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Depression-Era Radio: 1934 Grunow 460 Tombstone
I’ve had this Grunow 460 sitting in my queue pending for a little while and it’s finally on it’s way home looking great! There was a fun extra bit of detective work to identify and solve the issues that came up along the way.
The Grunow 460 was made by the General Household Utilities Company at the height of the Great Depression, in 1934. I follow eBay for Grunow radios specifically and saw a few of these for sale last year and have been curious what they’re like to work on. The cabinet materials do reflect being made at a time when many consumers were incredibly cost-conscious, and this was the most economical radio offering they made that year. The General Household Utilities Company did deliver an attractive design though with the contrasting color diamond in the center of the grill cloth and a light bevel around the dial face.
The design is a very simple 4-tube superhet receiver using the tubes 6A7 6F7 42 80. It’s functional, and would’ve worked well enough for daily use in cities with near-by radio stations. The 6F7 tube contains both the pentode section IF amplifier stage and a triode section Detector/1st Audio stage. There’s a lot of space under the chassis but a lot of components are stacked into one side.
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The chassis tags were in pretty good shape.
Unfortunately, one tube broke in shipping, the 80 rectifier. It turned out, with further testing, that all 3 other tubes needed to be replaced as well, so this radio got a full set of replacement tubes and should be good to go for many years.
Intake checks revealed a few problems and evidence of dubious repairs using highly variable components:
This connection to a floating ground lug wasn’t even soldered.
an IF transformer was loose
The exposed antenna coil was broken in several places and partially unwound. Fortunately, there are universal broadcast band antenna coils available, so I ordered one of those from my supplier and continued working.
I replaced the antenna coil with the universal model, attaching it to a terminal strip by a solder lug and reusing the existing screw and chassis hole:
You can clearly see where the power transformer was replaced previously in the radio’s life – the new one uses a vertical core; the stock Grunow transformers use a horizontal core and the bell housing end fits through the chassis. There’s no difference in how the styles work, just a different shape. The replacement is held on with only 2 of the screws since the base won’t line up anymore, but it’s a small transformer and the two mounts are plenty strong.
Also note there’s only one visible IF transformer can. Most radios have two (or more) of those cans which provide shielding and protection for the IF transformers. This Grunow has a single shielded IF transformer with the second IF transformer unshielded under the chassis, another way they saved costs in manufacturing and passed the savings onto the consumer.
This radio needs about a 6′ wire antenna attached to the white wire antenna terminal on the back. The radio was missing tube shields when it came to me, which were absolutely necessary for proper operation, but I was able to supply them from stock. They’re important to keep the tubes from picking up interference directly, or introducing interference to the other tubes – some generate a fair amount of noise if you leave them unshielded, and you’ll end up with a radio that squeals but can’t receive much else.
Not pictured, I also replaced the cord with a new polarized cord, the old one was cracking.
That did it! It was time for an alignment. I peaked up the IF trimmers which were off by a fair amount – the volume increased significantly after the adjustment – and fine-tuned the other adjustments. The dial tracks quite nicely within 10kc after the adjustments. I didn’t take photos of this process, unfortunately – doing an oscilloscope stage alignment on this radio would take a lot of time but not give any benefits over doing the classic signal generator/output meter checks.
At this point, I had the radio up and playing and was getting ready to send it along when it cut out. Back to troubleshooting. It turns out a section of the candohm resistor had opened. It’s likely it just failed at a terminal lug as nothing was shorting anywhere else in the radio to have caused a damaging current draw, it just died. Fortunately I had resistors on hand to replace it and mounted a terminal strip to use as new tie points.
The antenna coil is pretty cheaply made, and the solder lug was only weakly glued onto the cardboard coil form. The glue separated when being adjusted, so I was left with the last resort of wrapping it with electrical tape. It’s not readily visible and doesn’t harm the operation and is the most cost-effective fix for the problem.
This one will polish up nicely and look great on its owner’s shelf! I really like working on these Grunow radios – they have interesting cabinet designs, circuitry that usually has a couple of interesting tricks to it, and very good published schematics. I’ll be fixing up my World Cruiser when my workload dies down a bit.
Up next are two more Bose 901 Series 1 equalizers, a Philco 66, and a Silvertone 1708! Expect to see new articles more often than the last couple of months.
1937 Grunow 588 Radio Repair
I finally had a free day with no major commitments and was able to finally get some work done on my projects – it’s been a productive day.
I picked up this nice example of a Grunow 588 “Teledial” radio from the General Household Utilities Co. a few weeks ago and finally got a chance to work on it after a busy month. It’s a nice little five-tube radio that shares a lot in common with the Grunow 589s I’ve serviced in the past. It has a very nice original finish, original knobs, pointer and dial cover in good condition although covered in bits of packing material.
The Grunow 588 uses Chassis 5-W, which is also shared by Grunow 583, 585, and 586 radios. The tubes are 6A7 6D6 75 41 80. Nostalgia Air has the schematic for free from Rider’s 9-2.
The labels are in good condition for the most part:
The radio has all the tubes and shields intact. I tested the tubes and found 4/5 of them to be good, and the #75 tube had a good triode section but one marginal and one weak diode. I grabbed a NOS #75 from my stock but am keeping the used one as it probably has enough life left to be a bench sub.
The chassis is a little dirty but has been stored well for the most part, not much rust and no evidence of critters.
It’s been repaired a few times in the past. There are some ’40s, ’50s, ’60s and a ’90s capacitor present and a few different style resistors.
I tested the coils and found everything to be good, fortunately, so it was right onto replacing caps and checking resistors. Most of the resistors needed replaced as did all the caps.
I replaced all the paper and wax capacitors, and the 8×8 electrolytic with two 10uF discrete caps. I also eliminated the bias cell and associated 1M resistor and replaced them both with a single 10M resistor to provide bias. Bias cells were only needed for a couple of years, it wasn’t long before engineers figured out how to eliminate them.
Now, time to reassemble:
Back upside down for some final voltage checks from the bottom:
Now, time for alignment! For IF, I used the signal generator at 465KHz; for the oscillator adjustments I used known frequency stations at either end of the dial as it’s a bit faster that way. It was fairly out of alignment – I’d say I probably doubled the gain through the IF, and the RF was way off: 880 was coming in about 930! I was using a shorter antenna so didn’t pick much up on shortwave, but that’s to be expected.
Now, to put it all back in the cabinet!
This one was fun. These Grunow radios are easy to work on, have straightforward but well built circuit designs, and come in interesting styles. This Teledial table radio is unique for the era and it looks great on my shelf.
Feel free to write in or leave comments with questions!
What’s that piece of wire, some kind of gimmick? [Mailbag]
I was talking with a fellow antique radio hobbyist on the phone the other day about a repair he was working on. A TrueTone battery radio, he’d taken care of all the important steps – checking out the tubes, replacing capacitors, replacing the broken wires, that sort of thing. The radio worked before the service but wouldn’t play after and so we talked through the steps to see if there was anything he’d missed and get a second set of eyes on it.
This set had a lot of rubber covered wire which has degraded over time and turned into bare wire, so some of that needed to be replaced. This started to become a problem around the second half of the 1930s up through WW2 and sometimes after…you can replace the wire or unhook one end and cover it with heat shrink tubing. Everything sounded good until I heard about replacing some wires “on top of the variable capacitor.”
RF issues in these old things are insidious and tough to locate and it takes a lot less than you’d think to throw it off. Wire diameter, wire length, physical positioning and shielding are all involved to some degree. Wiring changes in the front end are the first thing to take a look at. He’d mentioned a broken wire on top that he’d replaced.
That rang a bell. My next question was, “Were they twisted around each other?” They had been. Problem identified.
This twisted-wire “fake” capacitor is called a gimmick and was a way to save a few cents on the manufacturing cost. It doesn’t take much to make a capacitor. All you need is a two conductive charged plates separated by something non-conductive; two wires twisted around each other don’t provide much but but can make a few pF. Just enough to couple a small bit of a high-frequency signal like in the RF or IF frequencies. It’s not always obvious that a bit of twisted broken wire is actually a circuit feature, though – especially if it’s in bad shape from age, so replacing it with a new piece of wire is a pretty obvious thing to do.
Unfortunately in this case, though, the circuit as connected is shorting the oscillator and antenna sections of the variable capacitor together and the radio won’t receive anything. It was a really quick fix, though. Just twist a little tighter and snip!
Doesn’t look like much but it gets the job done – the radio fired right up and received stations after the quick change.
He sent over some construction photos to show how you can make your own gimmick replacement:
Thanks Steve for the photos! I’m glad the TrueTone is back in operation.
I’m always happy to throw out some advice about antique radios and radio repair, so if anyone reading has any questions feel free to drop me a line through my About Me page, or make a comment reply and I’ll get back to you. Feel free to share photos and stories as well, I’ll post the most interesting ones on here so everyone can benefit.
1942 GE LF-116 Radio Repair Part 2 – Tool Prep
Part of a continuing series on restoring this radio:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement
Part 5 – Finished!
I’m continuing to work on my 1942 GE LF-116 radio. In part 1, I showed a few photos of it and talked briefly about the history and showed some photos of the different views of the cabinet and chassis. This radio has protrusions on all sides of the chassis – screws, metal seams, switches, an RCA jack, antenna and speaker terminals, and controls in the front – there’s no viable location to do my normal mounting trick of a C-Clamp and a small piece of plywood to form a stand, suspending the radio from each end. I searched the antique radio forums for a few ideas and came up with a simple one – a thick wood base with holes drilled and dowels inserted. The holes are drilled such that the dowel will land on an empty spot on the top side of the chassis, supporting it without damaging any components while it’s being serviced upside down.
I started with a pair of 2′ x 2′ x 3/4″ plywood sheets as the base, with them glued together it makes a 1.5″ thick mounting platform – should provide plenty of stability. This GE radio’s chassis is quite heavy.
I then clamped it to the edge of my coffee table and went to get lunch while it set.
The dowels are cut to size, 16″ each. I used a mix of 3/4″ oak (stiffer) and 5/8″ depending on where the dowel would touch the radio – if there were tight clearances. Faintly visible under the dowels are the guide marks for where to drill for this particular chassis.
I randomly drilled extra holes after the original marked ones were done just on expectation of using this with other chassis. I used an oversized base for the same reason – I might work on some bigger ones eventually. It came out okay:
And all mounted up:
Now the real work can begin. It’s not perfect, but it’ll do – the 5/8″ dowels and dowel holes fit together like I want them to – slightly snugly – but either the 3/4″ dowels or my 3/4″ bits are slightly off-sized as they fit only loosely. With 8 segments supporting it (including a few in between components where it couldn’t slip out) and the c-clamp arrangement in the back fixing its horizontal position, I’m satisfied it’s sturdy enough for light repair work as I’m doing. This won’t be a permanent solution, though. Steve Strong from the OKC antique radio club makes articulated chassis holders that mount to the existing chassis bolts and rotate 360° to provide perfect control. I’ll be picking one of those up in a few weeks, but sadly after I’m done with this.
Steve, e-mail linked above, is selling these for $57.50 + shipping and they’re custom made to order.
1942 GE LF-116 Radio Repair Part 1 – History and First Looks
Part of a continuing series:
Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuild
Part 4 – Capacitors and Socket Replacement
Part 5 – Finished!
I’ve had a GE LF-116 radio in my office for a while and now that I’ve started to clear my long backlog of projects it’s finally made it up to my bench. I’ve displayed it in my office holding up a monitor for a while but it’s made it onto my list of October projects.
It doesn’t look like much. Most radios after 1940 until this style died out seem to all look about the same to me. It has interesting enough features and is fairly rare, though, so it was worth preserving – AM Broadcast and Shortwave reception with a tuned RF amplifier, and early FM and a high-end audio output. Relatively few radios were manufactured with the original pre-war FM band, 42-50MHz and they tended to be high end models, this one no exception. With 11 tubes, including two 6V6GT tubes in push-pull output, a tuned RF amplifier on AM and high-fidelity system on FM.
The radio was designed at a time when television had just been invented and wasn’t widely adopted and before the U.S. became heavily involved in WW2 and domestic radio production was halted, and as I mentioned, at a time when FM radio was 42-50MHz instead of the current 88-108MHz. There was an exceptionally nasty series of court battles and “lobbyist activity” which ultimately resulted in the band being moved in June 1945 – and the first year major domestic mass production of radios returned was in 1946. A few niche-market or higher-end radios contained both bands for a very short time – like the Zenith 7H820 – but most stations on the old band quickly went bankrupt. and it was eliminated. As noted in the article, there were some physical constraints – that frequency range happens to have particularly bad interference characteristics and has been mostly abandoned today, so it’s not entirely lobbyists.
It’s important because they designed the most of the radio’s circuitry to be fairly broadly resonant – in this case, this radio is known to work up to around 99MHz of the modern FM band on strong stations by taking advantage of the 2nd harmonic of the local oscillator. There are some slightly different audio parameters between the two sets of broadcasts but the older circuitry is so broadly tuned it makes it work.
The preset buttons are labeled with local Seattle stations, although they’ve since changed frequencies over the years.
There’s a 12″ speaker driven by around 5W of power. The radio itself has 11 tubes, one of the more complicated ones I’ve tackled so far. The chassis also looks the least friendly, of course – but it’s in excellent physical condition at first glance, which will make it easier. It’s just very cramped underneath. With an 11 tube line-up of 6SG7 6SG7 7Q7 6SG7 6SH7 6SH7 6SQ7 7K7 6V6GT 6V6GT 5U4G there’s going to be lot of circuitry.
It’s also difficult to mount anything to the side of it for suspending the chassis above the work surface because of protruding screws and a sheet metal seam. I’ll have to build a new jig from an idea I read – a thick board drilled in a grid for dowels cut to length to suspend the chassis above a work surface by holding it up by the middle. It’ll cost around $20 in materials at Home Deopt, I imagine. That’ll be for the next segment. In the mean time, I’m fairly worried about this power transformer. Combined with the fact this radio came with every tube except the 5U4 rectifier, I suspect the previous owner may have damaged it by trying to plug it in – or else it just failed this way in its past life and was put into storage.
1953 Hallicrafters 8R40 Repair
Back in May, I dug up a Hallicrafters 8R40 from one of Seattle’s local electronics recyclers, 3R Technologies. It looked like it had been stored very well, had all the tubes, the dial strings intact, no rust to speak of and not even very much dust.
This one is a bit different from the others I’ve worked on, it’s a general coverage communications receiver as opposed to being a consumer/entertainment receiver. It’s built more sturdily, designed to be serviced more easily (down to the flip-up metal top for easy access to the tubes) and was designed with an eye for sensitivity and performance to receive ham radio signals. It receives the standard AM Broadcast Band + three shortwave bands covering most of the amateur frequencies from the 160M band (1.8MHz) down to slightly longer than 8M (44MHz). It uses 8 tubes, has a tuned RF amplifier stage and two IF stages (instead of one commonly found on consumer radios) and a device called a Beat Frequency Oscillator that means it can be used to receive Morse code, not just audio signals.
It’s really interesting to think about how capable of a device this is, given that its circuitry is of the same order of complexity of maybe a dozen transistors…about the same number a modern children’s noisemaker would have and nothing compared to the 731,000,000 transistors employed in a modern Intel Core i7 CPU.
An action shot of the underside, recapping in progress, starting from the right and working left. This radio was either built from bucket parts (unlikely, given it’s from a decent brand and wasn’t their cheapest model) or was repaired several times over its life, but always well. There were 6 different brands of paper capacitors: Aerovox, Micamold, Standard Condenser Co., Micamold, Cornell-Doublier, and El-Menco. I’ve replaced all paper capacitors with film capacitors with a uniform 630V rating ignoring the varying lower voltage ratings of the original capacitors. The small values were either replaced with 500V Mica or 1000V Disc capacitors of the appropriate capacitance.
Size comparison of old and new capacitors together.
All the old caps replaced, including the across-the-line cap (blue, top left) and the electrolytic filters (middle). I mounted a new terminal strip using a #6-32 screw through an existing hole in the chassis for two of the capacitors, and used an empty grounded lug from an existing terminal strip for the third. Some would remove the can capacitor, cut it open and re-stuff but I don’t find that to be a good use of my time.
A perspective shot showing the different components. It’s pretty busy down there!
All the replaced components. Four mica caps, one molded paper cap, 18 wax paper caps and two resistors.
So, flipping it back over for the first power-up:
First thing I noticed, the dial light is out. That’ll need to be replaced. The radio does crackle to life, though, and once I get the positions to life does weakly receive KIXI 880 in perfect alignment. But it has an overpowering volume-dependent hum and low sensitivity, missing many stations I know to exist there. We’re about six hours of hands-on labor to this point. Time for troubleshooting:
The hum is volume-dependent, which means it’s (almost certainly) not an issue with the power supply. Power supply wiring errors, like insufficient filtering due to a failed or missing filter cap, usually shows up as volume-independent hum. I’ll check the third filter capacitor, though, as it’s on a different ground lug. The radio has a Radio/Phono switch on the front which changes the audio source from the detector output (radio position) to the phono input on the back (phono position). When set to the phono position the hum disappears entirely. This indicates the problem lies in the RF section. I suspect the 6SG7 tuned RF amplifier has an internal heater-to-cathode short, which will put AC hum from the AC-powered tube heater into the DC cathode circuit. It could also be a short circuit to ground around one of the other RF tubes (converter, an IF amplifier or the detector) but I doubt this, a short around a functional tube would likely take it out entirely versus a leaking short internally.
Testing the tubes revealed they were all good. Several tested nearly new. All were identically branded Hallicrafters tubes to match the set which was even more interesting, I wouldn’t be surprised if they were the originals included when the set was sold. I relocated the ground from the above foreground electrolytic from the terminal strip to an unused solder lug but this didn’t correct the problem. Poking around, I discovered that the problem seemed to change with vibration and motion and the easiest place to see this effect was changing the position of the speaker. I initially suspected an intermittent in the output transformer (seen to the left of the black EPCOS capacitor):
but several tweaks around the transformer, including substituting another speaker, eliminated it from consideration. I desoldered the speaker leads and replaced with a bench speaker:
This didn’t fix the problem, so I reversed the modification and worked backwards from the output stage to discover that I’d missed a solder connection on the detector output. While there, I replaced two 200pF mica capacitors that were looking a little ragged for good measure.
With that repair made, I powered it up and it worked very nicely. AM comes on very well aligned with good tone and all the controls function. On the first shortwave band I picked up 5 stations but two of them are broadcast in Seattle. On the second, I picked up two copies of the BBC World Service and a news station in Spanish. On the highest shortwave band I picked up what I suspect are data signals and location beacons but no voice transmissions. These additional troubleshooting steps took about 2 hours, bringing the running total up to 8 hours of repairs.
I peaked up the IF transformers, but the RF alignment was “good enough” for me so I didn’t do a full RF alignment. This was built like a tank originally, and I didn’t need to replace any of the components in the RF sections, so it should be fine for quite a while. After reassembly, I put it up on my shelf:
Now I have a working general-coverage communication receiver. It’s right near my desk, so hopefully I’ll be able to spend some time cruising the dial and seeing what else is out there. Shortwave reception is heavily influenced by atmospheric conditions, it’s different every time!
Total project time for this one was about nine hours and consumed around $20 of on-hand consumable parts. For a parts list, see the service manual.
Antique Radio in “Firefly” Episode 2
I was watching Firefly the other day, and noticed something interesting. Firefly is set in the year 2517 centered around a small smuggling ship named Serenity manned by a crew of mercenaries, former freedom fighters, a pair of escaped convicts, a high class prostitute, and a comparatively normal mechanic and pilot. On one of their adventures in the second episode of the series titled “The Train Job”, space gangster Niska contracts the crew to rob medical supplies from a train. In the background of his office, you’ll notice something interesting:
In the lower left corner of the frame, you’ll see a 1940 Zenith model 10-S-464. It looks like it’s in good shape for being 577 years old, too. I wonder if there are any tubes left.
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