Vintage Motorola

My husband inherited this early 1950’s vintage Motorola AM radio from his grandfather. I’ve always thought it was really beautiful radio, but it spent 17 years sitting on our shelves as nothing more than a conversation piece. Recently, I decided I wanted to get it playing again.

This type of radio is called an All-American Five, so named because it uses five vacuum tubes, and American manufacturers mass-produced millions of radios based on the same basic design from the mid-1930s until the early 1960s. The radio was physically in pretty good shape before I started restoring it. It has always been kept indoors and never subjected to any abuse. However, I would soon learn that 60+ year-old radios that have been sitting idle for 20 years still require a lot of effort to get them working again.

Reading Up

I read all the restoration advice from Phil’s Old Radios and read a lot more about the All-American Five on Fun With Tubes before I got started. I also found the schematics for my radio (Model 5X12U) on a site called Nostalgia Air that provides PDF scans of the schematics and service manuals for antique radios. When restoring an old radio the schematics are nearly mandatory.

First Steps

In my initial assessment of the radio, I found that the power cord’s insulation was crumbling, it was missing two vacuum tubes, and it still had all the original wax paper and electrolytic capacitors. Capacitors are typically the least reliable parts of any electronic circuit, and most restoration experts recommend replacing them before you even try power on an old radio. I had already replaced one of the capacitors (the orange one) before it occurred to me to take a “before” picture, but you can still see most of the original components in the picture below.

(Almost) Before

I replaced all the paper and electrolytic caps, the power cord, and ordered replacement tubes for the rectifier (35W4) and the IF amplifier (12BA6) that were missing. I believe the remaining tubes (12BE6, 12AT6, and 50C5) were original to the radio. I checked them visually for oxidized getters and tested continuity their heaters, and everything checked out, so I decided they didn’t need to be replaced.

Before powering up the radio for the first time, I built a dim bulb tester. The basic idea is that you wire a light bulb in series with the radio, and if anything is shorted out, the light bulb limits the amount of current that can flow through the radio and prevents any major fireworks. If the radio does short out, the bulb will glow brightly, but if everything is OK, it will be dim — hence the name.

Full of anticipation, I hooked the radio up to the tester and powered it on. The the bulb glowed dimly — a good sign that I hadn’t botched anything — but then… nothing. All the tubes lit up but I got no audio — no static, no hum, just silence. This was a major disappointment and it took me a few days to figure out what to do next. Then I discovered that the 22 ohm series resistor in the B+ line had drifted up to around 70 ohms, which meant that the radio was getting less than a third of the current it was supposed to.

After replacing this, I tried again and the radio slowly crackled to life. But something was wrong… it definitely worked, but it was temperamental. The station would play for a while and then suddenly the volume would cut out to almost nothing.

First Audio

I checked all the other resistors and found several more out of spec, so I decided to replace them and ended up replacing all the other resistors while I was at it, but there was no appreciable difference afterwards. I tested the voltages at all the points indicated in the schematic, and most of them are within a few percent of the specified voltages. Unsure what to do next, I turned to the Antique Radio Forums for help. I made a several videos showing the audio problems I was having and people took a listen and offered their guesses as to what could be wrong. I owe a great deal to the friendly folks on the forum for all their guidance and encouragement, especially Peter Balazsy and Tom Bryant.

Silver Migration Disease

Peter identified the true culprit — the dreaded silver migration disease. Radios of this vintage contain RF transformers with integrated silver mica capacitors in the bottom of them. Together with the coils, these form a tuned circuit that filters out the other stations and helps the radio amplify the one you’ve tuned in. Over time, unfortunately, the voltage difference between the primary and secondary coils causes the silver on the capacitors to slowly migrate from one side to the other until it eventually shorts out. This causes what’s often described as a “crashing” sound. Fixing the problem requires disassembling the delicate transformers and removing the damaged capacitors, then wiring in external replacements.

I watched several videos about this, and came away quite intimidated. I held out hope that something else would fix it. I tried several suggestions, first replacing the strange ceramic capacitor combo pack that contained 5 capacitors in one with individual capacitors, then blowing out the tuning capacitor with compressed air and cleaning the tubes sockets with contact cleaner. Sometimes the radio would play beautifully for quite a while and then refuse to play at all. During its good periods, I preoccupied myself with taking photos and videos of the radio, including this footage of the insides with tubes glowing and the tuning capacitor moving while I tuned the dial.

Inside the Radio

However, the truth was inevitable and the radio never played well for long. Finally, I worked up the courage, de-soldered the transformers, and took off their metal covers. The most intimidating part of the operation is removing the rivet on the bottom of the transformer to expose the silver mica capacitor. In most of the videos I found, people used drills or dremels to grind out the rivet, which in my hands, seemed destined to destroy the fragile plastic frame or the hair-thin wires. After spending weeks scouring the forums for a better way, I came across the simple suggestion to use an Xcelite flush cutter to snip off the top of the rivet.

IF Transformer with Capacitor Exposed

I have to say, for anyone contemplating this surgery, that this is the way to go. The top of the rivet easily came off without even shooting off as the shears went through it. No fuss, no muss, far less drama than breaking out the power tools! Once the capacitor was removed, it showed a clear case of SMD. Note the black trail of silver oxide going from the capacitor on one side to the other.

Silver Migration Disease

After removing the capacitors, I snipped the contacts on each side so they wouldn’t touch each other, and then glued them back in place with a hot glue gun. I put the transformers back in the radio and soldered replacement silver mica capacitors across the contacts on the bottom.

Finally Done?

This picture shows the bottom of the radio with all the capacitors and resistors replaced.

After

After getting the radio back together, was relieved to hear that it still played although it was badly out of alignment. Only the two strongest stations were audible and even they were quiet. This was to be expected though, since the replacement silver mica caps I used were 100pf and the original ones measured closer to 120 pf. Properly realigning the a radio is a fairly complex procedure that involves using a signal generator to inject a signal of a known frequency and measuring the voltages while turning the tuning slugs in the transformers until the peak voltage is found.

Fortunately, after some feedback from the guys on the forum, I learned that you can mostly align a radio by ear just by tuning in a weak station and listening for improvements as you twiddle the adjustments on the transformers. I played with it for a few hours and got it aligned pretty well. However, I noticed that one of the tuning slugs was hitting the stop at the top of its range, so I decided to order some 120pf capacitors so I was able to align the radio more fully.

At this point, the radio played great for about a week, but then it developed another problem. The audio got really scratchy and distorted but this time it sounded much more like an amplification problem than a reception problem. I initially blamed the problem on the el-cheapo 50V capacitors I had used to replace the ceramic combo pack. People on the forum had warned me 50V was barely adequate for this application and that I should replace them when I got a chance. I put in some 1kV rated ceramics and I was convinced for a while that the radio was fixed, but the problem came back. My next suspect was the amplifier or the detector tube. Tom, who had also been helping me on the forums graciously sent me a few spare tubes he had on hand and I tried them, but the problem was still there.

Scratchy Audio

The problem turned out to be the speaker itself. The paper of the cone was very brittle, and I think the new activity after so many years was more than it could handle. With the cone warped, the voice coil rubbed against the pole of the magnet and shorted out. One suggestion from the antique radio forum was to dampen the speaker and leave it overnight to dry, which can cause it to straighten out and resolve the issue with the misaligned voice coil. Unfortunately the cone was so brittle that it basically disintegrated once I got it wet.

I searched Digi-Key and Mouser to find a replacement speaker that would fit and ultimately settled on the CUI GF1003M, which is a 4 inch speaker with a clear mylar cone. There were a few other 4 inch speakers available, most of them also from CUI, but this was the only one that I was sure would fit. After replacing the speaker, the radio sounded great again, but the new speaker looks a little out of place since it is so obviously not from the 50s.

Also, after putting the speaker in, the dust cap was touching the tuning dial assembly in front of it. To solve this, I pushed the dust cap in slightly to create a dent and prevent it from rattling when the speaker played. It looks a little ugly but you can’t tell when the radio is inside its case.

It’s a little disappointing that I couldn’t salvage the original speaker or find one that looked more like the original. I found a few services on the internet that will recone vintage radio speakers, so I’ve kept the metal basket from the original speaker and I may try to have it re-coned at some point so I can keep the radio a little more authentic.

Here’s the radio scanning the dial after replacing the speaker.

There’s also some bonus footage at the end of my newly acquired Commodore 64 and some breadboards I’m working on. More on those projects in a later post!

Safety warning: there are voltages inside these old radios that can kill you. Before taking on a project like this yourself, please educate yourself, understand the risks and know how to mitigate them.