KF7LZE's Blog

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.

February was a pretty slow month for projects, about the only one I managed to accomplish was fixing the Firestone car radio. March should be a bit more productive, though. I have a list of a few projects I’m planning to try and complete.

First up, a pair of Grunow radios that are vastly similar to the pair of 589s that I’ve fixed in the past – a Grunow 566 and a Grunow 588. The 588 even has a Teledial.

I’ll probably also start working on a a Jewel radio that was a Christmas gift:

I also have a kit amplifier left mid-project, I’ve mounted the iron and hardware but none of the connections or components yet. (It’s a stereo amplifier with a 6SN7 dual triode driving a pair of 6V6 in push-pull for each channel, and should sound pretty good when complete.)

Plus other projects that seem interesting along the way. Stay tuned!

Sorry for the slow month everyone! My job has been keeping me busier than normal, so I haven’t put anything onto the bench since finishing that Firestone.

A few things are upcoming, though. I expect to build a new computer before the end of this month, and in March anticipate repairing a couple of new Grunow radios I’ve picked up, and a handful of other projects as I come across them. Stay tuned!

A friend has a new business venture I’m really excited about and wanted to share with everyone here. He’s making multipurpose lavender sachets for indoor use in your bedroom, closet, car or anywhere really using California lavender assembled right here in Seattle. I’ve been playing with my sample sachet for a week now and it’s really a great product that I highly recommend. I’m not typically one for intentionally bringing new odors into my living space but the LavenHangers smell great and also aren’t overpowering.

You can leave the LavenHanger lying about, put it in a drawer, or slip it over the loop of a hanger to hang with your clothes in the closet. Lavender is a natural insect repellent and repels bed bugs, moths and other pests you wouldn’t want around and it’s all natural so if you’re using it to protect your wardrobe, you don’t have to worry about harsh chemicals like the petroleum naptha in traditional moth balls.

They’re 3 for $20 available over at KalbCo, made in the USA. You should go check it out!

A local friend is building a rat rod out of 1920s-1950s parts, a custom collection that ultimately will turn into a very fast car powered by a huge V8. He found a vintage car radio to go with it, the perfect addition and gave it to me to fix up. He requested to leave the metal cabinet alone so he could paint it to match after the car’s color scheme is finalized, so don’t worry too much about the finish.

This radio, the 4-B-31 “Roamer” was built by Firestone Tire & Rubber, the same company that today makes tires interestingly enough – they used to have a bigger product line when consumer buying habits favored combination stores. It’s a six-tube radio with a broad RF amplifier stage. Most likely the radio bolted up under a pickup truck’s dash and connected in the back to the firewall.

The tubes are 6SK7GT 6SA7GT 6SK7GT 6SQ7 6V6GT 6X5GT. The radio operates off a 6V car battery. With the low voltages it’s only about 1.2W of output power so will never be that loud, but when highways were new it was a lot quieter on the road and probably sounded better.

The battery directly powers the 6.3V filaments of the tubes, and the high voltage is provided with the help of a vibrator power supply. The 6V is fed into the electromechanical device which rapidly vibrates between two contact points turning the DC into a square-wave AC which is fed through a transformer to step the voltage up, then into a conventional rectifier power supply.

One of the pins was broken on this original vibrator, so it was the first to go. I replaced it with a solid-state replacement that uses a few transistors in a multivibrator circuit to accomplish the same effect, and should never need to be replaced again. I also replaced the 6X5 with a pair of 1N4007 diodes in an octal tube base, although this isn’t shown in any photos.

The chassis was decent to work on. It had open sides which made it easier to get things in with tight tolerances. The resistors tested decently, but all caps did need to be replaced as always. Several had blown their ends off already.

This radio was of course designed to be used in a car, and that means used with a car radio antenna which is a specific length and has certain transmission line characteristics – not quite as simple as just stringing out a long-wire. It’s a standard antenna, though, so I ordered a replacement that cost something like $10 with free shipping from Crutchfield.

It arrived in interesting packaging. The box was clearly broken in half, but both halves made it to my door without actually being connected somehow.

The antenna was in the bigger section. Go UPS?

A terminal strip in the radio was broken. This was a problem because the broken terminal happens to be the positive power lead-in and it couldn’t be salvaged. Only one terminal broke, though, so I improvised, screwing a screw lead to the mounting bracket and securing as shown, then running the wire out of the case.

Reassembled and testing with a bench power supply that was okay to check functionality. The switching power supply introduces too much hash to receive any stations, but it was good enough to do an alignment with a signal generator by injection. I then switched to a lantern battery for final tweaks which had a disappointing life of about 10 minutes. Clearly these were meant to be run off lead-acid batteries or linear power supplies only. It draws around 4A.

I also rewound the dial indicator. The dial tuning drum was still wound properly but the dial indicator string had broken so the pointer no longer moved. I used string that was a bit too thick but it worked out okay and is perfectly functional. No photos of that available though, it was pretty quick. The service manual had a full dial string diagram and pointer adjustment procedure. Unfortunately I ran into a problem as I was reassembling everything: the volume suddenly dropped off massively even with the control maxed out and it wasn’t coming back for anything. A check of the voltages showed that I had tens of volts on the screens of most all the tubes, where there was supposed to be a few hundred. I was at a loss about why this happened and finally resorted to the poke test.

The poke test is what it sounds like: poking or tapping on pretty much every part in the radio. I gave decent raps on all of the solder joints, tube pins, tie points and finally came to one that would make the volume cut back and forth: R7, the B+ dropping resistor for the screen voltages, a 15K 1W carbon resistor. Apparently it was internally cracked or otherwise defective. I replaced it with two 30K resistors in parallel to form a 15K 2W resistor, and a few others that shared the same tie point or were otherwise looking rattier than I really like even if they were in spec.

With that repair completed, the radio fired up perfectly with loud volume. This was a fun project, but power supply issues mean I don’t think I’ll take on too many of these in the future.

I’ve finally had the time to finish one of my long-running projects repairing GE’s most powerful radio offering from 1937, the GE F-135. I picked it up from Craigslist back in November but other responsibilities kept me from getting much done on it until the 1st of this year. Finally, after a few months waiting, it’s finished and playing!

The radio came to me complete and in decent shape for the age. It’s missing the glass for the dial, but is otherwise completely intact and the finish isn’t in bad shape despite a few scratches here and there.

This radio is the current king of my collection. The best radio GE sold in 1937, it has a lot of innovative features – early APEX hi-fi reception, dual 6L6 output tubes putting out 20W of audio to a large and rich-sounding 12″ speaker; a total of 13 tubes including a tuned RF stage, dual IF stages, separate oscillator and “station seeking” automatic frequency correction.

Under the chassis it’s in decent shape too. It looks like it has been worked on before a few times – there are some ’40s, a ’50s and a couple of ’90s capacitors installed. There’s a sticker from a Seattle-area Jukebox Repair store on the back which probably explains the more-recent repairs; I looked up the address and they don’t seem to be around anymore.

First thing first after doing the complete set of intake checks on the radio, I gave the cabinet a thorough scrubbing and hit it with Howard Restore-a-Finish and it came out beautifully. The intake checks were uneventful so I didn’t take any photos, but the power transformer, all the IF transformers, oscillator and antenna and RF coils and the speaker transformer and coils were just fine. After applying the Howard’s, it really cleaned up the fading and covered over a couple of small scratches and really brought out the natural shine on the wood.

Then I tested all the tubes and found that most were in good shape (especially the 6L6G tubes installed) there were a few 6J5 and 6K7 tubes that needed replaced. I had these in stock and so it was easy to solve that problem.

I worked on this one under my kitchen’s vent hood as it’s currently too cold to vent soldering fumes outside. The chassis stand is the right width though so that’s perfect.

Every paper and filter capacitor needed replaced, as well as nearly every single one of the resistors which had drifted as much as +100% in value in some cases. Fortunately the coils are all intact or this could’ve been a much messier repair.

I also found a couple of places where the previous repairman who worked on the set may have been dyslexic, as there were a couple places were numbered resistors were reversed – i.e. R23 being in R32′s place and so forth. I imagine that couldn’t have made it work any better, anyway. I tested and replaced going along from the bottom up as needed. These are in-progress shots, so you might see a couple places where leads aren’t trimmed or components aren’t yet soldered. All of those issues were taken care of but might not have made it into the photo series.

Small caps dealt with, it was time to replace the multi-section caps. There is a 4-section can, insulated from the chassis, mounted up top with a set of 2 filter caps and 2 cathode-bypass caps. These all ran to a hole in the chassis where they went above. I snipped the long cross-chassis leads and moved the components close to their intended locations. In this photo, I’ve replaced the 10uF cathode bypass capacitor with its replacement. I like using bipolar caps for the large-value cathode bypasses but that’s just my preference and what I keep in stock (I keep those values around for repairing crossovers in old speakers) but you could use a standard polarized capacitor there.

Here’s the totally-complete underside shot.

There’s still a matter to deal with above the chassis, though. This is an AFC radio which uses a special and complicated transformer heading into the diode which recovers the audio. And it has a small resistor which is reading double it’s value and needs replaced as well, or it won’t align right.

Then I sealed the can back up:

I hooked the speaker and pushbutton assembly up on the bench and gave it a test run – it fired up immediately and started pulling in a few stations even on the Shortwave bands. The dial was off alignment a bit though, so it was time for that.

For the alignment, I pulled up the signal generator and started with an IF alignment before going back to the RF stages. This radio has a special IF arrangement with a procedure, so I aligned the 3rd IF primary, second IF secondary and primary, first IF secondary and primary, then went back and aligned the 3rd IF secondary that feeds into the diode. Aligning that discriminator was a maddening 10 minutes spent trying to nudge the adjustment ever so slightly. My goal was to get 0V between two segments, but it approached that point at an incredibly steep slope. I managed to get it there, though.

The original 0V spec was made with a primitive meter; I’ll take 0.01 on a more sensitive modern instrument. That’s perfect IF alignment. It was definitely worth it though. Now onto the RF, which involved tweaking something like 16 trimmers in a precise order with an RF signal generator at various frequencies.

Finally, it was all set! Time to reassemble.

At this point, the radio plays beautifully and pulls in stations from all over, and I’ve added a line input to let me hook up an audio source. The hassle of the AFC calibration was definitely worth it, it’s nearly like magic to watch it work. With the switch off, the radio tunes sharply and a station comes in over just a few degrees of rotation. With the switch activated, it’s like an entirely different radio – the same station will come in across about a quarter-turn of the knob, 2 divisions in either direction from the center frequency and it will block quieter stations from interfering.

The radio sounds great with a pretty good frequency response and more volume than I know what to do with, too. The relay for the motor is burnt out, though. I missed that on the initial checks so when I went to test the pushbutton function…I got a whole lot of nothing. I’ll make another post here when I do get the motor resolved but for now I’m going to hang this one up and start playing it. This was a very fun and enjoyable project and I have a beautiful radio with a commanding presence to enjoy for many years to come.

 

I had an interesting call the other day with a gentleman about a radio he’s working on, among other topics:

That’s a Philco 46-420. They’re nice little bakelite radios with 6 tubes designed to receive the AM broadcast band. He’d come across some unlabeled wiring while repairing and had dealt with it but we were talking about what it’s purpose was.

I generally work on pre-WW2 radios so haven’t run into this particular arrangement personally, but I’ve read a few different articles by other collectors on this topic and recognized it immediately. The coil, wound 8 turns around the capacitor and connected at one end to the chassis, is a type of wave trap designed to cancel out the inductance of the old capacitor. This helps to prevent interference – both received, picked up through the cap as if it were an antenna, and radiated interference from the signal passing through the cap. Philco used these capacitor wave traps in most of their radios from 1946 and on. There’s an article at the Philco Repair Bench describing one style; this is a slightly variation with the same effect.

Modern caps are constructed out of metalized polymer films that have very little inductance, but these older capacitors were just concentrically coiled metal foil sheets with a lot of natural inductance.

Modern caps don’t have that physical property, so it’s safe to replace the wrapped capacitor with any modern replacement and either shove the new cap through the coil, or remove the coil entirely.

Thanks to Bob from Old Tyme Radio for these photos of his project, and for distracting me from being snowed in for a bit!

I’m always taking mail from readers with interesting anecdotes, photos and questions so feel free to send them over either as comments or through the e-mail address I’ve posted in my Repair Services page.

Following up on yesterday’s post, this photo of a sign posted downtown pretty well describes how Seattleites react at the first sign of flakes falling from the sky.

This photo courtesy of my friend Jamie Swedler.