|The Zenith 5R216 from 1938 is a 5-tube AC Superhet. It only receives the Broadcast band. The cabinet
is solid wood - no veneer.
The schematic for the 5R216 can be found on-line at Nostalgia Air. Any references to part numbers refer to that schematic.
The radio had seen minimal servicing in the past - all of the original parts (except 1 tube and the AC line cord) were still in place. I decided to try to maintain the set in its original condition to the extent possible, yet get it working.
My antique radio restoration logs
This radio was purchased on eBay. The cabinet finish, knobs, and grille cloth were original and in very good condition. The cabinet showed signs of finish oxidation, and was dark in places. But it was more than acceptable to me, and certainly did not warrant refinishing. The cord was cut off, and the radio sold as not working (the seller powered it up with a temporary cord and said it hummed, but no reception). As found, the chassis was very dirty and dusty, but with minimal rust.
All tubes except one were the original Zenith engraved base types with date codes consistent with the radio's manufacturing date. The 5Y4G had been replaced, or at least was not Zenith branded.
The line cord had been cut off, likely because it was unsafe. And it had been replaced more than once. There was no line cord chassis grommet or bushing present.
All original wiring, resistors, and capacitors were fortunately still in place, including the antenna and ground leads with their label tags.
My usual restoration procedure is to first make a complete survey of the condition of all components. The survey results guide my restoration strategy. I never apply power to a radio before restoration, even through a "dim bulb tester" or variac "to see if it works". If major and unique components are defective or missing and cannot be restored or replaced, I may elect to sell the radio for parts 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.
Before starting repairs I made BEFORE photos of the chassis top and bottom. I use these photos to ensure that replacement parts and wiring are placed as close as possible to their original positions. Some radios are subject to problems (such as oscillation) if wiring is re-routed or lead dress is not the same as the original. Plus, I prefer to keep the under chassis appearance as original as possible.
All tubes were removed. The tuning capacitor, dial assembly, and filter capacitor were removed. The chassis was very dirty, but had no rust. It was first cleaned off with an air compressor. The top and sides of the chassis were then cleaned with GoJo hand cleaner and 00 steel wool. Small bottle brushes and a toothbrush were used in tight areas. Since the steel wool may leave small fragments, I go over the chassis afterwards with a vacuum, a small magnet, and masking tape.
The AC line cord stub was replaced with a new brown vinyl cord. A suitable rubber grommet was used as a chassis bushing. I have no clue what the original looked like. None of the wiring in the radio needed replacement, since all wiring was cloth covered vs. rubber covered.
The pilot lamps were replaced with #44 lamps. The holes in the lamp diffusers were patched using small pieces of white thin plastic from an grocery store food container and Elmer's Stiks-All glue.
The original output transformer had an open primary winding. The transformer was mounted by extensions of the frame which are passed through slits in the mounting bracket on the side of the speaker, and then folded over (no screws or rivets used). So any replacement transformer would have to be close to the same dimensions. I maintain a collection of both new and used output transformers. I also mark all these transformers with their measured turns ratio. This is measured by connecting the primary to my HP 200A audio oscillator and driving the primary at about 12 volts at 400hz. The secondary voltage is then measured, the turns ratio being approximately equal to the primary voltage divided by the secondary voltage. The impedance ratio can then be calculated by taking the square of this ratio. In this case, the output tube is a 6F6, which needs a load impedance of about 7000 ohms. The speaker voice coil measured 4.8 ohms DC, or about 5.3 ohms impedance (adding 10%, usually a good estimate). I later actually measured the voice coil impedance at 5.47 ohms at 400hz. I measure voice coil impedances by connecting the voice coil in series with a carefully measured 5 ohm resistor to my HP 200A audio oscillator and driving the series combination at about 1 volt. By measuring the voltage across the voice coil and the resistor, the voice coil impedance can be calculated using simple ratios. The needed turns ratio was thus 35.77 (Square Root of 7000/5.47).
I then started searching my output transformer stock for a single ended transformer that was the correct size and had the correct turns ratio. I found a transformer that was in good condition with long leads and was the correct size to fit on the extant speaker mounting bracket, thus effecting an almost invisible repair. But its ratio was only 27.5 vs. the needed 35.77. This meant that the 6F6 would see a load impedance of 4126 ohms. But I really did not have another choice, and there were no commercially available new Hammond transformers that were compatible size wise. So I decided to proceed and use this transformer, suspecting that I would not notice any degradation in sound (confirmed). The transformer leads were spliced to the existing speaker cable, insulated using heat shrink tubing and then covered by a wrap of friction tape.
The original power supply filter capacitor C9-C10 was removed and restuffed. It was a 8+14mfd @ 450 volt capacitor. It was restuffed using 10mfd and 15mfd @ 450 volt capacitors. It was very difficult to remove the contents from the metal can. I first tried pulling out the original capacitor by its leads - no luck. I have had some of the same types where the contents pulled without difficulty. I then tried heating the can using a heat gun to melt the glue or tar retaining it. Again, no luck - it would not budge. I finally had to resort to drilling out the contents using 3/4" and 1" spade bits - a very messy operation! I finally removed enough of the contents to allow re-stuffing, but in the process dented the metal can in several spots when cleaning out the contents using screwdrivers. I had an identical empty can in my Zenith parts stock, so I decided to use that one.
All the original Zenith paper capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance. Only 7 capacitors needed rebuilding. My re-stuffing process is as follows:
The cabinet needed a good vacuuming inside and then cleaning on the outside with GoJo and 00 steel wool. The cabinet was then waxed using Johnson's Paste Wax. No further treatment was needed. The front grille cloth was loose from the cabinet when received. It was stapled back to the cabinet. Felt bumpers were added to the base. This set uses thick cardboard chassis spacers. These were loose in the cabinet as found. They were stapled to the base of the cabinet in their original positions (as indicated by staples still present in the spacers, and tell tale holes in the cabinet bottom).
Once the radio chassis 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 powered up and the audio section worked, but there was no reception. The B+ was much lower than expected: only about 185 volts at 110 volts AC input. And the negative bias on the 6F6 output tube was very high - almost 28 volts. In addition, the 6A8 oscillator grid measured +5 volts, and should have been negative if the oscillator was running. This indicated that something was pulling down the B+ (which would also increase the negative bias). There was no short or leakage from B+ to ground. My DVM measured resistance of more than 40 megohms from B+ to ground. I noticed that the B+ started very high at power on, and then rapidly decreased. This suggested that a tube was drawing too much current. All the tubes had been tested and were good, and there were no shorts indicated. I then started pulling tubes, beginning with the 6F6 output tube. Only a slight increase in B+ was noticed. I then pulled the 6A8 converter, and the B+ jumped to 220 volts! I then retested the 6A8 on my Hickok 800 tube tester. Again it tested good, and there were no shorts present. I then tested the tube for gas - BINGO!!! It was gassy, and I then noticed that there was a blue glow inside the tube elements. I substituted a good Zenith 6A8G tube and the radio sprang to life. It was then aligned, which is a very simple procedure since this radio does not have short wave. The set is very sensitive for a 5 tube set, and the tone was OK considering it only has a 4" speaker. The replacement output transformer worked out OK.