Before the second world war Radio was a revolution in mass-communication much like the internet today. Fortunes were made and lost, empires built, epic patent battles ensued, all of which resulted in the world being more connected than ever before, which makes for a really great story (and a great Ken Burns documentary).
Last month we showed you how to modify a vintage radio to play your own audio source through it while re-using the existing electronics and maintaining its functionality. In this post we will show you how to restore any vacuum tube radio. You will learn basic repair/restoration procedures from a different era when it was actually worth repairing consumer electronics. Plug into history and get your hands on the most influential technology of the first-half of the 20th century!
In the early days radios were very expensive consumer devices costing as much or more than an automobile or a house in some cases. For this reason many listeners resorted to building their own. Edwin Armstrong developed the heterodyne receiver that used frequency multiplication to shift the desired signal down to an intermediate frequency where it was filtered and detected resulting in reduced costs and increased access to radio technology. Heterodyne receivers cost significantly less than the previous architecture known as tuned radio frequency (TRF), where the radio was simply a very large tunable bandpass filter (3 or 4 sections typically) with amplifiers between each section that had to tune across the entire AM broadcast band. TRF architectures are expensive because to maintain a narrow 10 KHz band pass filter across one full octave is challenging and requires more precision than using only one fixed filter that can be mass-produced and is at a lower frequency.
Danger, Danger, High Voltage!
Be very careful when working with old radio equipment. Yes, these radios have very high voltage potentials inside. Many even tie the hot end of the line directly to their metal chassis (known as Hot Chassis radios), notably most of the post-war table top radios.
Antique radios can be functional statement pieces, showing both your appreciation for the old styles and your ability to repair just about anything. This hobby is well within the reach of anyone with basic electronics skills. Follow these general steps to restore your radio:
- Do not power on your radio. You could cause damage if you power it on before replacing the capacitors. There are several flavors of capacitors but the ones that typically decay with age are the electrolytics (usually found in power supply and on audio output circuit) and the wax-paper coupling capacitors.
- Find a service manual. Most are available online.
- Replace all electrolytic and paper capacitors with new ones of similar value and same or better voltage rating.
- Examine closely and replace anything that looks damaged, such as burned up resistors.
- Test the radio using a dim bulb tester, which is a 60 to 100 watt lightbulb wired in series with your radio’s line voltage. If the bulb glows dim after a while then there are likely no shorts across the B+ lines within your radio. You might even hear a crackle or some signals coming through.
- If it passed the dim bulb tests then try to power it up directly off the line. Most antique radios will work once the capacitors have been replaced.
- If it does not work then signal trace through the circuit by injecting a signal at the IF or RF, replacing resistors or occasionally a tube where needed. Old resistors have a tendency to increase in value with age. Often a bad stage in a tube radio is due to a resistor that has increased in value to the point of biasing the tube into cut-off. Tubes are usually not what caused the radio to be put out of service. The quickest way to find bad resistors is to probe the tube pin voltages. There will be a tube pin voltage chart in your radio’s service manual given a specific volt meter impedance. Wherever you find a voltage out of tolerance (greater than 10%), then check the resistors connected to that pin or to other circuits connected to that pin.
- If the stations are too weak then perform an alignment following the procedure in the service manual.
- Turn on the radio. It should work!
These basics are well covered in the video series that starts with this video:
RCA Radiola 18
Here is a fine example of a late 1920’s TRF receiver restoration. The power supply capacitor was replaced as well as a small number of paper coupling capacitors. Dust and grime were cleaned off. The wood enclosure was cleaned and waxed. For many TRF receivers like this one, an alignment is not needed. Performance was surprisingly good for a very sensitive receiver.
Silvertone Model 4686
Here is an example of the iconic American Console Radio. This radio, manufactured in 1937, was witness to much history: the Great Depression, the Second World War, and probably the Korean war War as well before it was put out of service. This generation of radio was the first to leverage heterodyne receiver architecture. For this reason, these radios tuned across a massive frequency range. Not only could they tune the standard AM broadcast band, they also covered shortwave frequencies from 2-18 MHz. Restoring these radios is not difficult. Millions were made. They are inexpensive to buy at flea markets and radio shows. If you can not find the right part buy another one and salvage it.
By the 1950’s radios were everywhere. FM broadcasting started to gain popularity with its high fidelity. Table top radios such as this one provided both AM and FM broadcast bands in a compact package with 1950’s styling much like automobiles, jet aircraft, rockets, tail fins and chrome, and other exciting technologies of the day. This radio in particular uses negative feedback in its audio output providing booming high fidelity audio. Unfortunately, it is a Hot Chassis radio, where the line cord is tied directly to the metal chassis inside so an isolation transformer must be used in-line with the power cord while servicing.
Learn About History By Repairing It
Time to hit the flea market and pickup a few old radios to work on! Teach yourself the skills to repair vintage electronics and get in touch with the past. Learn unique troubleshooting skills that will enhance your daily career. Tubes or transistors, a seasoned engineer is capable of working with technologies from today and yesterday.
- Join the Antique Wireless Association, read the bi-montly fascinating journal on the history of wireless technology.
- Get help from the forums: www.antiquradios.com
- bandersontv’s youtube channel.
My cousin, Juliet Hurley, MBA, MSF, MAC for type editing this post.
Gregory L. Charvat is author of Small and Short-Range Radar Systems, visiting research scientist at Camera Culture Group Massachusetts Institute of Technology Media Lab, co-founder of Hyperfine Research Inc., Butterfly Network Inc. , editor of the Gregory L. Charvat Series on Practical Approaches to Electrical Engineering, and guest commentator on CNN, CBS, Sky News, and others. He was a technical staff member at MIT Lincoln Laboratory, where his work on through-wall radar won best paper at the 2010 MSS Tri-Services Radar Symposium and is an MIT Office of the Provost 2011 research highlight. He has taught short radar courses at MIT, where his Build a Small Radar course was the top-ranked MIT professional education course in 2011 and has become widely adopted by other universities, laboratories, and private organizations. Starting at an Early Age, Greg developed numerous radar systems, rail SAR imaging sensors, phased array radar systems; holds several patents; and has developed many other sensors and radio and audio equipment. He has authored numerous publications and has received press for his work. Greg earned a Ph.D in electrical engineering in 2007, MSEE in 2003, and BSEE in 2002 from Michigan State University, and is a senior member of the IEEE, where he served on the steering committee for the 2010, 2013, 2016 IEEE International Symposium on Phased Array Systems and Technology and chaired the IEEE AP-S Boston Chapter from 2010-2011.