistor as well as carbon composition resistors. This "dogbone" resistor (R10) was out of tolerance and would be replaced with the same type resistor. I keep a stock of NOS and used "dogbone" resistors, and buy all I can on eBay and at radio swap meets (when reasonably priced)! Of course, most of these resistors, even NOS resistors, have also drifted in value and no longer have their marked values. My solution is to find a replacement resistor of the correct value and size as measured (ignoring the markings), and then repaint it to the needed value codes using enamel hobby paint! In the case of this radio R10 was replaced with a 250K 1/4 watt "dogbone" resistor that now measured 442K. It was repainted as a 470K resistor using hobby enamel paint. All the carbon composition resistors were within tolerance.
The filter capacitor was a dual 20mfd @ 150 volt twist lock type with a cardboard cover, since it is connected to one side of the power line (the chassis itself is isolated from the power line). It was rebuilt using two 22mfd 160 volt capacitors. After removing the capacitor from the chassis, the cardboard cover was removed. It pulled right off without too much difficulty. In some cases I have had to apply heat to the top of the cover using a heat gun, which melts the tar that secures the cover. The tar was then removed from the aluminum can using lacquer thinner, and from the cover mechanically (screwdriver). The crimp around the base of the unit was uncrimped using an old pair of diagonal cutters. The ground terminal ring was then removed. The insulator with two positive terminals was then removed by prying. In this case, the contents were removed along with the terminal board as a unit. In some cases I have had to clip the connections between the back of the terminal board and the contents and then remove the contents separately. In some cases, heat will release the contents from the can. In other cases I have had to remove the contents using a spade bit in my drill press and other tools!
The aluminum case was then cleaned. Small holes were drilled in the terminal board close to the two positive lugs and near one of the ground lugs (on the metal ring). I use small numbered drills that are only slightly larger than the lead wire that passes through the terminal board. The two 22mfd 160 volt capacitors were then mounted to the back of the terminal board. In this case I was able to use normal axial type capacitors as there was room inside the can (in some cases radial type capacitors must be used). Their positive leads were passed through the terminal board and connected to the two positive lugs. A common lead was passed through the terminal board near a ground lug but not attached until after the capacitor was re-mounted in the chassis and the mounting lugs twisted. The terminal board and ground lug ring were then reinstalled (there is a tab on the ground ring which lines up with a slot in the terminal board). The crimp around the base was restored using a small tack hammer. I did my best to smooth things out, but the base is hidden by the cardboard cover. The cover was reinstalled, but not secured with tar or glue for ease of future servicing.
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All except two of the original Zenith branded paper capacitors were rebuilt in their original cases using modern 630 volt axial film capacitors in order to maintain the original under-chassis appearance. Zenith tubular capacitors are easy to restuff. My restuffing process for these types is documented here. Two original Zenith capacitors had been replaced: C3 and C12. These were replaced using the correct original Zenith duds from my stocks, which were also restuffed. C2 (22-429AM) and C4 (22-1017AM) could not be restuffed - they were solid wax with only a paper cover, built like Solar Seald-Tite capacitors. They were replaced by Zenith 22-429E and 22-1017F from my dud stocks which did have the usual hollow cardboard tubes and wax sealed ends, and thus could be restuffed. The suffix AM, E and F apparently indicate the manufacturer of the part.
The original rubber power cord was frayed and bare wire was exposed near the fiber chassis entrance strain relief (electrical tape had been used to prevent shorts). The original cord was simply shortened and re-used, even though quite stiff near the chassis (perhaps due to chassis heat?)
The one incorrect pilot lamp bulb was replaced with the correct type (1490) which are available at 1000bulbs.com and other vendors.
The GT type tubes were replaced with the correct types: 35Z5G and 35L6G (both were Zenith branded).
The cabinet was in good shape, but was very dirty and the top had some sort of nasty film on it (tobacco smoke or grease?} It was cleaned with GoJo hand cleaner and 00 steel wool, followed by a coat of Johnson's Paste Wax. The film was very difficult to remove, but the original finish looked great after cleaning. The grille cloth was left as is. The knobs were cleaned in my old Heathkit ultrasonic cleaner followed by soap, water, and an old toothbrush.
After the radio was completely reassembled, power was applied through a wattmeter and fused Variac. Power was brought up slowly while monitoring the B+ voltage and the wattmeter. The radio came alive and worked - no assembly errors! The radio was then aligned. The IF transformers were very close to correct. The other trimmers were far off, but these had been disturbed during the tuning capacitor cleaning. The volume control was slightly scratchy, but responded to a spray of GC Big Bath. The tuning capacitor turned out perfect - no plates were rubbing, and no noise. The radio was quite sensitive. The tone controls worked well. However, there was some speaker rattle and distortion or muddiness on low bass on certain stations. I have had this same problem on other Zenith radios. Perhaps there was less bass when these radios were designed? There was no obvious problem with the speaker cone. It just appeared to be excessive cone excursion. The solution is to simply depress the BASS tone button or to reduce the volume when this happens. Some collectors have modified the tone control circuit to reduce this tendency.
Most of my restoration objectives were met, but not all. There was no intention of restoring the set to factory new appearance! My objective is usually to reverse any prior servicing and make the radio appear to have never been repaired. I do not go so far as to artificially "age" solder joints, as do some collectors! Nothing gives away a restoration faster than bright and shiny solder joints. Here are some of my "misses":
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