Diodes and demodulation

As we described before, a diode lets current flow in only one direction. This is essential for demodulation: the process of extracting information from a signal. To understand demodulation we need to first understand the first and simplest kind of radio transmission -- amplitude modulation.

The goal of radio is, ultimately, to transmit sound over very long distances. A band is playing in New York and I would like to hear it in San Francisco. One way to do this is to build a sound amplifier so loud that the sound waves themselves travel directly from the origin to my ear. This is obviously impractical: it would be intolerably loud at the origin and barely audible at the destination; furthermore, you could not have multiple radio stations broadcasting at the same time, they would all clobber each other in a cacophony of noise.

Another way to do this is:

• Convert the sound wave (anywhere from 1 Hz - 10 kHz) to another equivalent wave (measured in hundreds of kHz or even MHz).
• The equivalent wave, or modulated wave, contains the original sound wave information but in a different representation.
• The carrier wave is not audible to the human ear since it is in a totally different frequency spectrum.
• Sound waves travel by making the air vibrate. The amount of energy required to make air vibrate over long distances is enormous (think loud rock concert).
• Higher frequency waves are electromagnetic waves. They can travel much longer distances using much less energy.
• Once the modulated wave arrives at my radio's antenna, the radio translates this modulated wave back into a sound wave that I can hear. This is called demodulation, and it is made possible by diodes.

 Let's first see what a modulated wave looks like (courtesy of yourdictionary.com):

The carrier wave is basically the radio station frequency. When you tune into KFRC, you tell the radio to look for sound information embedded in carrier frequency 1550 kHz.

The modulating wave is the sound. This is what is embedded into the carrier frequency and, ultimately, the "information" we want to hear.

The modulated wave is the combined wave that travels from the radio tower to my radio. Visually, it roughly looks like a combination between the carrier wave and the modulating wave, which should hopefully agree with your intuition and some of the descriptions above.

Now, we wish to turn this modulated wave into sound. To understand how that works we need to first understand how a loudspeaker works, as shown on this diagram (courtesy of soundonmind.com):

• The magnet provides a fixed, constant magnetic field.
• The signal input provides the sound wave we wish to ultimately hear.
• When the signal input goes into the voice coil, the voice coil becomes an electromagnet.
• The voice coil's magnetic field "pushes against" the magnet's field, based on the strength of the signal input.
• The voice coil is attached to the diaphragm, which is basically a piece of cardboard.
• When the voice coil moves, the diaphragm moves, and pushes air to varying extents, generating a sound wave we can hear.
• If you've ever touched a loudspeaker that was playing music, you can actually feel the movement of the diaphragm with your fingers.

Let's step back and look at the complete picture:

• We have a modulated wave that contains the sound information embedded in a carrier wave. This modulated wave has very high frequency, measured in hundreds or thousands of kHz, so it is not audible by the human ear.
• We have a loudspeaker that can convert a wave into sound by vibrating a piece of cardboard.

What would happen if we feed the modulated wave directly into the loudspeaker? Think about it for a minute before reading on.

The answer is: absolutely nothing:

• The modulated wave has very high frequency, which means that the "peaks" and "throughs" come in rapid succession one after the other.
• When a "peak" arrives at the voice coil, it starts to move the voice coil out; this takes a bit of time, as the voice coil has to physically move in order to push the diaphragm and make a sound wave.
• However, a "through" quickly follows the "peak" and starts to pull the voice coil back in the opposite direction.
• The "peaks" and the "throughs" effectively cancel each other out as far as the diaphragm is concerned and no sound comes out the speaker.

What would happen if we feed the modulated wave through a diode first, and then feed the output from the diode to the speaker? A diode lets current flow in only one direction, so the modulated wave would basically be cut in "half":

Now, if we feed the demodulated wave into the speaker:

• The first peak will start to push the voice coil out.
• There is no through following this peak, simply an empty space, or "absence of signal".
• How is absence of signal different from a through?
• A through is a negative signal -- it starts to pull the voice coil in the opposite direction.
• Absence of signal is no signal -- the voice coil is left where it is will at most react based on its own inertia or the elasticity of the diagram.
• The next peak will start to push the voice coil further out.
• As you can see, the voice coil (and the attached diagram) react only to the peaks, in other words they both move according to a wave that follows the top of the peaks.

The wave that follows the top of the peaks is our original sound wave! Look back at the first diagram to see it, or just trace the peaks above with your finger.

The coil and diaphragm end up vibrating the air in accordance to the original sound wave, therefore producing sound we can hear. The diode demodulates the modulated wave back into sound we can hear.

Vacuum Tubes

Part of the reason I worked on restoring this old radio is because I wanted to learn how vacuum tubes work, as well as how radio transmission works.

The simplest vacuum tube is a diode -- a device that allows current to pass only in one direction:

• Green element = the heater. This is a filament that gets hot when current flows through it. Its only purpose is to radiate heat onto the red element = the cathode.
• Red element = the cathode, or the emitter. This is a filament coated in a special substance that can emit electrons when it's heated up.
• Blue element = the anode, the collector, or the plate. This is a flat piece of metal which collects the electrons emitted by the cathode.

The key feature of the diode is that current can only go from red to blue, not the other way. In other words, if the heater is hot, then current can flow from red to blue as shown above. However, if we flip the polarity of the battery (the + would be connected to the red element), no current can  flow.

Notes:

1. The heater circuit operates at low voltage, around 5V.
2. The anode/cathode circuit operates at much higher voltage, frequently above 200V (in my radio, the anode/cathode rail goes up to 750V). This is because, even in a vacuum, the voltage has to be high enough to force electrons to jump across the small gap between the two leads. This high voltage makes older appliances dangerous, they can definitely kill you if you're not careful.
3. The reason tubes are vacuum'ed is because the air molecules get in the way of electrons "jumping" from the cathode to the anode.
4. All vacuum tubes "wear out" in time: the substance that covers the cathode is literally stripped away and eventually stops emitting electrons entirely. This is why vacuum tubes in old appliances had to be replaced every so often (typically measured in years, but it depends on how heavily the device is used).

This is it. All vacuum tubes are variations on this theme. They contain various additional elements that serve to amplify or dampen the flow of current between the cathode and the anode, but the basic principle is the same.

So why is a diode useful? Who cares that we have a device that lets current flow in only one direction?

To answer that question we need to look at the simplest form of radio transmission: Amplitude Modulation, or AM. More on that in the next post.

Radio Frankenstein

About 6 months ago I wrote the first post about my classic radio restoration project. The project is done, the radio is functional, but I somehow never got around to writing anything more about it. I'm going to try to fix that in the next series of posts.

I read a few useful guides on how to get started restoring an old radio. Bringing a radio up to life for the first time is risky-business, since the radio components can become compromised over time, and you run the real risk of burning up the radio (literally) if you're not careful.

The most useful I found was from Phil's Old Radios, recommending roughly the following:

• Spot gross defects first: leaks, stains, smells, etc.
• The electrolytic capacitors are probably dead: replace them.
• Use a variac to slowly turn the radio on.

At the high-level, my Philco was in reasonable shape: it was missing two vacuum tubes, but aside from that it wasn't leaking or showing other signs of gross physical damage.

I proceeded to test all the capacitors in the radio: there's about 10-15 of them total, so it's not that hard. The most important test for a capacitor is to verify that it's not shorted, in other words the resistance between its two leads should be infinite. Over time, electrolytic capacitors dry-up (literally), and as a result short circuit the leads, which can be catastrophic (it can burn up the power transformer, which is hard and expensive to replace).

My Philco has two kinds of capacitors:

1. Two very large electrolytic capacitors (8 muF each). These are visible on the top of the main board, and serve to smooth the DC coming out of the rectifier bridge. It is essential that these not be shorted as I explain above. Sure enough, when I measured them, one was shorted, and the other was on its way. I bough two replacement Sprague Atom's, disconnected the leads from the existing electrolytics and connected the new capacitors in place. Incidentally, the Spragues are very solid, high-quality capacitors, great to work with. Here's a sample:


2. Bakelite capacitors: these look like a little "vat" with 6-7 leads. Inside it are 2-3 capacitors, sealed in a hard, black, gunky insulator. Fortunately, none of the bakelites were bad, they all tested good on the multi-meter and, fortunately, ended up working in the end.

Serious radio enthusiasts try to "hide" the new capacitors inside the old electrolytics, in order to make the restoration even more authentic. This is a process called "re-capping", it works like this:

1. Use a Dremel to cut open the bottom of the electrolytic capacitor
2. Gut its contents, leaving an empty aluminum shell
3. Insert the new electrolytic capacitor in the old shell, solder each end to the two leads
4. Glue the bottom of the electrolytic back with epoxy glue

I got so far as step 2 above: the "gunk" inside my electrolytics was a very nasty, black, foul smelling tar-like substance that wouldn't come out no matter what. I read that some people use a hair-dryer to literally melt this stuff out of the shell, but I decided that was too much for me, so I just put the electrolytics back on the circuit board (without the bottom) and left it at that.

The next step was to replace the missing vacuum tubes. I got the complete set of schematics and repair bulletins from the excellent Philco Repair Bench. It was easy to detect that the two missing tubes were:

• Vacuum tube 80 -- the rectifier
• Vacuum tube 42 -- the power pentode

These are ancient tubes, in fact the names alone should give you an idea: the tube manufacturers simply started to count at 1 and worked their way up to around 100 or so, each number representing one type of tube. There were only a handful of tube manufacturers at the time (Philco, Tung-Sol, RCA), and they made a majority of all the tubes on the market.

I thought I'd be totally out of luck finding replacements for them. In fact, it turned out quite the opposite: there a vibrant community of old-timers who sell tubes like these for cheap. I was able to find both tubes for around $15. Not only that, but I was actually able to find them NOS = "New Old Stock", which means that the tubes were in mint condition, they had never been used! Imagine: these are tubes made 80 years ago and they still work. Here's what the rectifier looks like:


Notice how it says "Made in USA" on it. When was the last time you saw that printed on anything?

With both tubes in their sockets, now came the real test: turning on the radio. I don't have a variac, and I thought it too expensive to buy one. So instead I built a cheap home-made one: a dim-bulb tester. I first tested the radio with a large 100W bulb, and then with a smaller 45W bulb. In both cases the vacuum tubes slowly started to glow, and no weird smells or pops came out of the radio, so I decided it was safe to plug it directly into the wall.

Imagine my surprise and awe when, after 15 seconds of warm-up, the radio actually went on and started hissing! I kept a finger on the antenna lead, and was able to tune into a few AM radio stations (one was broadcasting a baseball commentary, and another had a show about aliens invading the earth).

A beautiful thing.

How I chose the radio

Why did I choose this particular Philco model? The reasoning was not particularly scientific, but in retrospect it turned out to be a very good choice.

I'd spent some time browsing (and drooling over) various old radio galleries, to get a sense for what's available. Roughly speaking, here's my impression about the lay of the land:

• Wood radios vs. plastic radios. As much as some plastic radios are beautiful pieces of Art-Deco inspired art, I just didn't like them as much as the wood radios. Some particular brands of plastic radios, those made of the Catalin brand of plastic material, are extremely sought-after and expensive -- as much as $2000 or more for a radio in good condition!
• Cathedral vs. tombstone vs. console vs. tabletop. The console radios are quite large and unwieldy, so they were out of consideration for me fairly quickly. I was somewhat torn betwen cathedral and tombstone design -- I like them both, however, in the end I decided to go with a cathedral design since I find the design more appealing and timeless. The tabletop radios, while very nice in their own right, didn't quite have the vintage look I enjoyed.

vs.

• Manufacturer: Philco vs. Zenith vs. Motorola vs. RCA vs. Sparton. There is very large number of manufacturers that produced radios in the "golden era", most of them gone today. In the end, I decided to go with a Philco radio because it is one of the best and largest radio producers, they also produced many of their own radio parts (similar to RCA), and I quite liked their designs (the Philco 90, in particular, is considered by some "the" classic cathedral design).
• Last but not least: complexity. Since this was the first radio I'd try to restore, it was important that the electronics weren't too complicated. I definitely wanted a vacuum-tube based radio (as opposed to transistor), and also wanted a radio that was made in the late 20's - early 30's (earlier than that may have meant too much compromise on audio quality, which would have been a problem as I'd actually like to use the radio around the house).

In the end, the Philco 80 turned out to be a surprisingly good choice:

• It's a Philco wooden cathedral design. While by no means the most ornate, it still looks very good and embodies many of the design qualities I like.
• It's very simple electronically. It only has 4 tubes, compared to 7+ tubes for many of the "fancier" models. It has modest energy consumption (46 watts), and it is fairly well documented and understood.
• As it turns out, the Philco 80 Jr. was meant to be one of the cheapest cathedral design radios at the time -- which explains the complexity and power consumption above. The radio was introduced for $18.75, whereas other radios would start at double that.

Depending on how this restoration goes, I may look into a tombstone design next, as there are some very good looking radios in that category as well.

An old project about an old radio

For a while now I have been thinking of getting an old radio and restoring it back to life. The reasons are because I love the look of old radios, it would be a fun electronics project, and a good excuse to learn how more about how radio broadcasting works.

I finally took the plunge a few weeks ago and bought an old Philco 80 Jr from eBay. The radio is in great physical condition, especially for a radio made in 1933:

The internals also looked good (the seller made it clear that the radio was not plugged in and may or may not work upon arrival):

Upon closer inspection, the radio had a number of issues:

• It is missing two tubes: the power pentode (tube 42) and the full-wave rectifier (tube 80).
• The cap connector on one of the oscillators (tube 36) was detached and had come off.
• It had no power plug, and there were two strange wires coming out of the back of the radio.

You can see the real state of radio from this photo I took from my workbench (notice the missing tube sockets, and the broken cap):

Before getting the radio, I'd done some high-level reading about how to restore an old radio, in particular Phil's excellent guide, so I knew roughly what to expect, in particular not to plug it in until I can run a few tests.

One of the things I find amazing about such old radios (besides the fact that they're 80 years old) is the fact that this particular radio may well have played broadcasts from WW2, from the lunar landing, and other momentous times in history. I think there is something incredibly cool about that.

This post will be the first in a series that will document how I'm going to restore this old radio and what I learn along the way. Hope you enjoy reading it as much as I will enjoy writing about it!