VOLUME 15 AUGUST 1998 NUMBER 8
Radios That Are DOGS!Web Edition
BY FRED GEER
Using an apt metaphor, Fred Geer tackles those "dogs" in everyone's collection, offering suggestions to counter subtle problems he has met along his 50-year journey in radio. If a radio still doesn't work after you've corrected the obvious problems, you have a "dog" on your hands and may want to try some of Fred's solutions. (Editor)
My friend George and I were talking about radios that refuse to work properly. George said, "I know that some radios that I have worked on were shipped from the factory in nonworking condition. Too many sets that I buy to work on have so many things wrong with them that they must have been nonworking from the start."
After thinking over what George had said, I concluded it did have a basis in reality. A seller might just put such sets in a dark corner, take his loss, and forget them until a fellow collector discovers them years later.
Fifty years ago, I began tinkering with radios, adjusting those little screws that made them work better on certain stations. As I learned more about radio, it seemed clear that most nonplaying radios had only one bad part.
Now, 50 years later, that theory is no longer true. Heat, humidity and time have begun to have some strange effects on the radios that cross my workbench. The harsh climate here in Florida seems to be accelerating multiple radio problems making them all DOGS!
Over the years I have compiled a list of things that take the BARK out of those DOGS. Every radio is suspect now of having one or more defects beyond the obvious. Using a time proven technique, I find that 99 percent of the radios will work like new, cosmetics aside.
I use the following test instruments: an analog volt ohmmeter, a signal generator, a simple signal tracer, a signal injector pen, a resistor/capacitor substitution box, a good tube checker, contact cleaner, and plenty of alligator clip test leads to substitute other parts. A must is service information from "Rider" or "Sams."
Before You Buy
You can test that all important power transformer before buying a radio. Pull the rectifier tube. Power up the set, checking to see that the other tubes are glowing and that the power transformer is not making any strange sounds. Use a voltmeter to check AC voltages at the rectifier tube socket from the top of the chassis. If there is no voltage, run away before that DOG bites. If no AC power is available, use an ohmmeter to measure the resistance of the transformer windings. With all tubes and dial lamps removed, these measurements can be made at the tube sockets and the line cord's plug.
Taking the BARK out of DOGS
If it is old enough to be an antique, I suggest replacing every paper and electrolytic capacitor with a new exact value component. On AC sets, I use 600 working volt papers and 450 VDC electrolytics. In the past, mica capacitors have seldom presented any problems, but a new mica or ceramic disk won't hurt. AC/DC sets get 600-volt papers and 160-volt electrolytics. An electrolytic used in the audio section should be replaced with exact value and voltage.
Radios That Are Dogs.
[Although blanket replacement of all capacitors may be the best approach, A.R.C. believes that it may be overkill in some cases. Instead, follow the guidance provided in items 1 and 4 below. Replacing only the critical capacitors will get the radio playing. Whether you decide to replace all capacitors as part of a complete restoration will depend upon your skill level. If you do replace all of the capacitors, it's a good idea to replace them one at a time and one lead at a time to prevent wiring mistakes. When in doubt, make a sketch of the capacitor's connections before proceeding. A loud hum may indicate an immediate need for replacement of the electrolytic capacitors. Shorted paper capacitors may cause resistors to overheat. Such obvious capacitor problems should be corrected before proceeding with the other troubleshooting techniques.]
Simple Tests First
1. You are now ready to find out what else is wrong with the radio. In AC sets, pull the rectifier tube before powering up. Let the remaining tubes heat up and then insert the rectifier just long enough for it to become active. Watch the rectifier for evidence of excessive current. Symptoms of excessive current or a short circuit may be bright flashes, a purple glow, or overheated plates that glow red. If any of these symptoms appear, cut the power or pull the tube fast and check over the components in the power supply. Most of the time, if you have replaced all of the capacitors, the set will play like new; however, at this point, the DOGS begin to BARK.
2. The best test for tubes is direct substitution with a known good tube. Some tubes refuse to work in certain circuits, so try several. The tube pins, as well as the tube socket, may have corrosion or rust on them. Clean them with contact cleaner and fine sand paper. The wires inside the tube's base may have a poor solder connection that keeps the tube from working. Remelting the solder in the tube pins can make a bad tube good. Poorly made sockets may have a break between the tube pin connection and the solder lug. Rock the tube around in its socket or jump with test lead from the lug to the tube pin.
3. Missing tube shields can be replaced with aluminum foil wrapped around the tube and grounded to the chassis. A shield is used to stop hum and undesirable interstage coupling. Example: An Emerson U5A would not play anything but squeals when on a station. I found that the original shield was missing, and that a foil cover made the radio play as it should.
4. You must use all of your senses. Look for a wisp of smoke, and smell for overheated parts. Listen for strange sounds. Hopefully, none of those symptoms show up. If they do, cut the power and do some detective work with the ohmmeter looking for shorts in wiring, coils and resistors. Look especially for parts that connect from B+ to ground. It is OK to power up from time to time to help isolate the offending part.
5. Once everything seems to be working, put your finger [via a metal probe] on the center volume control lug. Does it give a robust hum? If it does, the audio section is working. This test will give you an idea what next to consider.
6. Does the oscillator work? Get another radio and set it to a clear spot on the dial around 1200 kHz. I use a transistor radio and hold it close to the offending radio. Tune the suspect radio, listening for its local oscillator on the transistor set as you tune up and down the dial. A faint whistle indicates that the radio's oscillator is working.
7. Offending resistors may check good but may change values under load. Resistors made from 1942 to about 1950 are the worst for value change. Those from 100,000 ohms up love to go up in value. I suspect internal heat does this. Find the bad ones with an ohmmeter or with a resistor substitution box.
Be very careful selecting replacement resistors. Use the same value and wattage rating. Sometimes the working temperature is very important. Example: a 1948 Philco FM/AM/SW oscillator section had 1/4-watt 1-meg ohm resistors that ran very hot. They were replaced with 1/2-watt resistors. However, I now had a DOG with frequency drift. The 1/4-watt resistors' heat stabilized the circuit by preventing drift.
Resistors that go down in value are harder to find. Take voltage readings at the tube socket. When improper voltage is detected trace back to the voltage source. All resistors in that line become suspect. Replace them one at a time with a known good resistor.
Check any wire replacements with an ohmmeter to be sure it is just wire and not a resistor that looks like a wire.
When resistors make noise (crackling, popping and static), wiggle every resistor to see if the noise can be produced at will. My signal tracer has a noise-generating voltage. With the set powered down, hooking the probe and ground connection to each part sometimes causes the part to act up, be it resistor, transformer or any part.
8. Wires and solder joints can help a DOG BARK. Some strange sounds can be generated by a wire that is incorrectly located in the chassis. Just a slight wire movement during the work process might cause hum, motorboating and squeals to appear or disappear. That goes for parts location too!
Over time, rubber insulation develops cracks and crumbles, exposing the wire to hidden shorts. A worse offender is wire shielded with a braid or spring. The internal conductor may touch the shield causing a short. Also heat can melt the insulation.
9. Fine wire and old solder in the presence of humidity can sometimes cause an acid that eats fine wire. Reheating the joint cures it most of the time. Otherwise the wire must be cleaned, tinned, and resoldered. This happens in audio transformers, IFs and RF coils.
Here's an example of a new problem with audio transformers: The radio played fine at very low volume but became very annoying at room-filling volume with static, crackling, and a raspy sound on high notes. It checked perfectly, but the noise tester indicated that the static sound was coming from an interstage transformer. Unwinding the transformer revealed that several turns had insulation breakdown, but perfectly good wire [no copper oxide].
10. Moving parts, including switches, variable condensers, volume and tone controls, and trimmer condensers, have hidden defects that will drive you crazy. I hate rotary switches the most. Some have silver contact points that turn black with silver oxide and other corrosion effectively stopping the radio from working. Ideally taking the switch apart and polishing cures the problem. I leave that job to the truly dedicated. Tuner cleaner spray works for a time, and working the switch will keep it clean. Switches are designed to be self-cleaning through use.
Variable condensers may corrode (a la Philco), develop aluminum oxide (the white bumps and powder), and have bent plates. Those with very close spacing may have metallic dust from the atmosphere trapped between the plates. Disconnect the wires going to the variable condenser. Hook an ohmmeter between the stator and rotator plates. Moving the condenser through its range will give an indication if and where a short exists.
For example, an RCA Model K-80 uses a very large condenser frame with a tiny, closely spaced variable condenser and mechanical push buttons. Repeated hard use of the push buttons causes the frame to warp throwing off the plate alignment. The cure is to cover both sides of each plate with scotch tape -- a really trying job.
Taking the condenser off the chassis and soaking it in Fantastic overnight will loosen all the trash. Scrubbing with a stiff artist's brush will remove the difficult dirt. Rinse and let dry.
Volume and tone controls make a scratching sound. Replacing with new ones would be great but is usually out of the question. Take them from the chassis and remove the back cover. Dust the resistive element with a soft brush, and with a 6B lead pencil, coat the resistive surface. (A 6B pencil is a form of conductive graphite that lubricates the surface and stops the noise.) Check with an ohmmeter to see if the wiper arm is insulated from the case. Should a short exist, the control must be insulated from the chassis.
A wire-wound volume control is hardest to fix because the wire can't be soldered. Make two "U"-shaped metal clamps to hold the broken ends of the resistive element in place. Then solder a wire between the two clamps. When it is more than a break and some of the wire is missing, use the clamp to make a solder joint. Measure the total resistance, and then use a fixed resistor to make up the difference.
11. Speakers with broken cones just need a little help to sound good once more. Kleenex and Liquitex Acrylic Gloss Varnish, medium, (buy it at an art supply store), and a white milky glue -- something like Elmer's Glue -- can bond thin Kleenex to cover holes and tears in the cone. Painting the whole cone with this varnish will help preserve the paper.
If a voice coil (VC), whose spider is held in place by a screw, is rubbing the pole piece and needs to be centered, it can be recentered using three shims made from thin metal. After loosening the screw, push the shim between the voice coil and the pole piece, centering the cone. Once the cone is centered, retighten the screw, and then remove the shims. The cone should work smoothly. Trash lodged in the VC post gap on an electromagnet speaker can be dislodged with the shim. On a permanent magnet type -- good luck!
What is a true DOG?
A chassis with all its parts removed and without a speaker is a true DOG. The only cure is to replace it with a new transistor radio cleverly hidden under the nonworking chassis.
I hope that these "BARK Suppressors" help tame all your DOGS!
(Fred Geer, 6042 Brookridge Rd., Jacksonville, FL 32210)
Fred Geer's radio collecting began in 1953 with a Shure microphone and an RCA R-32. In addition to his 300-plus collection, he owns around 2,000 old-time radio shows, which he can broadcast to any radio in his house. Through his articles, he enjoys sharing various aspects of radio collecting and history.