KF7LZE Radio & Technology

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 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.

 

It’s the winter here in Seattle and it gets cold outside occasionally. Cold enough that I can’t vent my soldering fumes out an open window like I normally do most of the year, but I don’t want them in my apartment. Fortunately for me, my chassis stand is the same width as my stovetop and sits neatly under my externally-venting range hood so I can move it over there for ventilation.

The stand can hold the chassis up at any angle and supports a lot of weight. Every time I use it, I’m impressed with the build quality and flexibility. It’s not as nice working in the kitchen as it is on my normal workbench, but it gets the job done.

 

I was cruising eBay randomly the other day and made an incredibly lucky find…a 1934 Simplex Model P Dual Band tombstone radio had just been listed. I have a pretty low maximum that I’m willing to pay for any radio, as I only buy fixer-uppers and not completed radios, so don’t buy them all that often but this one jumped out at me.

It’s dirty, as can be clearly seen in the photos. It could use some attention on the trim, but the finish is in pretty decent shape; there are some veneer issues on the bottom as well that will be a new skill to learn about fixing for me. And the knobs are wrong. Electrically, it’s your standard 1930s “Parallel AA5″ design: 6A7 6D6 75 42 80. But, it has a lot of potential once it’s cleaned up and fixed up and will display very nicely.

This one makes the cut on the display shelf, so I’ll be fixing it up sooner than later.

A client came to me with another Grunow 589 needing repair. I’ve seen this model radio before in my shop and it was pretty exciting to work on another identical one. This particular one came in excellent shape – it looks like it was stored well for most of its life. The chassis arrived with no dust, no rust, a shiny coating, and not even any cadmium flaking off under the chassis.

The radio looks like it was well loved during its time. Built in 1937, it has evidence of being repaired several times all the way up through the 1950s. The UL Reexamination Service sticker is somewhat of a testament to this as well, and the fact that it’s in nearly perfect shape.

I performed the customary intake checks – all coils, transformers, tubes and controls. This one was in pretty good electrical shape. Only two of the tubes needed to be replaced after the years, testing very weak. The volume control was in bad shape, though – same as on the other 589. Unfortunately though for this particular radio, it couldn’t be saved and needed to be replaced. The output transformer connecting the radio to the speaker was also open and needed to be replaced before the radio would run again.

I sourced a volume control identical to the original (250K with a tap for tone compensation) and a functional replacement output transformer. With these parts on hand, it was a simple matter of performing the replacements. All capacitors were replaced with brand new models, and any resistors that had drifted in value by more than 10% were also replaced. On this radio, the first audio output tube (type 76) used a bias battery to establish a control voltage. As circuitry evolved, engineers discovered this was no longer necessary; I updated the circuit design to eliminate the battery by changing the 500K grid resistor to a 10 Meg grid resistor and jumping across the battery.

I also installed interference-suppression capacitors on the incoming power line, and a 0.75A inline fuse under the chassis to protect against a tube shorting and taking out the power transformer. With these modernizations, the radio will play beautiful and should perform without maintenance for many years.

A few parts were replaced during this operation:

The radio’s owner sent me a photo of it installed back in its cabinet after he received it from service. It looks – and sounds – great!

I provide affordable antique radio repair services in the Seattle Metro area and beyond if you’re willing to ship the guts. Check out my portfolio and information page for more details!

I picked up a very nice 1937/1938 General Electric model F-135 radio from Craigslist a few weeks ago, and have been starting to dig into it. It’s one of the highest end radios they produced that year for the 1937 holiday season and 1938 production year: with 13 tubes, automatic frequency control to eliminate station drift, experimental hi-fi reception in 4 bands from 550KHz (AM) up past 40MHz (VHF) continuously, it offers a motorized tuning with 13 station presets and push-pull 6L6 tubes pushing 20W into a huge speaker. It should sound incredible when it’s all fixed up, and it’s the most complex and highest-end radio in my collection.

I found an ad mentioning my F-135, and showing the next lower model the F-96, in the Dec 1937 issue of Radio Retailing:

I used to have an F-96, actually, but sold it unrestored for the same price I paid for it without touching to free up some storage space – a decision I still regret, but have rectified by getting a new F-135. It’s a beast of a radio, stored fairly well but with some rubs, scratches, water rings and a cigar burn on the top.

I vacuumed miles of cob-webs and a 1/4″ thick layer of dust before even attempting to remove it from the back. The radio has a unique power transformer style, only GE used “bathtub” power transformers like this. Most are normally square with square cores…this one’s an oval. You can see the motor preset assembly and the pair of output tubes as well.

The chassis is enormous and has a lot going on.

Underneath, it looks like someone’s taken a crack at it in the ’80s or ’90s. There are a pair of orange drops, and a pair of modern electrolytic capacitors – and a few 1940s/1950s era replacements – but other than that it’s all original. There’s a decent amount of crumbling rubber wire that will need to be replaced, and one of the coils has a broken solder lug that I’ll have to fix, but it’s in shockingly good shape otherwise and should be easy to service. Not many things are buried.

The cabinet cleaned up very nicely after intensive scrubbing with cleaning wipes, Goop, and plain water and rags alternately. Following that, two applications of Howard’s. It brought out the shine on the finish while still leaving the patina around the corners, and blending the scratches in the original finish. I lightly sanded the rings on the top, but not all the way, and left the cigar burn – I didn’t want to remove all of the “character” in the cabinet.

The chassis doesn’t have any good mounting points, so I bit the bullet and ordered one from Steve Strong who makes custom radio chassis stands. They’re huge, high-quality, fully articulated and can support a lot of weight. Arrived some assembly required:

All assembled, it can rotate 360° including suspending the chassis upside down, locked in with the chassis retaining bolts that normally hold the radio into the cabinet.

And locked in ready to go:

Most of the coils in the radio test fine, with the exception of the output transformer which has one half of the push-pull primary open. That’s bad news, but fortunately 6L6 tubes are pretty commonly used today in guitar and hi-fi gear, so new-stock output transformers are easy to come by. The original was rated for 20W; the closest size available for the replacement is an Edcor 30W transformer. The new transformer might even improve the sound quality a bit – losses in transformers were one of the sources of poor fidelity in older radios (and even in modern hi-fi gear); the new transformer is flat from 20Hz-20KHz.

Edcor builds the transformers to order, so it won’t be here for another month or so, but I’ll be starting on the rest of the repairs before then. Stay tuned!

In “House”, Season 8 Episode 5 (aired 11/7), we can see that the good Doctor has a 1937 Philco 37-620 Radiobar in his office. Unclear whether or not it has the original glassware. I’d love to have one of these in my collection, but they’re quite rare and expensive.

Philcoradio.com has some more about the history:

This Philco Radiobar sold for $252.50 in 1937 dollars, around $3800 in today’s money. This was a high end luxury for the wealthy to have in their homes, but out of reach of most consumers of the time.

Just a short update to show what I’m working on right now. Other priorities have gotten in the way of actually updating this site over the last week, but things are still going on in the background!

One project up on my bench is a 1951 car radio, a Firestone 4-B-31 “Roamer”. It’s a 6V radio that uses a vibrator power supply and 6 tubes to receive the AM band, and should clean up pretty nicely once it’s finished. I’m repairing this one for a client I met locally who is building a rat rod out of 1920s-1960s car parts.

 

Part of a continuing series that is now finished – see the previous installments:

Part 1 – History and First Looks
Part 2 – Tool Prep
Part 3 – Capacitor Can Rebuilding
Part 4 – Capacitors and Socket Replacement 

After a brief delay while I waited on parts from my supplier, I’m finally set to finish up the GE LF-116 that I’ve been working on this month. It’s going to be returned to my office in playing shape very soon!

I powered up the radio for the first time with the power supply activated to get a feel for it, and played an MP3 through the line input. It played great – for about five minutes, until it heated up, then a nasty 60Hz hum took over. I’d let the set sit for a few minutes, then power it back on and it was fine for a minute or two before humming again. This was very clearly a thermal short, heater to a cathode. There are four tubes in the radio that are prone to H-K shorts: two 6SG7s and two 6SH7s. One of each tested totally dead, and the others were weak. So was the 7Q7 oscillator. I ended up ordering a new set of NOS tubes for the RF stages, and installed them; the radio played perfectly at this point although not very well aligned.

First, I peaked up the IF transformers with a 455KHz reference signal from my generator:

This step could be done outside of the cabinet, but the RF alignment required putting it back in and attaching the antennas.

With the installation  complete, I switch over to RF calibration, first with a 17.8MHz reference signal, then a 1500KHz, finally a 580KHz signal to align the Shortwave and AM bands.

I peak the alignment trimmers with the radio installed in the cabinet using a non-inductive alignment tool:

The trimmer locations were clearly called out on the service documents, so it was easy to find where to adjust even though it’s not the easiest to reach. It aligned up nicely! The dial tracks perfectly now. And speaking of the dial scale, it’s interesting – the tuned station is indicated by a thin line of red light surrounded by shadow.

And that’s it! Back to the office it goes. I’m not planning to do much with the cabinet as it’s not that bad, and it’s going to be in a bit of a high-traffic location so if something happens to the finish I won’t have to worry about it.