KF7LZE's Blog

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 1987 and I’ve been needing a new laptop. After a careful process of comparison shopping I settled on my Sharp PC-4501 from Montgomery Ward. Let’s open it up and take a look!

For only $699.99, it’s a state of the art machine. A 7.16MHz processor, 640KB of RAM, and mine comes with an optional second 3.5″ floppy disk drive! Now I can work on a program and save its data at the same time. It was designed to be especially easy to use.

Good thing they provide unpacking instructions inside the box.

It comes with an instruction manual and the operating system disk.

I’ll make sure to return the warranty card.

Power up…time to go through the BIOS setup!

One thing I don’t like so much is the screen is front-lit with ambient lighting, and is glossy. Makes it a challenge to see what’s being displayed sometimes.

We’ll try out a productivity application.

Looks productive!

I can’t wait to begin editing documents and spreadsheets from my own home. It’ll be so much easier than my old typewriter!

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!

Antique Electronics Supply, one of my go-to vendors for supplies for working on antique radios, is in a festive mood this season.

That’s a chocolate vacuum tube along with my order of various dial lamps and a handful of replacement tubes. Very neat!

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.

The last of the three radios I picked up from an estate sale a few weeks ago, I spent about an hour this evening bringing it back to life. I’m still waiting on parts for my GE LF-116 – my supplier sent me the wrong tubes – so while that’s on hold I’m checking off more projects from my list. This was the most recent one:

The RCA Globetrotter A-20 radio is a Canadian design, not sold in the U.S. but quite a few have made it south of the border over the years. It was designed by famous industrial designer John Vassos for RCA which makes it a desirable and very attractive, if not terribly well-performing, radio.

It’s a simple five-tube transformer radio with a ready input for hooking up an MP3 player or other external audio device, controlled by a switch on the back. It has interesting shortwave bands – on U.S. radios, the wave bands are usually 1.8MHz-5.5MHz and 5.5MHz-18MHz; on this one it’s 2-6.7MHz and 7-22MHz to match what the Canadians were using at the time.

The previous owner had replaced about half of the capacitors with late ’80s or early ’90s Orange Drops; I replaced five more capacitors he’d left in place as well as the electrolytic filters. Then it was time to power on and see what happened:

Powered on nicely! A bit more hum than the other sets I have, but not enough that it’s malfunctioning – it’s just a radio with simple circuitry.

It was tricky getting the dial scale back in alignment with the pointer, as the dial stays in the cabinet and a slug fits in between two pins on the pointer to move it. It took a few tries to get it aligned right, then I tapped it into place with a long-handled screwdriver.

The alignment is spot on, receiving KIXI 880 AM exactly where it should be on the dial, no further repairs necessary. Including time spent removing material from the cabinet, this was about a two hour project. I skipped a lot of the initial checks and just started working on this one, since it’s for my own collection, which cut down on the time significantly.