1938 Zenith 9-S-262 AM/SW Robot Dial Console
I've had this set hanging around for about six or seven years, and I think it came from a junk store over in Virginia. It's a motorized robot dial set so I definitely want to restore it.
The biggest issues with it are the damaged finish on top of the cabinet and its missing the original speaker. I looked for an original speaker online but saw some really stupid prices, so I'm going to use a 14" Zenith speaker that came from a later model console, probably a 10S599 or 10S690. I just need to change the output transformer to make it work, and I already have the transformer on-hand, so no extra expense involved in that. |
The original grill cloth is Zenith's wave or swirl pattern. The exposed parts are a brown-gold color, but in areas where the wood protected the cloth from exposure it's actually a little on the greenish-gold side. It's still in decent shape I guess considering its age, but its dirty and unfortunately has holes in it caused by a couple of the speaker mounting screws.
I ordered a replacement cloth from Richmond Designs. Their wave pattern is very similar to the original, but the colors are much more brown-toned and lack the gold tint of the original. It would look nice on the set, but I kept looking around and found another wave pattern cloth at Radiogrillecloth.com. They call it a swirl pattern and it appears to be a better color match, so I ordered it, too. I'll use whichever is closest to the original. I'll also save the old cloth. I might be able to use some of it on some other set. |
The top of the chassis is very dirty and there are several rusty spots probably from mouse pee. The three-section tuning capacitor also has a good bit of rust on its frame.
The original 6T5 eye tube has been replaced with a 6U5, but its screen is very dark, indicating it's probably bad too. I also tested the power transformer with the tubes pulled and all the voltages look good, so I'm not seeing any major issues with the chassis. I haven't tested the tubes yet, but when I get to them I'll try to make up a full set of shouldered "G" style tubes from my tube stock. |
Under the chassis is still mostly original, but it has been serviced in the past. I see that two waxed paper and foil capacitors were replaced, and most of the Candohm resistor sections are open and have been bypassed with resistors. One section of the Candohm going to ground is also open but not yet bypassed. There is even a service sticker on the rear panel for the Radio Service Station, 124 Broad Street, Milford, Conn.
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Maybe I shouldn't plug this one in for a quick test. The dry rotted cloth-covered power cord is not original. The original cord was rubber-coated and someone cut it off and pigtailed the cloth one onto it. They didn't even solder it, just twisted the wires together and taped it up with bandage tape and old-style friction tape.
Since this is a robot dial chassis I'll go the extra step of restuffing the paper capacitors for it, and I also found original replacements for the two non-original caps. I'll restuff the metal filter can containing three of the power supply electrolytic capacitors, but the single capacitor can I'm just bypassing under the chassis. I'll leave the old can installed for originality.
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Someone has tried to get the radio working before I got it. The 16uFd electrolytic capacitor (listed as 12uFd on the schematic) was cut out of the circuit, and I discovered that the 15KΩ 1-watt resistor on the schematic that provides power to the 6J5 oscillator was missing, plus the 10KΩ 1-watt resistor in series with it had drifted high. As it happened, the 10KΩ had drifted to almost exactly 15KΩ, so I removed it, repainted it as a 15KΩ, and soldered it back in place of the missing resistor. Then I installed a new 10KΩ power resistor. In the photo I've also Installed the replacement for the 16uFd electrolytic.
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This is the 14" replacement speaker and is marked as part number 49H442. Its original output transformer was for push-pull 6V6 tubes, but I replaced it with a single-ended output transformer for a 6F6.
Another issue is the field coil is only approximately 600Ω so I added a 400Ω power resistor under the chassis in series with the coil to match the 1000Ω resistance of the original speaker's coil. |
The chassis is finished and I'm just waiting on some tubes. One of the two 6K7 tubes, plus the 6L7, are very weak. I found a good 6K7G in my stock but I had to order a 6L7G. The 6F5 and 6J5 tubes test good but are metal tubes so I will try to find some glass tubes to replace them, too.
I found my stash of NOS drive belts and replaced both broken ones. I thought the motor drive belt might be a little loose, but turns out it works fine. Now I'll start cleaning up the cabinet. |
I think the original finish is still in decent enough shape to save, with the worst areas being on top where it apparently has some water damage, maybe from a flower pot sitting on it. I managed to remove the milky white water spots by wiping the affected areas with alcohol, letting them dry, and repeating multiple times. The spots eventually faded. But part of the spot closest to the middle had no finish left and needed some extra work. The top's lacquer was also very dark and/or dirty so that needed some attention, too. The final staining and lacquering has not been done yet, still just working on bad spots.
The molding around the bottom is a bit beat up as well, and there are some scratches in the finish around the cabinet that will get worked on next.
The molding around the bottom is a bit beat up as well, and there are some scratches in the finish around the cabinet that will get worked on next.
Most of the parts replaced are shown. Missing are the 4 electrolytic capacitors, the Candohm resistor, the eye tube, and the power cord. And of course there's the replacement speaker, the audio output transformer, and the speaker cloth. I also added a fuse holder and fuse to the AC line. The pile includes 13 resistors, 16 capacitors, hardened grommets plus deteriorated gum rubber chassis and speaker mounts, rusty hardware and 2 of the 3 weak tubes.
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The three dial faces are all in very good condition. I only had to do some minor touchups, and then each face was waxed with an automotive paste wax. This deepened the black and really made the faces shine. It actually made it harder to take photos without numerous reflections in the glass and dials.
The replacement tubes arrived and the radio was powered up using a variac and dim bulb tester. The power supply and audio amp section are both working well, voltage measurements look good, and nothing is overheating The audio output (using an injected signal) is very loud and undistorted, but I'm not receiving any stations or hearing much static. This may just mean the radio alignment is way out, so a complete alignment is the next step.
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A look at the underside of the finished chassis. The tuning of both the RF and IF sections was indeed off, and an alignment brought the radio back to life. I also found two issues, first was the eye tube is very dim. I already figured this would be the case and have a new one ready to install.
Second, the local/distant switch didn't work, but it turned out to be an easy fix. One side of the switch is grounded to the chassis through its threaded collar and nut, but it wasn't making good contact because of the painted chassis. I added a toothed lock washer under the nut to fix it. |
The Candohm in the 9-S-262 is a five section wirewound chassis mount resistor strip. In this radio almost every section either measured as a high impedance or completely open. Even the section that still measured near its original value drifted around a bit. Most of the sections are not really open, they just have poor connectivity between the nichrome resistance wire and the connection tabs. It would be a mistake to just parallel new resistors between the tabs, so I mounted turret terminals on each tab except the ground.
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Circuit-wise, the Candohm is divided into two sections. The two 11KΩ resistors form a voltage divider across the B+ to ground. The other three resistors on the opposite end form a negative bias divider circuit between the high voltage winding's center tap to ground as shown in the drawing.
Also shown are voltage drops, current flow, and power dissipation of each resistor as it applies to this particular radio, which is not 100% stock, because of the replacement speaker. There may also be very small errors due to fluctuations and rounding off. |
The voltages were measured, but the wattages and amperages were calculated. The actual wattages of the resistors chosen are multiple times the calculated values. For example, the first 11KΩ resistor needs to safely dissipate 2-watts, so I installed a 7-watt resistor to keep its surface temperature from approaching that of the sun. A ten-watter would have been even better, but I only had a seven on hand.
After some more testing and just listening to the radio, I discovered two more issues. First, I noticed the broadcast band was still not quite right. I have a great deal of AM interference in the shop which was masking a problem. The second issue was touching the chassis was causing popping and cracking noises, and even some howling. Something's loose somewhere.
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The solution to the popping, cracking and and howling was actually really simple. The tube shields twist lock on the tube bases, but they were all quite loose. I slightly bent out the tabs around the bases, so the shields fit snugly over them, and it instantly solved the problem. It seems the shields were only intermittently grounded which was the root cause of the noise.
As to the other problem, after fixing the local/distant switch I can hear static on the broadcast band, but the local AM station is very low and I actually get a better signal just by putting my finger on the grid cap of the 6L7. So something is still not right with the RF amp.
As to the other problem, after fixing the local/distant switch I can hear static on the broadcast band, but the local AM station is very low and I actually get a better signal just by putting my finger on the grid cap of the 6L7. So something is still not right with the RF amp.
After a little more troubleshooting I see there's no B+ on the plate of the 6K7 RF amp when the band switch is on the broadcast band (also the second shortwave band). After looking over the schematic it appears the RF choke (part no. 14) must be open. Both its coils are in series with the B+ on the the AM band, and the smaller coil is inline on the second shortwave band (1.8Mc to 5.5Mc). The entire choke is bypassed on the first shortwave band (5.6Mc to 18Mc) so that band is working fine.
The choke is a wooden core with two coil bobbins and it's screwed to a phenolic base which has the connection terminals. If you loosen the screw the core with the bobbins can be rotated. And that's exactly what someone has done in the past, ripping the fine connecting wires right off the coils. I spent a couple of hours performing "surgery" and managed to find and reconnect to all four broken leads. Now all the bands are finally working properly. |
The chassis is finished. The radio delivers good, clean audio output, and thanks to its RF amp is very sensitive on all three bands. The geared vernier makes fine tuning a station a breeze, and the electric motor drive gets from one end of the band to the other in a jiffy. All that's left now is finishing up the cabinet and putting it all back together.
The cabinet is coming along, but still needs a little more work. There's a couple of missing braces that need replaced, and I need to rub down the cabinet and give it a coat of wax. The cabinet is not perfect, but I don't think it would look right if it were. I want the radio to still have a bit of character, it is over 80 years old after all. |
I've done about as much as I can with the finish, so I reinstalled the radio back in the cabinet. I decided to use the grill cloth from radiogrillcloth.com because I liked the colors better. As I mentioned earlier, I left some imperfections in the finish so it doesn't look "too good" for its age, and I'm happy with the way it turned out. It's going beside my rocking chair in the lab, so that it's within easy reach. It taking the place of my 1939 Zenith 9-S-367 Robot Dial console, which now needs a restoration of its own.
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Page created 5/7/2024
Last updated 7/14/2024
Last updated 7/14/2024