Crystal Radio Construction Hints

Front Panel Prep

With this page, I will show you some of my ideas that I use to make my crystal sets. I will start with the preparation of the garolite panels. The panels are cut to size with a table saw. Before I make a cut, I put masking tape over where the cut will be. This is to reduce the chipping of the edges. Then I use a file and a sander to smooth the edges. Garolite is pretty easy to work with and so far I haven't ruined any due to cutting.

Next I cover the front with wide masking tape. This is so I can mark where cuts are to be made. I use an adjustable square to measure for the holes. Rounding the top corners is easy to do and makes the set more attractive. I use a round object to trace the cutting line. The cuts are made with a hacksaw, making three cuts on each corner. The first cut is made at a 45 degree angle. I make two more cuts on each edge to give a start to the rounded shape. I use a file to finish the corner. Stepping back and looking at the edge will give you an idea where to continue to file. Don't try to make it too perfect. Every time I try to make that "final touch", it turns out worse than when I began that cut.

The capacitor holes are drilled from the back side. Frist, measure up from the bottom and draw a line. I used 3-1/2 inch on this project (set#24). After marking where the capacitor shafts will be, I use a compass and a protractor to mark the holes for the mounting screws. Make sure you have the angles correct. I measured wrong the first time. But before I started drilling, I took that second look and compared it with the capacitor. Oops. Measure twice and drill once. I also draw a little circles around each correct mark as there are sometimes a lot of marks on the panel.

All holes are pre-drilled with a 1/16 inch hole. This makes drilling the bigger hole a little easier. A 1/2 inch forstner bit is used to drill the hole for the shaft and bearings.

Large Litz

The proper soldering of litz wire is essential to your crystal set working properly. Litz has many very small insulated wires within a bundle. All these wires have to be soldered, making them conductive. This section pertains to litz that is "solderable". That is, the insulation will melt when heated.

Larger litz, such as the 3/54/38 (162 strand 38 gauge) is difficult to solder all the conductors properly when all bunched together. I separate the three bundles and solder them separately. This soldering is called "tinning". The smaller bundles can be heated so that even the conductors in the center are tinned. After I have tinned the bundles, I use a small amount of 30 gauge silver plated wire tightly wrapped around the three bundles. Heating this joint and adding a little solder gives you a manageable end that can be soldered to where ever it is to be connected.

The soldering iron can be a pencil type but should be able to heat to 900 degrees. I use an Ungar UTC100 to do the tinning. I crank it all the way up and that way there is enough heat to do the job.

If you want to solder litz to a ground bus, unwrap all the bundles and fan out the wires. Then wrap the wires around the bus. The soldering iron tip then can be placed on top of the wire and then solder can be applied.

You might want to buy a small amount of rosin soldering flux to aid in the tinning of larger litz. The pictures below show some of the details of large litz wire soldering.

Best wishes and 73 from Dave, N2DS.

Schematic Drawings

It is always nice to have a great looking schematic drawing to go with that nice new set that you built. I struggled with this for a while until I found Gary's Tubepad page. Basically what this is a selection of small symbol drawings that you can drag onto a paint program. Then you add the line drawing wires and text. You have a near instant schematic diagram.

I had a little trouble with one symbol overlapping another, thus cutting off part of a symbol. I took Gary's drawings and trimmed them down to the bare symbol. I also converted them to jpeg, but for no good reason as it turns out. I am offering these symbols to you for your free use and distribution. Gary tells me that the paint program has to be set to "transparant" and the original symbols should work fine. He also mentions that once he is done with the schematic, he converts it to gif format for web use. It works better with line drawings.

symbols.zip (about 250k) If you make some new symbols, e-mail them to me and I will add them to this bunch.

I use a graphics program that came with my scanner. It is called PhotoImpact. Also handy is having a dual monitor computer system. One monitor has the file names opened up in Windows Explorer and the other display has the PhotoImpact program running. I can drag and drop the symbols where they are needed.

I like to use 360x270 pixel size for my crystal sets up to 640x480 for the larger diagrams. Each drawing program operates a little different, so experiment.

Radio Restoration Hints

Grille Cloth: I have found that most grille cloth does not need replacing, only cleaning. If it isn't ripped, remove the cloth and squirt some Formula 409 on it. Wait a minute or so then gently rub the 409 around in the cloth. Then take a paper towel and blot up the liquid. Let the cloth dry and take a look.

Paper Capacitors: If your old radio has these, they are probably leaky (a high resistance across the terminals). It is recommended that all these capacitors be replaced. Newer style mylar capacitors may be used. The most important capacitor to replace is the one that connects to the audio output amplifier control grid. These are always a leaky. Put your voltmeter on the grid of the audiio output tube and see if there is any voltage. There should be no DC voltage on the grid (unless the tube is biased through a resistor). But change it. If there is positive grid voltage on this tube, more plate current will flow, causing distortion, extra heat and shortened tube life. The AGC action of a radio can be improved by changing those capacitors. It is always good to replace the paper capacitors on the B+ and screen grid supplies. If those short, it can get ugly. When you have replaced them all, your radio will be more reliable, run cooler and sound better.

Soldering Schottky Diodes

Those little schottky diodes are darn small! Even with my so-so eyesight, I am able to easily solder little wires on the surface mount packages. You need a soldering iron that isn't real hot. Some silver plated kynar wire is used for the leads. A little masking tape is also needed.

Take a small piece of masking tape and make a small loop with the sticky side out. Stick that on your work area. Take a diode and place it on the masking tape with the top side down. Strip the insulation off a piece of the silver plated wire.

Next, touch the soldering iron to the wire and add a little solder. After that, move the part of the wire you want to solder about a quarter inch (6 mm) from the terminal you wish to have that wire soldered to. Once the wire is aligned parallel with the terminal, touch the iron to the wire and move both, touching the diode terminal.

After a very brief time, remove the soldering iron. The wire should stick to the diode terminal. This is called tack soldering. Turn the diode around and do the same thing to the other side with another piece of wire.

Once that is completed, take your diode and test it on a working crystal set. I like to use thumbnuts to fasten the diode to the circuit for easy and fast changing. If you solder the diode in the circuit, leave lots of lead length so the tack soldered end doesn't come off while soldering the wire to the crystal set.