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.
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.
It’s interesting working in a radio that’s been repaired before in its life. The quality of the work varies greatly. I’m working on a Grunow 460 (which will be written up later) and I found a late 1930s repair attempt which hadn’t been soldered – just wrapped around the end of another component’s connection to a lug.
I doubt this helped the radio’s operation much.
I picked up this Grunow 750 “World Cruiser” radio from eBay a little while ago for an incredible deal and now it’s time for it’s turn on the bench. These radios are fairly uncommon and frequently sell for several hundred dollars, so I was excited to be able to pick one up for under $100 with shipping.
It’s in remarkably good shape, despite the eBay seller packaging it in form-fitting cardboard with no padding whatsoever and the chassis unsecured in the cabinet. The fact it arrived as anything other than a pile of broken wood, bent metal and shattered glass astounds me – it was by far the worst packing job I’ve ever seen an Internet seller provide.
The radio looks like it sat somewhere very dirty, and possibly was briefly inhabited by a rodent. There are a few chewed-on spots, and some fiberglass insulation was dragged into the cabinet. It doesn’t look like whatever lived there was in it very long, however, as there’s no rust, the damage is very minor and there wasn’t a lot of “fill” material brought in.
I set to cleaning and examining. One IF transformer is missing it’s grid cap, that’ll be a bit annoying to replace.
You can see around the edges where it looks like a rodent did some chewing.
It looks like it also chewed through the output transformer leads.
This is a big radio with a big chassis to match, accepting 7 tubes 6D6 6A7 6F7 75 76 42 80. It can receive 2 shortwave bands and the AM Broadcast Band, features a tuned RF amplifier, and double-tuning on the broadcast band for extra selectivity. For the double-tuning, it uses a 4th segment on the tuning capacitor. It’s very rare to see a 4-segment tuning gang on a superhet and it’s a definite indicator of quality.
The underside is built a bit like a tank, with multiple sets of shielded coils. Fortunately, the sides of the chassis are bolted on allowing easier access to the components. It would be impossible to work on otherwise.
With the sides off and the coil covers removed, it’s a lot easier.
I’ll be working on the radio this week, testing all the coils and transformers and then replacing the out of tolerance resistors, new capacitors, and repairing the IF transformer grid cap and output transformer leads. The radio will also need a new cord as the old model was badly frayed and chewed and so it was discarded.
I’m getting ready to sell some of my radio collection to make room for new ones, but until that happens I’ve been under something of a self-imposed moratorium on new purchases. This one came up on eBay a little while ago for a steal so I cheated a bit and had it sent my way.
This is a huge tabletop radio – a “grande” coffee from my local coffee shop is provided for scale. It’s a 7-tube model using the tubes 6D6 6A7 6F7 75 76 42 80 – the AM Broadcast Band and the lowest Shortwave band are double-tuned with a single-stage RF amplifier for extra selectivity; on the two higher shortwave bands the second stage of tuning is disabled to increase sensitivity. With an 8″ speaker, it should sound pretty great. There are a couple of minor scratches on the front that will buff out nicely, and it’s otherwise original – this should be a nice, fairly quick radio to bring back to life.
I started this radio a few days ago in Part 1. I left off with the radio up on the bench ready for service. It had a lot of issues with the power supply – and as I discovered while working, some shipping damage: the volume control wires, and one wire from the power switch, had snapped. There was a fair amount of bad workmanship in the radio everywhere which contributed to that happening…you can’t expect much from a 50-year-old cold solder joint after all. I repaired it back to schematic along the way.
You can see some of the damage here where a 150 Ohm flex resistor broke. This could have been shipping or age but the combination sure didn’t help it.
I went to the Mike and Key Electronics Show and Flea Market today down in Puyallup to replenish my stock of parts and hopefully find some new ones…actually, I’m pretty disappointed. I had a very long list of parts I was hoping to find, the sort of things that are in everyone with a real workshop’s stockpiles. Quite a few had some elsewhere, or didn’t think to bring any, but I ended up not finding anything. I did pretty well though getting four front end sets (6A7 6D6 75) and four of each #41 and #42 output tubes, as well as a bunch more of various numbers mostly from $1 working-pulls bins. I don’t have any photos inside, but here’s one of the line to get in.
I pulled out quite a bit from the radio.
And for the first power-up.
I wish I could say it worked at this point, but it didn’t. I took the tubes out to test and found a 6A7 that tested just marginal but with no other issues. This really should not have been a show-stopping issue, but it seems it was as when I replaced it with one of the tubes I’d picked up earlier in the morning it fired right up.
Another shot for fun of the bottom:
At one point in the radio’s past, the IF transformers were replaced with service parts – in this case with Meissner universal coils. They work just fine and I’d really like to know what happened to the radio to need both replaced. It’s the first radio I’ve worked on which was in the shop so many times for what looks like major surgery.
Time for an IF alignment. I decided to broadly tune these for a little better fidelity at the expense of some selectivity which was going to be bad anyway. I tuned the secondaries to 465kHz and the primaries to 455kHz. This should give about 10kHz of bandwidth to pass as much of the signal as possible.
With that, I reassembled it and took some photos in front of a makeshift backdrop made out of cardboard boxes.
This one came out with an interesting camera glitch, but it’s below the radio itself so I’m leaving it.
This one needed some interesting troubleshooting to get it to work but most of the issues were things I spotted while looking it over initially, so I didn’t waste a lot of time on it. The radio has a really big speaker and is very powerful and sounds as good as it can sound so I’m glad I broadly tuned the IFs. This was a fun project with the bit of detective work involved in fixing mistakes.
I’m off to a good start for my March projects, fixing up the Grunow 588. Next up is my Grunow 566 from the year before, from 1936. I picked this one up from eBay in shockingly good original condition – it looks almost brand new. Digging into it, though, it turns out I have a bit of work ahead of me. Someone’s been in this one – several times – and has made several modifications of somewhat questionable workmanship, to put it nicely.
This one is an even simpler radio than the model 588; it’s still a superhet but this model has no AGC (automatic gain control) on the front end. The tube line-up is 6A7 6D6 76 41 80. Most of the other radios I’ve serviced have a #75 in the 3rd position (a double-diode + triode tube); the #76 is a single triode only. The lack of the two diodes accounts for the lack of AGC. This does make it a simpler circuit to work on, though, as there are several fewer capacitors and resistors that would otherwise be on the AGC line.
The chassis is very clean and both tubes that need shields have the originals present. Finding missing tube shields is a real pain. The chassis tags are in good shape too, this was the only model to use the chassis 5-S:
Pulling the chassis out, it’s easy to see what I mean about it being messed with before. The electrolytic cap which usually clamps to the top of the chassis is missing to the left of the front coil:
There’s also something about that speaker…it’s been replaced! The original, correct, speaker for this model is an electrodynamic speaker with a field coil. Field coil speakers were used before materials science advanced to the point of giving us reliable and powerful permanent magnets, where the magnetic field for the speaker was powered by a high voltage passing through a large coil. Electrodynamic speakers required four wires to operate (two for the field coil, two for the voice coil). This one only has two!
Someone replaced the speaker with a rather large permanent magnet speaker. The original speaker had a hum-reduction coil which doesn’t exist on this speaker, so it’s been eliminated as well. Granted, this one might actually sound a little better than original, but it’s a big change. Swapping a field coil speaker for a permanent magnet speaker also leaves the issue of the field coil, which is a part of the power supply circuit and is necessary for the radio to work.
The previous owner replaced the field coil with what looks to be a much-too-small resistor in its place:
I’ll have to check the tube datasheets to see what kind of current they’re drawing, but that may need to be replaced for safety and reliability. Even worse than these mods, though, is what happened with the power supply. Parts failed a lot back then, it was just a fact of life. When the filter caps died the first time, the external filter was replaced with one under the chassis:
When this cap failed, it was replaced by two more caps – put in the circuit across the old caps, without removing them.
When one of those caps started failing, it was replaced by another also just tacked in across the failed point. And arriving with one end broken off.
I’m somewhat curious what “Semi-Polarized” means, but suspect it’s just a quirk of older manufacturing methods. Regardless, though: this radio which is supposed to have only two electrolytic filter capacitors currently has five installed. I’ll be cutting them all out and sanity checking the entire power supply.
The age of the parts used in these repairs leads me to believe it was modified like this sometime in the late 1940s up through the ’50s, which makes it somewhat authentic if not original. I’m going to fix it up as well as can be but don’t plan on reversing the speaker-swap and we’ll see how it goes. Stay tuned!
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!
Keeping my word about actually trying to accomplish projects in March, I started tearing into the 1937 Grunow 588. It’s vastly similar to the Grunow 589s that I’ve repaired in the past, the main difference being that it’s a table model radio and not a large console.
It’s also a five-tube radio with the line-up 6A7 6D6 75 41 80.
The chassis stickers are in great shape. It’s always nice to find them in good condition. I also like how on this model, the antenna and oscillator trimmers are on the side near the transformer (for adjusting the alignment) – on the 589, they’re up in the front partially under the dial face which is more difficult to access.
The chassis is pretty clean, just a little surface rust on the transformer and the tube shields with the rest in good shape.
The underside shows evidence of having been worked on a few times. There are some of the original Grunow caps, and several ’40s, ’50s replacements, a ’60s or ’70s replacement and one from the ’90s. Other than needing to verify the wiring to ensure past repairs were correct, it’s nearly the same radio as the others and should be an afternoon’s project at most, maybe two if I take my time on it. I’ll be getting started on that in the next few days.
And we’re done! The saga of restoring the Grunow 589 is coming to a close, as I’ve sorted the last issue, performed the final set of adjustments and verified correct operation of all its features.
If you’re just joining in, take a look at the previous entries to see what happened so far (it’s a lot!):
Part 1: Identification and Task List
Part 2: Intake Checks
Part 3: Capacitor Replacement
Part 4: Resistors and Controls
Part 5: First Power-Up
Part 6: Socket Replacement and First Alignment
Part 6.5: Diagnosing an RF Intermittent
I’d completely electrically restored the radio and completed the first phase of alignment after the socket replacement, but the radio was acting up, oscillating and cutting out intermittently. It turns out that a wire had broken under the cloth insulation. I located the break (near an end of the wire), stripped off the insulation back to a good segment and reconnected to restore proper operation.
I re-peaked the alignment just for good measure, this time using my non-inductive alignment tool.
Finally, at one very narrow range down at the low end of the AM broadcast band, the oscillator was cutting out for a very narrow range – only about 580-600 KHz. Because of the nature of the failure it was easy to determine that it was caused by shorting plates in the tuning capacitor. Shining a bright light on the plates, it was easy to see where they were just barely scraping, and very carefully bending it outward solved the problem completely. I used a cut to size piece of a business card for this task.
The radio receives stations across the entire dial and the tone control nicely compensates for proper sound depending on the style of music being played. Radios this old weren’t built to “hi-fi” specifications as they are now, so the frequency response is somewhat chopped compared with a modern system – it rolls off the highest highs and distorts on big bass hits – but it sounds incredible playing period music, talk, sports, and lighter ambient choices.
I replaced quite a few pieces during this restoration:
Dial string, tube socket, bias cell, one tube, 13 paper capacitors, 3 mica capacitors, 2 electrolytic capacitors (not pictured), and 7 resistors.
Photos of the completed chassis:
A video of it playing through the AM broadcast band, and scanning across the shortwave, is available from YouTube:
I was able to weakly receive a single shortwave station, this one at the high end of the dial, while scanning today. Shortwave reception is so variable, depending on atmospheric conditions, time of day, local electrical noise and RFI, and antenna setup but I’ve been able to receive a few other stations at different times of day with this one. As this is an entry level model lacking a tuned RF amplifier, its shortwave performance will never rise above mediocre at best but it probably performed acceptably in days of less noise and more stations.
This process dragged on longer than I usually like due to a couple of issues that cropped up in the middle, but on most old radios there are no problems that time and patience can’t solve. Another successful project, done!
Update: The series is complete. For more articles and information, visit these other posts:
Part 1: Identification and Task List
Part 2: Intake Checks
Part 3: Capacitor Replacement
Part 4: Resistors and Controls
Part 5: First Power-Up
Part 6: Socket Replacement and First Alignment
Part 6.5: Diagnosing an RF Intermittent
Part 7: Conclusion
We’re nearing the end of this particular project, and are just down to some touch-up work. The Grunow 5-U chassis I’m working on has had a lot of work done on it so far, please visit the previous entries if you need a recap or want to see some more photos.
Now we know the radio is in good shape, it’s time to start working on the alignment. The radio receives stations but with a bit of distortion, so the first steps are to peak up the IF transformers. Using my signal generator set to the intermediate frequency of the radio and verified with the frequency counter, I’ve hooked the generator’s output to the grid of the converter tube. This will introduce a warbling signal to the radio.
Using an insulated screwdriver and listening for peak output, I adjust the IF transformer trimmers to bring them into alignment at 465 KHz starting with the secondary of the second IF transformer, then the second IF transformer primary, and finally the secondary and primary of the first IF transformer using an insulated screwdriver (as the primary of the IF transformers carries the high voltage, so if this were to short to the can it could damage or destroy components.)
It’s a little messy, but here you can see them all chained together as the process gets underway.
Unfortunately, though, the alignment procedure revealed a weakness that I hadn’t previously detected. It turns out that the 6D6, IF Amplifier tube, was extremely picky about its orientation during operation.
It turns out the socket cracked and was no longer making good contact with the tube, causing intermittent operation. That’s unacceptable, so the socket had to be replaced. Socket replacement isn’t terribly difficult, but it is tedious and time-consuming as all the connections must be carefully removed from the tube and preserved, the socket rivets drilled, a replacement socket mounted and connections re-established. This frequently requires a realignment, too, as components being physically moved (lead dress) can change the alignment.
Using my hand drill at a slight angle, I attack the rivets holding the socket in. The rivets split into three pieces instantly, and the socket was free.
Clearly it wasn’t held together by anything except those rivets,as it disintegrated immediately.
The new socket mounted up easily with #6-32 machine screws. I replaced the 470 Ohm cathode resistor as well, as it broke during removal.
The top view after final socket replacement. Pretty much all the work under the chassis is done at this point, although I’ll be taking another crack at the alignment for good measure to peak it up after replacing that socket. The 6D6 now properly mounts and its shield covers it. The shield is necessary or the radio won’t play. The only evidence of the replacement are the small brass screws. The radio plays much better now, and isn’t sensitive to bumping the table, walking by, looking at it wrong, or any other contact-related annoyance.
All that remains on this radio is to complete the alignment and re-string the dial.