2016 February 25 Electronics Radio
A reader emailed saying he has a WWII radio. It's a BC-342, an HF receiver that was used by the US Army Signal Corps.
When he plugs the radio in, he's reading 120 volts AC between the antenna connector and the GND connector. (It drops to zero when the GND is earthed.)
He wants to know if this is supposed to happen. I want to know if this is supposed to happen, too! So let's try to figure this out.
He also wonders, Is this supposed to be a "hot chassis" radio? Would an isolation transformer help? Do you need to put a three-prong plug on the radio?
Let's address these questions.
Some of this will be specific to the BC-342, but many of the basic troubleshooting principles will apply to other radios also.
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In This ArticleSafety
Inspecting the Radio
Two Prong vs. Three Prong
Voltage Blocks & Shunts
One More Thing
Don't even think of working on old radios unless you understand the basics of voltage, current, resistance, etc.
Don't work on old radios if you lack hand-eye coordination.
Don't work on old radios if you don't fully appreciate the dangers.
Don't work on old radios when you are distracted.
Most of us eventually get unsteady hands in our old age, so here's a safety tip: concentrate on doing as much testing as possible on radios that are OFF and UNPLUGGED. If you don't have a steady hand, you could accidentally short something live with one of your DMM test leads. That's usually bad.
In some articles on this website I've been known to test live electronics with DMM's, but radios can have high voltages. Be careful!
Keep one hand in your pocket at all times when working on an energized / live / plugged-in radio.
Even so, make sure you get an isolation transformer that's made for working on radios. This will eliminate one particular type of lethal danger: namely, sudden electrocution by grounding yourself on something else in the room.
Don't over-rely on an isolation transformer. Any component with more than about 30 volts on it can be dangerous. And it's still somewhat dangerous, even if you isolate the radio from earth-ground reference.
The reader said he already has a multimeter, and he just picked up an isolation transformer. Both are essential. Just be sure your favorite manufacturer didn't change their design and tie the primary and secondary grounds together, or something. I can't keep track of what all these companies are doing. You've heard the saying "too many cooks spoil the soup", and I think the corporate model does that to good products sometimes. If you want to be sure the isolation transformer is the right kind, go with a get vintage one that was definitely made for radio and TV repair.
Also get yourself a high-ohm resistor, preferably high wattage, and a couple of alligator clip test leads. Some people use a 10K-ohm, two-watt resistor. It depends on the voltage and how fast you want to empty the capacitor.
Keep that resistor handy. Some components will retain lethal voltages even after the radio is off / unplugged. You will need to drain them with that resistor before you proceed. Use the multimeter (carefully) to find them.
Since the reader already powered up the radio, this next piece of gear is not going to help much. But if you're tackling an old radio of unknown status, here's what I'd do.
Make yourself a dim bulb tester. Or, get yourself a variable autotransformer.
These methods work differently, but either or both can be used to power on a radio "gently".
The dim bulb tester limits the current, in case the radio has a short in the power circuitry. (In another article we'll look at a couple ideas for building a dim-bulb tester.)
If the bulbs are running at full brightness, it means the radio has a direct short, and it's not safe to power it up by itself. (Just make sure you're using a bulb that's higher-wattage than the radio, or it won't be a reliable test.)
If the bulbs stay dim, it means the radio is acting as a bottleneck for current, which is a good thing. That's what it's supposed to do.
If the bulbs don't light at all, it means the radio has an open circuit, which could just be a fuse, or a broken wire. But then if it's the fuse, you'd have to ask yourself what caused it to go...
The autotransformer allows you to ramp up the AC voltage. This is not good for some types of circuits, but it usually won't harm an old radio, as long as you don't leave it that way for hours at a time.
One more thing. So that you don't engage in the dread practice of Golden Screwdrivering, make sure you get yourself a couple of good books on radio repair. Read them, then read them again. Talk to the old pros, read the books they write, and learn all you can about working on radios.
The best approach to fixing old electronics is to be conservative and cautious. No one says you have to repair a radio in one evening. Some of them take months, by the time you get the replacement parts.
Inspecting The Radio
If you ever get a radio where the power cord has been clipped, it was probably done for a reason. The person who had it might have done that because of an unsafe condition. But just because the radio has a power cord, that doesn't mean you should plug it in right away.
There are several tests you should do before you even plug in a radio. I know it's kind of a drag to have to remove the chassis cover and inspect everything... but this is what you should do before even powering up a tube radio.
Look for anything that's blackened, bubbled, melted, sooty, or excessively dirty. Look for broken connections and frayed insulation.
Obviously-bad components should be addressed before you even power up the radio. Doesn't matter if someone says "That model rarely has problems". You could have gotten the one unit that had a lightning surge go through the chassis.
I would also check the resistance between B+ and chassis ground; we'll revisit this topic later in the article.
Two Prong vs. Three Prong
Old radios have two-prong plugs. To a radio that uses a transformer, there is no difference between the "neutral" and "hot" lines. The only real difference is to humans, because the "neutral" is bonded to ground at the service entrance of a building.
The third wire ("safety ground", usually green) was supposed to provide an uninterruptible link to the earth ground, just in case the neutral ever got interrupted (or there was a short to the chassis). Remember, neutral and ground are bonded at the service entrance. And usually, that third wire goes to the chassis of an appliance.
So, should you install a three-prong plug on an old radio?
Generally, it's not necessary. A polarized two-prong plug is fine. Many modern appliances have polarized two-prong plugs.
Polarized: not because it matters to the function, but because it matters to your safety. It's best to have the switches and fuses on the "hot" side, while keeping the neutral line clear.
"Hot chassis" radios will tend to have the chassis live when the plug goes in one way, but not when it's reversed. If you wire such a radio correctly with a polarized plug, you can keep the chassis from ever being live at all. That's a good idea, I would say.
A "radio repair" isolation transformer may have three-prong outlets, but that's only so it can accept three-prong equipment. The third or "ground" prong doesn't actually do anything there.
That's because a real isolation transformer will keep the primary ground / neutral isolated from the secondary ground / neutral.
A real isolation transformer is not supposed to provide any earth-ground reference. The electronics don't care, as long as they're getting the correct AC voltage.
Correct VoltagesThe reader said his BC-342 was showing 120 volts AC from the Antenna connector to the Ground connector.
He said that when he hooked the Ground connector to earth ground, the voltage went to zero.
Does this sound right?
Before we answer that, a couple things to consider.
1. If you measure the voltage at two different points on a wire, it will show as zero, even if the wire has a couple hundred volts on it. (A common rookie mistake when using a voltmeter; however, anyone can forget that momentarily. Just don't get electrocuted thinking a line is cold when it's actually hot.)
2. In other words, voltage is all relative. With AC, normally we measure the voltage relative to earth ground. With DC, it's typically measured relative to circuit ground, or the negative terminal of a battery. The grounds can be tied together, or not.
3. There are lots of places in a radio where you could read a potential difference of 120V AC, but that doesn't mean they're all energized to 120 volts with respect to ground.
Now, with all this said... the voltage he measured seems like it shouldn't be there.
A receiving antenna and chassis ground should be at about the same potential. That means if you read the voltage between them, it should be zero volts. I can't see anywhere on the schematic where an AC line would connect directly to chassis ground.
In fact I know it shouldn't, because...
The radio has test points which are on the Group 6 terminal board:
And here's what the voltages should be, with radio on and volume control at max:
Test points 1 and 4 should always be at 0 volts relative to Test Point 8 (chassis ground). No matter which way you have the 2-prong plug oriented. (If SW12 is open, points 1 and 4 would be at 120V AC relative to each other.)
Look at the diagram shown below. Test Points 1 and 4 are on the primary side of the transformer. The chassis ground (Test Point #8) is on the secondary side.
Notice the lines that I highlighted. On this particular radio, you would choose "110" or "120" depending on your AC line voltage; both of these go to one of the prongs of the plug (orange lines).
The red lines connect to the other prong of the plug.
Neither one connects to chassis ground, not even through a cap or a resistor. As long as there's no short, the mains and chassis ground don't reference one another; hence no potential across them.
If they did reference one another, which they would do when you connect the GND terminal to earth ground... you still shouldn't be reading a significant AC voltage between Antenna and GND.
Voltage Blocks & Shunts
Inside a radio, various components are used to keep voltages limited to certain areas.
You would not want the 60Hz mains voltage to be in your audio circuit, for example. That's not only because it might burn stuff out, but also because it would get amplified into your audio. 60 Hz doesn't sound really that great over a loudspeaker.
And 120 volts is orders of magnitude "louder" than the tiny RF voltages that are received by an antenna.
Capacitors, inductors, and resistors are three kinds of components that are used to keep everything where it should be.
Capacitors, when they work, are amazing. Thanks to frequency vs. impedance curves, you can use capacitors to filter out AC line voltage while letting radio-frequency through. Or, you can shunt radio-frequency somewhere, while allowing AC line voltage to pass.
Brilliant, until the capacitors go bad. In a 70-year-old radio, there are certain types of capacitors that are almost always bad.
That means there will be voltages where they don't belong.
A radio would normally have the B- voltage tied directly to the chassis ground. If it doesn't do that, it's a "floating ground" radio, which means you could get weird voltage readings if the radio is not hooked up properly (or even if it is).
Best I can determine, the BC-342 is not a floating-ground radio. It shows B- connected to chassis ground at terminal #8 of the Connection Strip in the schematic.
I do know the BC-342 was designed to be earth-grounded when in use. The "GND" connector on the front of the radio should be hooked to an earth ground whenever you use the radio. That would put chassis ground and AC neutral on the same ground reference. It still shouldn't affect the potential between GND and the receiving antenna; that should always be about zero volts.
We're talking about a receive antenna and a chassis ground, neither of which should be energized with line voltage.
One More Thing
Some radios have filter capacitors on the primary-side wiring. Sometimes these are called "line bypass" caps. You'd have one or two capacitors with a connection to ground. You could measure 120V AC across there, easily.
The BC-342 has no line-bypass caps. Capacitors C78 and 79 look like an EMI/RFI filter network (chassis ground connection between them), but they're on the secondary side of the transformer.
Capacitors C89 and 90 also connect to chassis ground. They're also filtering capacitors, but again... secondary side of the transformer.
The reader found that earthing the GND connector made the voltage go away, but it seems to me there could be a short somewhere. This, again, is why I'd be first checking the voltages at Test Points 1 and 4, relative to Test Point 8.
Three things, first:
1. With radio UNPLUGGED, check the ohms between B+ and chassis ground. Normally that reading would be either megohms or open-line (OL), but some radios might have a few hundred K ohms.
BC-342 has resistor R40, which does go between B+ and the chassis ground. R40 is only 65K ohms. I don't see anything else that should act as a resistor in parallel to ground; double-check the schematic. R40 probably drifted upward; look for a reading maybe 50K to 100K ohms from B+ to GND.
2. With radio UNPLUGGED and "GND" terminal not connected to anything, check the ohms between the mains side of the transformer and the chassis ground. Far as I can see on the schematic, that should be high-ohms or open-line. If not, there's a short somewhere.
3. Look for frayed wires, debris, obvious shorts, or damaged components.
4. Pull the tubes and make sure none of them have shorts. (Use a tube tester. If you're looking for one, buying it through this link will help keep my website on-line.) Don't forget to check the rectifier tube. (It's a VT-97; I think that might cross-reference to a 5W4.)
If these things don't show any problems, then I would check the test points. Or, I might check the test points first, but only if I knew the radio already powered on without overheating or shorting.
Capacitors are almost always bad in these old radios. Fan out from the power circuitry, inspecting and replacing anything that might be in the path of 60Hz AC. Remember, the transformer secondary is going to produce 60Hz, even though it's not at line voltage anymore. From the schematic, it looks like two of the secondary voltages are at AC 60Hz; they bypass the VT97.
Many old radios have a dead-short condition between B+ and ground, due to failed capacitors. There could also be something forming a voltage divider, giving you less-than-B+ voltages, but still significant.
If the primary-side wiring is shorting to chassis ground somewhere, that could cause all kinds of problems. It could be even a single strand of wire or a splash of solder somewhere. (That why item #2 on the list.)
Sorting out grounds and unexpected voltages can be a challenge. I don't have a BC342 here to test, so one of you readers might know something about it that I don't. Maybe I even overlooked something on the schematic.
That said, it would be quite a design stretch to have a receiving antenna at 120V AC with respect to chassis ground... especially on a radio that was made to be used in World War II.
When trying to figure out a radio, it always helps to have a schematic diagram. If you don't have that, start with the principles that apply to every old radio. In this article we've looked at a couple of those.
Whatever you do, I highly recommend some basic equipment and a couple of good books. Be cautious, be safe, and enjoy the great hobby of old radios.
If you found this article helpful, please help me out by purchasing your stuff through these links. (Use this link to shop for a BC-342 radio and it really helps me out.)
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