1) Use a polarized plug, and rewire the radio so that the hot side of the line is switched.
2) Use an Isolation Transformer. This "Isolates" the chassis from the AC line.
3) Consider plugging AC/DC radios into a GFCI outlet, or installing one of those plugs you see on hair driers that have a built in GFCI in it. This way, if someone gets shocked, the circuit will be broken. Note: if also using an isolation transformer, the GFCI needs to be on the SECONDARY side of the line for protection.
:Is it possible to fully eliminate the hot chassis issue on AC/DC radios. I have followed the advice on a couple of sites and still have the hot chassis.
Poston
:These radios are by design, less safe than ones with a power transformer. However, the following can be done to improve safety. Note that this does not completly eliminate all voltage to the chassis.
:
:1) Use a polarized plug, and rewire the radio so that the hot side of the line is switched.
:
:2) Use an Isolation Transformer. This "Isolates" the chassis from the AC line.
:
:3) Consider plugging AC/DC radios into a GFCI outlet, or installing one of those plugs you see on hair driers that have a built in GFCI in it. This way, if someone gets shocked, the circuit will be broken. Note: if also using an isolation transformer, the GFCI needs to be on the SECONDARY side of the line for protection.
::Is it possible to fully eliminate the hot chassis issue on AC/DC radios. I have followed the advice on a couple of sites and still have the hot chassis.
::These radios are by design, less safe than ones with a power transformer. However, the following can be done to improve safety. Note that this does not completly eliminate all voltage to the chassis.
::
::1) Use a polarized plug, and rewire the radio so that the hot side of the line is switched.
::
::2) Use an Isolation Transformer. This "Isolates" the chassis from the AC line.
::
::3) Consider plugging AC/DC radios into a GFCI outlet, or installing one of those plugs you see on hair driers that have a built in GFCI in it. This way, if someone gets shocked, the circuit will be broken. Note: if also using an isolation transformer, the GFCI needs to be on the SECONDARY side of the line for protection.
:::Is it possible to fully eliminate the hot chassis issue on AC/DC radios. I have followed the advice on a couple of sites and still have the hot chassis.
If its OK then maybe you made a mistake installing your polarized plug. I don't know that radio but, if one side of the power cord goes directly to the chassis, then it must be the one that connects to the wide blade in the plug.
Some radios have the switch inserted between the line and the chassis. In that case do this:
1. Find where the AC line cord wire that does NOT go to the switch is connected. Disconnect the line cord from there and run a new wire from there to one terminal of the switch (disconnect the wires from the switch first).
2. Connect the wire from the narrow blade on the plug to the other terminal of the switch.
3. Connect the wire from the wide terminal of the plug to the chassis.
Tony
:I did all this but when the radio is on, I still read 120v from the chassis to ground....
:
:::These radios are by design, less safe than ones with a power transformer. However, the following can be done to improve safety. Note that this does not completly eliminate all voltage to the chassis.
:::
:::1) Use a polarized plug, and rewire the radio so that the hot side of the line is switched.
:::
:::2) Use an Isolation Transformer. This "Isolates" the chassis from the AC line.
:::
:::3) Consider plugging AC/DC radios into a GFCI outlet, or installing one of those plugs you see on hair driers that have a built in GFCI in it. This way, if someone gets shocked, the circuit will be broken. Note: if also using an isolation transformer, the GFCI needs to be on the SECONDARY side of the line for protection.
::::Is it possible to fully eliminate the hot chassis issue on AC/DC radios. I have followed the advice on a couple of sites and still have the hot chassis.
I assume that this isn't one of those sets designed with a hot chassis. I think those were post-war.
Anyway, there is often a capacitor between line and chassis. This can cause 120V a.c. to be measured from chassis to ground when using a modern high-impedance voltmeter, but it is usually is a phantom voltage without a shock hazard. You can connect a 120-V light bulb from chassis to ground and see if it lights - it probably won't.
In such a situation, you want to switch the hot-side of the line and have the neutral on the side with the capacitor to chassis. Sometimes, you physically can't accomplish both without rewiring the set.
Here is the link to the schem: http://www.nostalgiaair.org/PagesByModel/179/M0014179.pdf
:I couldn't find your schematic - perhaps you can link us to it?
:
:I assume that this isn't one of those sets designed with a hot chassis. I think those were post-war.
:
:Anyway, there is often a capacitor between line and chassis. This can cause 120V a.c. to be measured from chassis to ground when using a modern high-impedance voltmeter, but it is usually is a phantom voltage without a shock hazard. You can connect a 120-V light bulb from chassis to ground and see if it lights - it probably won't.
:
:In such a situation, you want to switch the hot-side of the line and have the neutral on the side with the capacitor to chassis. Sometimes, you physically can't accomplish both without rewiring the set.
I think you can avoid this by moving the switch into the leg that goes to the resistors where the neutral is now. Keep the hot side with the switch.
You have the hot going to the switch. Thats good. Hopefully there is only one wre going to the other switch terminal. If so, you can disconnect it. The other end of that wire goes to the floating ground. That will connect to a bunch of places. Tie the neutral side of the line cord to any one of the places on that floating ground. Remove the wire stub that used to go to the switch. Now, take a new wire and connect it to the empty switch terminal and put the other end where you had the neutral before.
I hope thats not too confusing. The bugaboo will be if there are several wires going to the second switch terminal. If so, those all need to remain connected to each other but not to the switch. That can be a pain. Hopefully that is not the case.
Tony
:Thanks for all the replys. I do have the hot side wired to the switch. The neutral hooks to a terminal strip that is a double resistor. This strip is not tied to the chassis.
:
:Here is the link to the schem: http://www.nostalgiaair.org/PagesByModel/179/M0014179.pdf
:
::I couldn't find your schematic - perhaps you can link us to it?
::
::I assume that this isn't one of those sets designed with a hot chassis. I think those were post-war.
::
::Anyway, there is often a capacitor between line and chassis. This can cause 120V a.c. to be measured from chassis to ground when using a modern high-impedance voltmeter, but it is usually is a phantom voltage without a shock hazard. You can connect a 120-V light bulb from chassis to ground and see if it lights - it probably won't.
::
::In such a situation, you want to switch the hot-side of the line and have the neutral on the side with the capacitor to chassis. Sometimes, you physically can't accomplish both without rewiring the set.
Either make the wiring changes that Tony proposes, or just live with it.
If you're going to work on the radio and you can't work in an insulated environment (say on the living room carpeting), use an isolation transformer. Otherwise I don't see why everyone makes a big deal about AC-DC radios. There isn't a need to modify them. I see more logic to the removal of old fashioned playground equipment (merry-go-rounds, etc.). Still, it saddens me whenever things like slides and merry-go-rounds are removed. Tires are great, but when all you have to play on is tires, life is boring. At my gradeschool we even had a no touch rule. We couldn't even play tag. Bizarre! The voltage present on an AC-DC chassis isn't going to jump out and bite you, just like if you drive a Corvair at a sane speed around corners, it isn't going to flip over. The only time the car ever flipped over was when it was being driven improperly at unsafe speeds.
Thomas
Thomas
My mother solved the problem her own way. Her little one tuber that she had as a kid had a band-aid over the end of the tone arm. When I inherited it, I took the ugly band-aid off and then found out why it was there.
Thomas
It's the way things are these days. Everything has to be "safe". When I was a kid I played around with radio and TV circuits and later as a Ham operator, I made my own tramsmitters. I got zapped many times and I grew to fear it less and less and to respect it more and more. Electricity and me developed a mutual understanding, so to speak. My kids are scared to death of the slightest tickle (they are 26 and 28 years old). Now, I don't go looking for a shock, but I don't think I have the same fear as they do because I was allowed to experience it.
On the other hand, if one person has his heart stop from touching part of a radio that wasn't supposed to be touchable, or because he forgot it was plugged in, it is probably worth a little effort to make the radio safer. All it takes is to move a few wires around.
I miss that part of the good old days. I loved riding my bike without a helmet, and swimming in the pond. But it just isn't that way anymore. In some ways, these radios keep me in touch with those days, even if I do rewire them and put polarzed plugs on.
Tony
:An annoyance, though, is a portable phonograph. You take that thing outside, and the metal tone arm will give you a shock whenever you try to put on a record. Usually these are isolated by a condenser, but the shock is nasty. I recommend using an isolation transformer on a phonograph whenever it is used outside.
:
:My mother solved the problem her own way. Her little one tuber that she had as a kid had a band-aid over the end of the tone arm. When I inherited it, I took the ugly band-aid off and then found out why it was there.
:
:Thomas
My wife and I have traveled in Mexico extensively. There are many very interesting tourist attractions that would be roped off in the U.S. There are pedestrian crosswalks where you better look both ways - twice!
I think the U.S. has gone too far and Mexico maybe not quite far enough. It's hard to say where the happy medium really is.
Regarding Mexico, yeah, a lot of things are kind of interesting down there. Things aren't quite as regulated as up here. I have "family" down there, so I go there from time to time. They still wire houses to this date that don't have grounded receptacles. Playground equipment certainly is more dangerous (and fun) down there than what we find at most schools up here. I did see a school recently (in Oregon), that is from the turn of the 20th century, which had its original playground equipment still in use. Some of that stuff is scarry, but cool.
One very interesting thing I find in Mexico is the use of these little metal lumps as speed bumps and larger ones as lane dividers, etc. The larger ones really throb the tires. Keeps people in their lanes, though. I originally thought it was a good idea to some degree, but then realized what would happen here in Milwaukee with the snow plows.
Thomas
One more question... The field coil on this bad boy is shot. It's wide open. For those that have looked at my schem. can I throw a 1 to 2 K 25W power resistor in place of the field coild and run a PM speaker off the output trans? I really don't like the thought of trying to rewind the one I have and figure I can run the PM till I find a suitable replacement for the original.
:Is it possible to fully eliminate the hot chassis issue on AC/DC radios. I have followed the advice on a couple of sites and still have the hot chassis.
Maybe I missed it, but what is the model number? Maybe 1K would be reasonable to start with, and check the B+ voltages.
:Yes, I think the resistor substitution should work. There will be more DC ripple and maybe a little hum?
:
:Maybe I missed it, but what is the model number? Maybe 1K would be reasonable to start with, and check the B+ voltages.
Just guessing, I'd start with 1000 ohms for the substitute resistor.
However, you could calculate the size of resistor required. The set has a half-wave rectifier which, ignoring ripple, will put out 170V peak with a 120V line voltage, less the drop thru the rectifier. Look up the plate current for all the tubes at the plate voltages given on the schematic, and add them up. Then figure out the required voltage drop thru the resistor to give 100V output.
With many more modern AC-DC radios, the plate of the output tube is tied directly to the cathode of the rectifier. If you want to modernize your radio with a PM speaker, this is what I recommend you do: The field coil was a choke. It adequately reduced hum. Therefore no additional filtering was necessary for the RF and 1st AF circuits. If you look at 42-PT-10 (and possibly your schematic if you can find it), you can take note of this. However, you'll also note that the plate and screen grid of the output tube are tied after the field coil in order to load down the field coil enough that it becomes a strong magnet. The output tube then receives less voltage than is ideal. If you are going to put in a PM speaker, you might as well modernize the power circuit since you are going to alter it anyway. Connect the screen grid and the output transformer B+ lead to the rectifier cathode. The output tube does not need much filtering because it is not feeding any further stages which would amplify hum. As I said before, the only reason why it was connected after the field coil was for loading purposes. Now, where the field coil was, put in a 2.2K ohm 1 or 2 watt resistor. Start with this value and work upward until there are about 95 volts on the IF tube plate(s). There should be an electrolytic before and after this resistor as there was before with the field coil. Adequate filtering is necessary for all of the RF stages and the primary AF stage. This is why they must be coupled through a resistor. If any of these stages have hum, each successive stage will amplify the hum.
If you have a bit too much hum with the output tube connected directly to the rectifier, you may insert a 100 ohm resistor (2 watt) on the rectifier cathode (this will feed the entire radio, not just the output tube). Place a 40 to 50 MFD condenser before and after this resistor.
If you do not feel comfortable with this modification, then do not perform it. Simply replace the field coil with an appropriate resistance or reactance.
Also, keep in mind that Quam field coil speakers for AC-DC radios are sometimes found on eBay. Furthermore, before you trash that speaker, I recommend that you open up the paper on the field coil where the wires go in. It's broken anyway, so why not? The coil often breaks right at the lead wires. If you can find a break, mend it and you'll be back in service.
Thomas
Not to beat a dead horse, But: I agree that the DC resistance of speaker field coils was often in the 500-650 ohm region. But, I figure the inductive impedance will also come into play by blocking some of the ripple peak, and thus reducing the output voltage from the pi-type power supply. Also, with the higher modern line voltage, you'd possibly want further voltage drop through any resistor replacing the field coil. I don't have any numbers to back this up, so if you decide to go that route, just experiment until you get about 100V DC output. If you do what Thomas suggests, then as he says, you still might have to do a little experimentation
During the transition period in the 1940s when AC-DC radios lost their field coils, they used special output transformers that had hum bucking coils in the primary. Works well. The primary is tapped into two sections. On one side of the center tap is a large section. This is for the output tube (audio). On the other side of the tap is a small section. The center tap is connected to the rectifier. The output tube plate is connected to the end of the large coil, and the rest of the radio (including screen grid of the output tube) is connected to the end of the small coil. Since the two coils are out of phase with eachother, hum to the screen grid of the output tube and the other tubes cancels with the hum in the output of the output tube (from the plate). The only drawback of this system is that the hum bucking coil absorbs some of the audio. Naturally audio is inducted into this coil. Since this coil is part of the rest of the power supply, the electrolytic in that section of the power supply absorbs some of the audio. It isn't much, though. These radios perform well (like my RCA 55X2, which came after the first 55X models which had field coil speakers....this radio is also "Golden Throat" certified, and sounds beautiful for a bakelite radio).
Thomas