Antique Radios Chassis Restoration
ęStan Watkins 1997-2000
this link to a page of case histories of problems I have
encountered in my restoration projects
Ultrasonic Cleaning and Nickel Plating Follow these links to find out how to clean and restore those difficult parts.
Special thanks to Robert Lozier for contributing these articles!
Atwater Kent Audio Transformer Replacement An easy and inexpensive way to replace bad AK audio transformers.
These are the steps I recommend for Chassis Repair and restoration. I will add details for each step and a few pictures also.
Removal from cabinet
Determining model or chassis number
Assessing the condition of the chassis
Filter capacitor replacement
High voltage bypass & coupling capacitor replacement
Resistance tolerance check
Speaker cone Repairs
Additional repairs and troubleshooting
Installation in cabinet
1.Removal of radio chassis from cabinet
This step can be very difficult or very easy. Some radios require only removal of the bolts fastening the chassis, others require removal of lots of other parts before removing the chassis. Start with the knobs. Before you pull too hard, look for set screws and loosen them first. You can usually leave pushbuttons on the chassis. Sometimes the knobs are really stuck, do not pry them off with a screwdriver, this mars the cabinet and can break the knob as well. When faced with a troublesome knob, I pull firmly and evenly (sometimes with both hands!). If the knob still refuses to budge, I use a little WD-40 on the shaft.
Now that the knobs are off, determine what you must disconnect from the chassis before removal. Is the speaker fastened to the chassis? If not, disconnect the wiring if possible(usually a plug or terminals). Disconnect any antennas, carefully noting which wire connects to which terminal. On some radios (certain Zeniths and others) the pushbuttons assembly fastens to the cabinet separately from the chassis, this necessitates removing the wires connecting the pushbuttons to the chassis. On these same Zeniths, the tone control assembly is fastened to the cabinet and wired to the chassis without any plugs. You must turn the tone control sideways and push it back into the cabinet before chassis removal.
OK, all the extra stuff (if your radio has any) is disconnected. Find the bolts holding the chassis to the cabinet and take them out. Gently pull the chassis out. Does it slide out easily, or does it seem to be glued firmly to the cabinet? Often I find that the rubber feet on the chassis have deteriorated, gluing the chassis the cabinet. If your radio chassis seems glued to the cabinet, make SURE you have all the bolts loose, then gently pry up the edge of the chassis until the gooey rubber feet let go. Once the chassis is out, remove the speaker from the cabinet (if the speaker is not mounted to the chassis). Note: if your speaker is wired directly to the chassis you must remove them at the same time. Be careful not to poke holes in the speaker cone.
My favorite tool for radio chassis cleaning is an air compressor. I use 100 psi air with a trigger operated air nozzle. The high pressure air removes the thick layers of dust. I use a 1" paintbrush to help loosen the dust and dirt as I blow compressed air on the chassis. I remove the tubes and tube shields and clean them separately. Careless or inexperienced use of high pressure air damages radios, you may want to start with 40 psi. Improper use of high pressure air ruins mica trimmer capacitors, bends tuning capacitors and breaks speaker cones. If you use compressed air to clean a chassis, do not blow air directly on the speaker cone or trimmer capacitors. The sheets of mica in the trimmer capacitors are fragile and will shatter if you blow high pressure air on the edges. High pressure air is the most effective method I have used for removing small conductive particles from between the tuning capacitor plates. It works great as long as you do not blow on the flat sides of the plates (it can bend them sideways). If you do not have an air compressor, you can use a vacuum cleaner along with a paintbrush to loosen the dust. You have to work harder with the paintbrush when using a vacuum cleaner.
I clean tubes with 0000 extra fine steel wool. The steel wool makes the glass look new very quickly. Steel wool will completely obliterate the numbers and any other writing on the tube, so do not rub the numbers! Bakelite tube bases clean well with a mild water based cleaner.
The best advice I have for dial cleaning is: BE CAREFUL! Many dials will loose all their numbers when cleaned with plain water! Some glass dials clean up very well with steel wool, but this could remove the numbers, it depends on the dial. If you must clean a dial, experiment with a small part that will not show when assembled. Frequently, I clean only the side without the numbers and leave the numbers alone.
Ultrasonic cleaners are best for cleaning corroded and dirty metal or ceramic parts. See the article by Robert Lozier: Ultrasonic Cleaning of Old Radio Parts
3. Determining Model Number
So you think this is easy? Sometimes it is. The most common locations for model numbers are the rear of the chassis, the top of the chassis, and the inside of the cabinet. If your radio is one of these and the numbers are still legible, then you don't need to read the rest of this section. Unfortunately, papers come unglued and fall out of cabinets, and numbers printed on chassis become illegible. Before you give up looking for a legible number, look ALL over the inside of the cabinet and the chassis. Model numbers sometimes appear on the bottom of the cabinet or the front of the chassis. Still no model number? Read on.
The first step in determining model or chassis number is finding out who made the radio. Look at the name on the front of the radio. The name on the front will tell you who made the radio or give you a good clue. Next, find out how many tubes the radio has, and the type of tubes (e.g. 6A8, 6K7, 80, 6Q7, 6F6). Some radios have tube numbers stamped on the tube sockets. The type of tubes will give an approximate date. Now you need to get access to a set of Riders Perpetual Troubleshooters Manuals or some other resource listing tubes and model numbers. The rest is a process of looking at many schematics or tube listings and finding one that matches your radio. You can double-check your results by determining if the radio really includes all the parts listed in the schematic. Riders Manuals are now available on CD ROM. Consider purchasing a a set of Riders if you are serious about repairing or collecting antique radios.
4. Assessing the condition of the chassis
You can get a rough idea of the scope of repairs your radio needs by a good visual inspection. You have removed all the dust at this point, so let's start looking at what was underneath all that dirt. First, is there anything obviously missing? Look at the top of the chassis. Large holes in the chassis and empty tube sockets are a good indicator that something may be missing. Some radios came with extra unused holes in the chassis, so this is not always a good indicator. I've seen many radios with holes in the chassis where the old electrolytic capacitors were removed and replaced with newer units located under the chassis. Look for loose ends of wires, another good indication that something is missing. Double check the schematic for the parts that the radio should include. Does it include a power transformer? If so, does the transformer look like it has been overheated? Burnt paint, and excessive amounts of tar or wax drips are signs of overheated or failed transformers. How many coils should the radio have? (some coils live on top of the chassis, others lurk underneath) Look for the audio output transformer, it may be under the chassis, on top of the chassis, or on the speaker frame. While you look for the audio transformer, look at the speaker also. Is the speaker cone intact? You can repair even large tears. If much of the cone is missing, you will need to have the speaker re-coned.
If your radio is complete and free from obvious problems, you can move on to preliminary testing!
5. Preliminary Testing of your Antique Radio Chassis.
Now comes an exciting step! When I began restoring antique radios, I skipped some of the necessary safety precautions and just plugged radios in. I always asked the questions- will it work?, will it just be dead?, or will it smoke and spark? I ruined a number of rectifier tubes in this manner and started a few small fires! So, before you start, make sure your radio has a good line cord, and read Safety and Antique Radios. When you come back to this page we will get started on testing your Radio.
The most important piece of equipment in this step is the Variac. The Variac is an adjustable transformer. Given a 115V input, my Variac will give me a variable output from 0 to 140V, continuously adjustable by a single knob on the front. The Variac is useful since it allows you to apply a low voltage input to the radio for testing. This low voltage allows you to test your radio without damaging it - even if there are electrical shorts in the radio. You can order a Variac from Aircraft Spruce and Specialty.
Set the Variac to zero and plug the radio into the Variac. Remove the rectifier tube. The first test assesses the condition of the power transformer. If the radio does not have a transformer, skip this step. Turn the radio on and slowly bring the voltage up to about 70 volts. Make sure nothing is overheating. Carefully measure the voltage from the power transformer a the rectifier socket. The high voltage will be around 200 to 400 volts. If nothing looks or sounds suspicious, adjust the Variac to 115 volts. Leave the power on for approximately one-half hour. Watch the radio to make sure nothing is getting too hot. The power transformer should be warm, but not hot in this test. A shorted power transformer will heat up even with the rectifier tube removed. If the transformer passes the test, proceed to the next step.
Connect an antenna to the radio if the radio requires an external antenna. Turn the radio on. Slowly bring the power up to about 50 volts. Make sure the rectifier tube is not getting too hot (if the plate glows a dull red, it is definitely too hot!). Check the B+ (high voltage from power supply). The B+ voltage should be about one third of normal. If it is zero, turn the radio back off. If all is well bring the power up to about 70 to 80 volts. Check the rectifier tube again and the B+ voltage. If the radio works at all, you will hear it begin to play. If the radio plays, then you may only need to replace capacitors for it to operate safely and reliably. If the radio makes no noise at all, but has reasonable B+ voltage, the audio transformer may be bad or one of the audio circuit capacitors may be shorted.
"But I don't have a Variac, what can I do?"
If you do not have a Variac, you can wire a 250Watt heat lamp in series with the radio for testing. The lamp should not light up, a little glow might be OK.
6. Filter Capacitor Replacement
Filter capacitors smooth out the waveform of the DC current produced by the power supply. Many old radios came equipped with liquid filled filter capacitors. Filter capacitors are frequently labeled "electrolytic" on the schematic diagrams. My rule of thumb for filters it to replace them unless someone else has already replaced them recently. Shorted or leaky filter capacitors ruin power transformers, rectifier tubes and speaker chokes - often difficult and expensive to repair.
Check the schematic diagram to find out how many filters the radio has and what the values are. Use the same value filter for the capacitor closest to the rectifier tube. Using a lower value will increase the hum, a higher value will cause the voltage to be higher than normal. You can replace the other capacitors with ones of equal or slightly greater values.
Do not discard the old wet capacitors from the radio chassis. Leave the original cans on the chassis for the sake of authenticity. You can mount new capacitors under the chassis, or you can cut open the old capacitor and install the new one inside it. Either way, make sure the repair is neat and safe (insulate all connections). When I install new capacitors inside the original cans, I cut the can open at the top, or wherever a cut is the least noticeable. Once the new capacitor is inside the can, I glue the top back on using gap filling super glue or Duco cement.
Potted (encased in wax or tar filled cans) capacitors present a little more challenge. First, remove the can from the radio, marking all the wires as the can is removed. Many potted capacitors are soldered closed, so the first order of business is to get inside the can. Dynamite is the quick and easy method of opening potted cans. Dynamite is a tad dangerous, so I recommend a large soldering iron or a small propane torch. Once the can is open, you still have to get the potted capacitors out. The best way to get the capacitors out is by heating the entire assembly in your oven. Before you heat the capacitors, send your wife out shopping. Heat the can at 200 to 250 degrees F. When the wax or tar starts to become soft, remove the can from the oven and slide or dig the mess out. Make sure you clean up every trace of this mess from the oven! Use aluminum foil and an old cookie sheet to keep potting material off the burner. Fire is always a hazard in this type of operation, so keep the heat low, and have a fire extinguisher handy.
Once you have and empty can, install new capacitors (make sure the wires are not shorted to each other or the can). If you are a real purist, refill the can with the wax or tar you removed earlier.
7. High voltage bypass & coupling
Most Antique radios contain quite a few capacitors manufactured if wax, paper, and foil. These wax and paper capacitors fail with age and use. The wax tends to flow out, allowing moisture in the capacitor. As the capacitor ages and absorbs moisture, it starts to act like a resistor, or even worse, like a piece of wire. Leaky or shorted capacitors in high voltage circuits overheat and destroy resistors, field coils, power transformers, tubes, and other components. So, which ones will you replace? You could just replace them all (many choose this option) or you could replace all that must take high voltage (more than 20V), or you could pick a strategy somewhere in the middle. Lets look at these three options:
- Replace all wax/paper capacitors: A good option from the safety and reliability point of view. Easy from a technical point of view, you don't have to know which capacitors are most likely to fail. Disadvantages: more work and cost, less original parts left.
- Replace only high voltage capacitors: Easier if you know which ones see high voltage. Careful analysis of the schematic will tell you this. Disadvantages: You have to know how to read the schematic very well. Sometimes low voltage capacitors will cause trouble also.
- Replace all high voltage capacitors and some others: I replace all high voltage capacitors along with most of the capacitors in the audio section of the radio. Additionally, any capacitors that look bad (lots of wax leaking out or otherwise damaged) or test bad (shorted) are replaced also. This alternative offers reliability without replacing everything. Disadvantages: More technical knowledge and experience required to know what to replace.
Make sure you replace capacitors with ones close in value to the original (in Microfarads) and with an equal or higher voltage rating.
8. Resistance Tolerance Test
Most resistors remain within tolerances, but enough of them change value to make testing worth the effort. The standard tolerance for resistors in most old radios is 20%. Replace any resistors that are more than 20% off of their rated value. Replace any burnt resistors, even if they still have the proper resistance. Shorted and leaky capacitors overheat the resistors. Atwater Kent radios made in the 1930s normally contain many out of tolerance resistors. The AK resistors change value over time. Any high value resistor should be suspect as they tend to change value more than the lower value resistors. Radios with tuning eyes usually have a bias resistor in the socket of the tuning eye. I have found the tuning eye resistor bad in 80% of the radios I restore. When replacing resistors, use the same value in ohms (or close) and an equal or greater wattage rating. A resistor with a lower wattage rating will overheat.
9. Speaker Cone Repairs
Most speakers need minor repair or adjustment. I always try to repair a speaker before resorting to re-coning. You can repair large tears and holes in the cone as well as loose cone edges. An old speaker rebuilder once told me to glue torn speaker cones with RTV (silicone caulk). I thought this was messy and "unprofessional" , but I found that it works great! RTV looks really bad, but unlike other glues, it remains flexible. Gluing a speaker cone with a rigid glue often just results in another rip since the glue will not flex with the rest of the cone.
Speaker cones frequently come loose at the outside edge where the cone meets the frame. Loose cones edges cause loud rattles in the speaker. The chief difficulty in re-gluing the cone edge lies in getting the glue in the right spot. I use a syringe and needle to get glue under the edge of the cone. Woodworkers glue works well for this since it flows easily and cleans up with water. Other suitable glues include super glue, Duco cement, or RTV.
Many speakers use a spider to center the voice coil in the magnet. Often, the speaker voice coil is rubbing the magnet, causing buzzing and distortion in the speaker. Sometimes, you can cure this by adjusting the spider. Carefully loosen the bolts holding the spider in place, and move the spider to a position where the voice coil does not rub the magnet. Retighten the bolts to finish the job. This may take quite a few tries to complete a successful repair. The speaker spider is located in the center of the speaker where the coil meets the cone, or on the outside of the voice coil where it meets the cone. Spiders on the inside of the cone use a single bolt fastened to the magnet, spiders on the outside of the cone fasten to the speaker frame with several bolts. Newer speakers (1940's and later) use a combination spider/dust cover on the outside of the voice coil. These covers are glued to the speaker frame. When the glue ages and fails, the loose cover rattles and buzzes. Re-glue these using the same techniques as for re-gluing the cone edge. Make sure the voice coil is centered before gluing.
10. Operational check
All the repairs done? Will the radio work? Now it's time to find out! Hook up your variac, set it for zero volts. Turn the radio chassis on it's side or in some position where you can measure the high voltage in the power supply (other wise known as the B+ voltage). Plug the radio into the variac and turn on the radio. Slowly bring up the voltage to 80 volts. At this point, your radio may begin to play, if not don't despair. Measure the B+ voltage, it should be around one half of the normal reading. If the voltage is zero or very low, turn the radio off and look for the cause. If the radio seems OK, examine tubes, resistors, transformers, capacitors, and the speaker field coil for overheating. Once everything passes your scrutiny, attempt to tune in a strong local station. Gradually bring up the voltage to 100volts (I'm assuming a line voltage of 120volts). The radio becomes louder and more sensitive as the voltage comes up.
11. Additional Repairs and Troubleshooting
If the radio did not work properly, then it's time to dig a little deeper to repair the radio. I start at the audio section and work back through the radio. If the radio has a first audio tube with a grid cap (75, 6Q7, 6F5, etc) touch your finger to the cap and you should hear a loud hum. If there is no hum, the problem is somewhere between the first audio tube and the speaker. The best way to check the audio stages is with a signal generator that supplies a modulated audio signal.
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