|The USA stock market crash of 1929 and the following Great Depression
resulted in a market mismatch with Zenith's early 1930's line of expensive radios.
So Zenith developed a low cost product line in order
to better match market needs. In order to distinguish those radios from their
high-end line, they branded them "Zenette" rather than
Zenith. Although supposedly a lower-end radio, the build quality of my
radio is very high.
The circuit is an 8-tube superheterodyne that receives only the broadcast band. It features a tuned RF amplifier and separate oscillator and mixer stages and a separate AVC tube (unusual for early 1930's sets). It uses 2.5 volt tubes, typical of the early 1930's.
The radio had seen some servicing in the past but had not been hacked excessively. This being the case, I decided to try and retain the original top and bottom chassis appearance if possible.
The schematic and a partial parts list for the radio can be found on Nostalgia Air.
My antique radio restoration logs
I had previously restored a Zenith Model 216-2 Zenette (the "-2" indicates a 25hz model). The model 230 Zenette is quite similar, but has 8 tubes and a more complex circuit (the RF, First Detector, Oscillator, and IF amplifier stages are very similar). The 216-2 uses a conventional type 55 duplex-diode/triode as the second detector, AVC, and first audio amplifier. The 230 uses a type 57 pentode as an anode bend or biased type detector/amplifier, with a separate type 57 AVC amplifier tube. Due to the need for additional voltages for the AVC tube, the 230 has TWO wirewound metal voltage divider resistors (similar to the Muter Candohm type). The 230 power transformer also has two separate filament windings: one for the RF and IF tubes, and a separate supply for the AVC tube, detector, and audio power amplifier tubes. The center taps of these windings go to different reference points. The RF/IF center tap reference is to the chassis ground. The AVC/detector/audio reference is to their cathode potential (at C- potential). Perhaps this was to reduce hum?
I noticed little or no difference in performance between the 7-tube 216-2 and 8-tube 230 for this all this additional complexity.
The radio was purchased at a Southeast antique radio swap meet rather than being shipped from somewhere. The only clues I found were service tags glued to several original Zenith tubes. Those tubes were tested and noted as "fair" on August 8, 1935 and again on January 16, 1941. The service tag read:
Replace with TUNG-SOL radio tubes
Ben's Radio Service
Dial 4-0028 Free Inspection
721 S. Union
Des Moines, Iowa
So apparently the original owner lived in or near Des Moines, Iowa and twice took advantage of a local radio shop's offer to inspect the radio and test the tubes FREE (when I checked the tubes, they checked GOOD!)
The original filter capacitor block had been replaced (fortunately the metal cover remained).
One of the original Dubilier Cub paper-wax capacitors and one dogbone type resistor had been replaced with more modern components (not recently - an older repair).
The volume control and switch had been replaced (the original was an Allen-Bradley BradleyStat type, in which the switch often fails).
The original type 280 rectifier had been replaced by a type 80 (not Zenith).
All the remaining tubes were Zenith branded and likely ORIGINALS (all had similar engraved bases, for example "Zenith type Z-58"). The type 56 was a globe type.
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 and cannot be restored or replaced, I may elect to sell the radio rather than restore it. I assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original components if possible). And most original resistors would have drifted and be out of tolerance range.
The chassis was VERY dirty, and there was some surface rust. There was considerable corrosion and pitting of the aluminum shield cans. The power transformer covers and laminations were very rusty.
The dial drive cable was missing (a bronze wire type).
Two of the included knobs were incorrect.
The speaker cone was damaged.
The cabinet was in poor shape, with some loose joints and missing veneer strips on the back of the arch.
The grille cloth had been replaced. The grille cloth was glued to a solid piece of cardboard - no hole for the speaker! My guess is that the seller did this so that the speaker cone damage would not be noticed (light shining through the holes!).
The type 59 output tube was missing.
The power transformer was OK. It drew less than 10 watts unloaded, and all voltages were correct (the high voltage was balanced on each side of the center tap).
The speaker field and output transformer were OK.
All RF coils were OK.
The input IF transformer had an open primary. The output IF transformer was OK.
The original power cord was not safe and had several breaks. The plug was OK, and an older style, but perhaps not original.
The speaker cable also had several breaks and the cloth covering was worn away in spots. It appeared to be original, but I noticed it had SIX leads (only 4 were used).
The tone and sensitivity controls were intermittent and needed cleaning - they were BradleyStats, which are difficult to clean.
7 original dogbone type resistors were out of tolerance by 30-100%.
The 6-tap metal clad lower voltage divider resistor (bias divider) had one section that was 40% high.
The 5-tap metal clad upper voltage divider resistor (B+ plate and screen divider) changed values when the terminals were moved.
When the capacitor 22-175 (0.002mfd/600 volts) between the plate and cathode of the AVC tube was removed, it was noted that a type 22-115 (.1mfd/200 volts) was installed. This was perhaps an error during assembly - mistaking 22-115 for 22-175. This capacitor was restuffed with a 0.0022mfd capacitor.
I normally clean the chassis before starting restoration. I first blew off the above and below chassis dust with an air compressor. The chassis was then partially disassembled for access and cleaning. The tuning capacitor and dial drive mechanism was removed as a unit after unsoldering the leads and ground braids. I had to remove the mixer coil shield cover in order to gain access to the screws and spacers that held tuning capacitor shield to chassis. The filter capacitor block cover was removed. The volume and tone controls and their bracket were then removed after disconnecting all wiring from the controls. It was very difficult to manipulate the controls and bracket out of the chassis - there was no room to remove the controls before removing the bracket, and the bracket could not be removed without first removing the controls! So I had to loosen the control nuts, leave the controls in place, and bend the bracket slightly in order to remove it.
The very rusty power transformer was removed for wire brushing and repainting. In order to gain access, I had to remove the 6-tap metal resistor (bias divider) on the side of the chassis, which was riveted to the chassis. This gave me a chance to "squeeze" the sides of the metal-clad resistor unit in my large bench vise. This resulted in all resistances being spot on and stable. Crimping the case on each side of the terminals tends to restore the pressure of the contact bands on the resistance wire. The bands will tend to loosen over time perhaps due to shrinkage of the insulation material on which the resistance wire is wound, or gradual expansion of the bands themselves. Both metal resistors were checked at 450 volts for leakage using my Sprague TO-4 capacitor tester before being reinstalled in the radio.
The chassis and top components were cleaned using GoJo, steel wool, and toothbrushes. The tuning capacitor was removed from its shield cover and cleaned with soap, water, and toothbrushes and then dried using a heat gun and lubricated. The trimmer micas were removed prior to cleaning (I count the "turns to tight" on each trimmer so that the trimmers can be returned to their original position prior to alignment. The complex brass helical ring-and-pinion gear dial drive mechanism was removed, disassembled, cleaned, and lubricated.
The missing dial drive cable was replaced with braided bronze cable. Fortunately, I had notes from my Zenith 216-2 restoration documenting how the cable was installed. I also installed a suitable spring (I have a set of dial cable springs I purchased on eBay.)
The power cord was replaced by new brown cloth covered wire and the original plug (perhaps NOT original to the radio). The speaker cable was replaced by a piece of 4-conductor brown cloth covered cable. A new rubber grommet was installed for the speaker cable.
All paper capacitors (13) were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance. The radio used Dubilier Cub type capacitors, which are difficult to restuff. My process for restuffing Cub capacitors is documented here.
All the Cub capacitors were originally marked with paint: either ORANGE (for .1mfd), YELLOW (for .05mfd), or GREEN (for .5mfd). Cleaning the cardboard case removed most of the paint. I suppose this was to simplify assembly, since the person assembling the radio would not have to read the label on the part! I did not repaint the capacitors since some of the original paint did remain.
I fabricated a replacement main filter block using two 10mfd/450 volt tubular electrolytic capacitors. The original block was two 8mfd/500 volt capacitors. There was evidence remaining that the original capacitor block had terminal lugs (there were remains of the lugs on some of the wiring). The "repairman" had simply cut the lugs from the original capacitor and removed it. He then simply soldered the leads of the replacement capacitor to the remnants of the lugs and taped over the splice. The replacement was fabricated as follows:
The other filter capacitor, rated at 2mfd, was rebuilt in its original cardboard case using a 4.7mfd/450 volt capacitor.
All original resistors more than 20% out of tolerance were replaced (there were 10). I used dogbone type resistors as were used originally. I picked out NOS and used dogbone resistors from my stock and junk box that had drifted to the correct needed resistance and then repainted them to match the original resistor's color codes. The replacements may continue to drift, as would most new carbon composition type resistors. But to me, maintaining the original look is more important than long term reliability of the radio.
|Here are some replacement dogbone resistors,
ready for installation in the radio. All have been repainted using
The item at the top is a rebuilt Dublilier Cub capacitor. The procedure for rebuilding these is documented here.
The volume, sensitivity and tone controls were of the Allen-Bradley BradleyStat type, which are difficult to repair or clean. The original volume control had been replaced. I found a used BradleyStat control in my junk box that was the correct value (1 megohm), had a switch, and had the correct shaft length. But is was a rheostat and not a potentiometer.
My process for restoring them is as follows:
While opened for cleaning, the rheostat with switch was converted to a potentiometer for the volume control by drilling a small hole in the bakelite adjacent to an in contact with the end of the resistance element. A piece of buss wire was then forced into the hole. A solder lug was attached to the back cover of the control and the wire was connected to the solder lug.
The primary of the input IF transformer was open. The secondary was OK. I removed the transformer from the radio for investigation. I was hoping that the break would be on the OUTSIDE of the coil, so its leads were disconnected and a few turns removed. I had no luck! No immediate break was found. I then attached my DVM to the outer lead and probed the coil with a sharp Exacto knife. I found continuity down about 2/3 of the way, so it appeared that the break was on the INSIDE. At this point I removed the coil assembly from the transformer (after carefully documenting where each lead went). Using a heat gun, I was able to remove the primary coil from its cardboard tube. I then started removing turns from the INSIDE of the coil. A break was found almost immediately. I then fished out another turn using my Exacto knife and continued removing turns. Another break was found. But again, no continuity! This was repeated several times with no luck. At this point I figured I would have to remove too many turns to be able to use the coil (it would likely NOT be adjustable to the IF frequency).
I replaced the cardboard coil form and its two coils using a similar assembly from a 175KC modern IF transformer. The original trimmers were 70-160 pfd, and the donor trimmers were 20-200 pfd. So hopefully I would be able to properly align the transformer (confirmed!)
The frayed grid lead was replaced while the transformer was open.
Both metal clad wirewound multi-tap resistor units were removed from the radio and their cases "squeezed" in my large bench vise (on both sides of the terminals). The result was that both units were now mechanically stable, had no leakage to ground, and all values were almost spot on! However it was later noticed that the remaining insulation material on the 5-tap unit was breaking off, likely due to heat and age. This meant that there was now only about a half millimeter gap between some of the lugs (at B+ potential) and ground! It was decided to replace the upper 5-tap voltage divider with individual resistors. The lower 6-tap divider was left in place, since it is a bias divider, has only about 70 volts across it, and if it fails would not result in catastrophic damage to the radio.
A replacement was fabricated using a piece of insulating material salvaged from an old Tektronics 'scope, solder lugs attached with 6-32 nuts and bolts, and individual wirewound resistors. The Zenith data sheet for this part listed the maximum dissipation of all resistors at 2 watts. So I used 5 watt wirewound units for most sections, and a 7-watt unit for the 3300 ohm section (based on voltages listed in the schematic, the actual dissipation of the 3300 ohm section was more like 3.5 watts!)
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. Normal B+ was reached with only 100 volts applied, and the radio worked. I assume that the high B+ was due to my using a 10mfd input filter capacitor instead of the 8mfd originally used. While the B+ was correct, many of the other voltages were NOT correct. Also, I noticed that the AVC voltage measured at the cold end of the RF and First Detector coils was POSITIVE! This told me that the AVC amplifier stage was not operating correctly. I then measured all the voltages with the following results:
|Voltage Measured||Spec||Actual||With Shunt|
|59 Output Screen||165||161.7||178|
|IF Amp Screen||95||100||108|
|Oscillator Screen||70||22 **||23.7|
|First Detector Cathode||-2||-3||-2.2|
|59 Output Cathode||-70||-92||-73|
** Likely due to a design change - several resistors were not installed and wiring slightly different from schematic.
So while the B+ voltages were close, the negative voltages were way off, even though the bias divider values agreed closely with published Zenith values. It appeared that the B- voltage was way to high. Not knowing anything else to try, I tried shunting various resistors from B- to ground (essentially in parallel with the bias divider) until the output tube cathode voltage was correct (-70 volts). That turned out to be 4000 ohms (in parallel with the 2405 ohm bias divider). All the remaining negative voltages were now close to correct. The positive voltages were slightly higher. The AVC tube was now operating correctly, delivering wide swings of negative voltages to the RF and IF stages as stations were tuned. I never figured out what was throwing the voltages off, and left the radio with the 4000 ohm 5 watt resistor installed.
The radio was then aligned. It performs very well - it has excellent sensitivity (due to the tuned RF stage) and good tone also.