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The Zenith model 7S433 (7-S-433) "Zephyr" from 1940 is a small tabletop
7-tube AC superhet circuit radio with a unique cabinet design based on the
railroad train with the same name.
It receives the standard broadcast band and two short wave bands, has
"automatic" or push-button tuning, and the Zenith Radiorgan
tone control (but has only two controls rather than the usual 5 or 6) The radio had seen extensive servicing and "electrical restoration" in the past - likely by a collector - and very sloppily. I decided to try and restore the original top and bottom chassis appearance if possible and to reverse previous repairs and restorations to the extent possible. The schematic for the Zenith 7S433 Chassis 5721 can be found on Nostalgia Air. Any part numbers mentioned will refer to numbers on that schematic. |
My antique radio restoration logs
There are two different chassis used in model 7S433: chassis 5721 and 5724. My example used chassis 5721. Chassis 5721 uses a voltage doubler rectifier circuit with two 25Z6G dual-diode tubes and a 25AC5 direct drive triode audio power amplifier. Chassis 5724 uses a power transformer with a single 6X5G rectifier. In order to maintain 7 tubes, and thus the 7S433 model number, Zenith replaced the 6Q7G triode-duodiode with a 6J5G detector (diode) and 6F5G audio amplifier and added an RF amplifier stage. I would guess that the 5724 chassis would have much better performance, although there are still only two tuned circuits used (the converter input is untuned on the 5724 chassis).
The radio worked when received, although the seller stated that it should be serviced before use. This is a very difficult radio to service. The chassis is very compact, and the automatic tuning unit is under the chassis. Some parts cannot be accessed without partial disassembly.
The radio had been partially recapped, likely by a radio collector. However, capacitors that would have been difficult to replace due to their location were NOT replaced.
The voltage doubler input capacitor C19 had been replaced, but the main filter capacitor C20/21/22 was original.
A new AC line cord and plug had been installed
All the tubes were replacements, except for the Zenith 6AF5G, which was good.
The shield base for the 6K7G IF amplifier tube had been wrecked. Originally the shield base would have only accepted G type tubes, and metal or GT types could not have been used. So likely sometime in the past a metal or GT type tube would have been used with the existing shield (since Zenith usually did not ground pin 1 of the tube - again to prevent use of metal or GT type tubes). A 6K7G tube was installed when I received the radio.
The volume control circuit had been modified. The capacitor C12 which should feed the hot end of the volume control was instead connected to the CENTER wiper terminal, along with the capacitor feeding the first audio amplifier. A 1 meg dogbone type resistor was connected from the tone control tap on the volume control to the hot end - nothing else was connected at that lug. I also noticed that the lug for the hot end of the volume control was mostly missing (broken). I am really not sure why this modification was done. Since a dogbone type resistor was used, I suspect this was not a recent modification. The volume control was good (but needed cleaning).
All original chassis bolts and rear cover finishing washers and screws were either missing or replaced with generic hardware.
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 automatic tuning unit (push button assembly) was removed in order to gain access to the other components. Fortunately, only two wires had to be disconnected. I found:
All tubes and shields were removed. The automatic tuning unit was removed. The dial scale and pointer were removed (the remaining dial cord was taped to the pulley for ease of reassembly - the dial cord would be VERY difficult to replace in this radio - access to the dial drive is blocked by a coil). All non-original parts and both filter capacitors were then removed. Since 10 original Zenith capacitors had already been replaced, I did not pay much attention to the placement of those replacement capacitors, except to where they were connected. The volume control was removed for cleaning and to check out the broken lug. The oscillator trimmer capacitors and oscillator coil were moved out of the way for access. They do not have to be disconnected, but the ground buss to the trimmers and a couple of leads from the band switch to the Wavemagnet socket on the rear chassis have to be disconnected. I then took photos of the chassis bottom so that routing of wiring and component placement can be restored. Lead dress is often critical in radios. 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. Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component. At this point, the chassis appeared as follows:
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The volume control switch was flooded with Big Bath cleaner and cycled many times. The switch eventually worked. The automatic tuning contact springs were cleaned with lacquer thinner on a pipe cleaner followed by DeOxit and then more lacquer thinner and a pipe cleaner.
The broken Radiorgan tone switch pull was replaced using one from a donor switch assembly in my parts bin. The original phenolic link was first fully depressed and then cut about 3/32" from the switch frame using a hobby razor saw. The phenolic link on the replacement was then cut at the same point, but leaving it a little long! The two parts were then filed smooth and measured to make sure the switch pull projected the correct distance from the switch body when pressed in. The two parts were first joined using super glue, and then epoxy.
The top of the chassis was cleaned with GoJo hand cleaner and 00 steel wool.
All original Zenith paper capacitors remaining 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 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 types) wax/paper capacitors for use when the originals are missing. Zenith schematics in Riders Manuals indicate the Zenith part numbers of the capacitors used. I was able to find the correct part numbers for most of the missing original Zenith capacitors in my stocks. In some cases I did not have the correct part number dud in stock, but did have a Zenith part with the same value and voltage rating. In a couple of cases, I was forced to use a part with a different voltage rating. For example, the original might have been a .01mfd at 600 volts, which was replaced by a .01mfd at 400 volts. In all cases, these capacitors were restuffed with modern 630 volt film capacitors and resealed.
One original Zenith filter capacitor C19 had been replaced - and the replacement did not have a cardboard cover. I found a dud FP type electrolytic capacitor in my junk capacitor box which had the correct diameter and length and had a cardboard cover. This capacitor was opened, its contents removed, and was rebuilt using a modern 33mfd/160 volt electrolytic cap. The dud FP capacitor I used had 2 lugs. One was removed. I did NOT attempt to modify the label on the cardboard cover. The original main filter capacitor C20/C21/C22 was 30/20/10mfd at 350/250/250 volts. It was rebuilt as 33/22/10mfd at 350/250/250 volts using new modern radial lead electrolytics.
The two dogbone resistors that were out of tolerance were replaced by a NOS dogbone resistor (that was in tolerance) and another that had drifted to near the correct value. That one was repainted using hobby paint to the correct color codes. While this resistor may continue to drift, so will the others in the set. I wished to maintain the original above and below chassis appearance.
Correct G type tubes were installed. I had a new Zenith 25AC5 in stock, still in its original Zenith container. I also had one Zenith 25Z6G in stock. A second 25Z6G was found on eBay. I had a good 6Q7G and 6A8G in stock. I attempted to straighten the damaged 6K7G shield base as good as possible. In any case it is hidden by the tube shield.
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 - no surprises. The radio is difficult to align because the loop antenna and speaker must be connected while it is aligned. One trimmer is under the chassis! The IF transformers were way off - set to about 470Kc. The broadcast band peaked up nicely. The push buttons were adjusted to local stations.
The radio performs well, and has very good tone on the broadcast band. The short wave performance was disappointing. However, this is really just a 5 tube radio with no RF amplifier stage.
In the BEFORE chassis photo, the push-button tuning unit and tuning capacitor have been removed for access.
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Chassis Before Restoration |
Chassis After Restoration |
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