Restoring a Magnavox radio gifted to me long ago.
Operationally I know the two conductor power cord doesn’t care about polarity, but looking at the schematic attached, isn’t one way better than the other?
Currently the load lead is going to the “SW On Tone Control” the on/off switch. I did that assuming I didn’t want the chassis “live”.
Is this best? Or is there a better, safer way to wire this input circuit? By the way I have an inrush current limiter inserted into the load lead ahead of the small coil.
I must admit I have trouble envisioning the power flow in this radio given that the neutral line is grounded at my mains breaker panel and the switch when on is sending the load line to the chassis.
Thanks for any clarification offered.
Jeff
Operationally I know the two conductor power cord doesn’t care about polarity, but looking at the schematic attached, isn’t one way better than the other?
Currently the load lead is going to the “SW On Tone Control” the on/off switch. I did that assuming I didn’t want the chassis “live”.
Is this best? Or is there a better, safer way to wire this input circuit? By the way I have an inrush current limiter inserted into the load lead ahead of the small coil.
I must admit I have trouble envisioning the power flow in this radio given that the neutral line is grounded at my mains breaker panel and the switch when on is sending the load line to the chassis.
Thanks for any clarification offered.
Jeff
Attachments
put a new polarized 2 pin plug on it. connect the return to chassis (wide pin wire)
reconnect the switch to the hot wire (narrow pin) and R27 and rectifier.
reconnect the switch to the hot wire (narrow pin) and R27 and rectifier.
Radio's without mains transformer are usually designed such that you can't come into contact with the chassis during normal use, so then it shouldn't matter which side is live - assuming the insulation is good enough to be trusted (so no plastic knobs or screws replaced by metal ones by previous owners, among other things).
However, if the capacitors that insulate the inputs are antique paper capacitors, you can be almost certain they don't insulate properly anymore. You would have to replace them with something safer, preferably modern Y capacitors.
Regarding your question, I really don't know. If C43 should fail, it is safest if it is connected to the neutral rather than the live side. If you don't trust the insulation between the chassis and the user, then the radio is quite dangerous anyway, but it would be slightly less dangerous with the switch connected between the live wire and C42 and the neutral always connected to the chassis.
However, if the capacitors that insulate the inputs are antique paper capacitors, you can be almost certain they don't insulate properly anymore. You would have to replace them with something safer, preferably modern Y capacitors.
Regarding your question, I really don't know. If C43 should fail, it is safest if it is connected to the neutral rather than the live side. If you don't trust the insulation between the chassis and the user, then the radio is quite dangerous anyway, but it would be slightly less dangerous with the switch connected between the live wire and C42 and the neutral always connected to the chassis.
Neutral and Ground are not the same thing, except back at the Mains Panel.
I have seen as much as 8VAC difference at the user outlet, due to voltage drop from current on the neutral wire, but no current on the ground wire. You do not want any current in the ground wire, that is only when you have a fault (and something has to be repaired/fixed, or removed from the mains.
This was a radio that was designed for no user contact to the chassis, and no user contact to the wiring.
If you will keep the radio as original and no user contact with the chassis nor with any wiring, that is one way to restore it.
Otherwise, you are talking about adding an isolation transformer, 3 wire IEC cord, and power mains outlet that has a 3 wire standard.
There probably is no room for an isolation transformer. That means the radio needs to be kept in original condition of No User Contact with chassis, and no user contact with the radio wiring.
So, you can not modify it to use as a vacuum tube amp this way.
Remember, some outlets in homes may be keyed to fit plugs that have both the wide and narrow blades, but some of them were wired by un-knowing or un-careful persons.
That means the wide and narrow keyed outlets may be dangerously wired, with the wide section connected to Hot (very bad!).
Without correctly wired 2 wire outlets, you have to retain the original design.
Old radios were designed for a reasonable amount of safety, but only when used on the properly wired power outlets of the day, and when there was no user contact with chassis or wiring.
I have seen as much as 8VAC difference at the user outlet, due to voltage drop from current on the neutral wire, but no current on the ground wire. You do not want any current in the ground wire, that is only when you have a fault (and something has to be repaired/fixed, or removed from the mains.
This was a radio that was designed for no user contact to the chassis, and no user contact to the wiring.
If you will keep the radio as original and no user contact with the chassis nor with any wiring, that is one way to restore it.
Otherwise, you are talking about adding an isolation transformer, 3 wire IEC cord, and power mains outlet that has a 3 wire standard.
There probably is no room for an isolation transformer. That means the radio needs to be kept in original condition of No User Contact with chassis, and no user contact with the radio wiring.
So, you can not modify it to use as a vacuum tube amp this way.
Remember, some outlets in homes may be keyed to fit plugs that have both the wide and narrow blades, but some of them were wired by un-knowing or un-careful persons.
That means the wide and narrow keyed outlets may be dangerously wired, with the wide section connected to Hot (very bad!).
Without correctly wired 2 wire outlets, you have to retain the original design.
Old radios were designed for a reasonable amount of safety, but only when used on the properly wired power outlets of the day, and when there was no user contact with chassis or wiring.
To be clear, this is an electrically hot chassis, which is very dangerous. Many wall outlets are wired backwards,
which is dangerous even when the plug is inserted as intended (if a polarized plug is still on the cord any more).
At this point in time, you can't really safely use this radio, unless it is plugged into an isolation transformer as mentioned.
which is dangerous even when the plug is inserted as intended (if a polarized plug is still on the cord any more).
At this point in time, you can't really safely use this radio, unless it is plugged into an isolation transformer as mentioned.
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How much room is left in the cabinet? You may be able to find a spot to bolt in a 1:1 torroid. I have several hot chassis radios, but I know all my outlets are wired correctly. You can put on a polarized plug neutral on chassis but make sure you only use the radio with an outlet you verified is wired correctly.
Wow! Thanks for all the prompt feedback.
I check all my outlets and they are wired correctly. I have already replaced most of the ceramic caps with safety caps, but not C43. There is room for a toroid but despite not really having any collector value, there is a lot of sentimental value as it was a gift from a friend who is no longer with us.
I now see the wisdom of swapping the load to the switch and sending the neutral to the chassis. That is exactly the kind of advice I was looking for. Unfortunately, if I am to remain original, polarized cord on the radio end is out as the old brown pressboard backing (keeping the chassis safely at bay) has an integral unpolarized two conductor captive connector. But if I polarize the plug and mark and install the captive connector onto the back properly, I should be good to go. The rest of the wood box and phenolic insulators are in good shape and once tuned up there will not be a need to expose the chassis again. I do have an IT for my bench which is where I listen to this radio so I could use it if I have to.
Thank again, all.
Jeff
I check all my outlets and they are wired correctly. I have already replaced most of the ceramic caps with safety caps, but not C43. There is room for a toroid but despite not really having any collector value, there is a lot of sentimental value as it was a gift from a friend who is no longer with us.
I now see the wisdom of swapping the load to the switch and sending the neutral to the chassis. That is exactly the kind of advice I was looking for. Unfortunately, if I am to remain original, polarized cord on the radio end is out as the old brown pressboard backing (keeping the chassis safely at bay) has an integral unpolarized two conductor captive connector. But if I polarize the plug and mark and install the captive connector onto the back properly, I should be good to go. The rest of the wood box and phenolic insulators are in good shape and once tuned up there will not be a need to expose the chassis again. I do have an IT for my bench which is where I listen to this radio so I could use it if I have to.
Thank again, all.
Jeff
Yes, the power switch in "AC/DC" transformerless radios is normally in the to-chassis wire.
This is an artifact of older radios having a 3-wire cord, the third wire a resistance element to waste-off excess 110V to a heater string (before the 35 and 50 V types were developed).
If you think in *complete circuits* there is no contradiction, the radio will work.
This is for RADIO, where only radio waves come in and only sound waves go out, NO human-touchable metal. You can't use it for a hi-fi or guitar amp without further safety measures.
This is an artifact of older radios having a 3-wire cord, the third wire a resistance element to waste-off excess 110V to a heater string (before the 35 and 50 V types were developed).
If you think in *complete circuits* there is no contradiction, the radio will work.
This is for RADIO, where only radio waves come in and only sound waves go out, NO human-touchable metal. You can't use it for a hi-fi or guitar amp without further safety measures.
It is bad radio design to use a potential live mains connection via a capacitor no matter how small a value to help with the signal strength .
Done --I can assure you for cheapness /more profit --outlawed in the UK .
110 volts AC across your heart might not kill you ( unless you have a heart problem) BUT 240 AC certainly will.
The automatic continuous heart beat is interrupted in many cases -- other cases just killed the users .
I am with Chris ---- NO information on converting or using NONE mains transformer isolated radios or TV,s of that era should be given out and I hope DIY Audio holds the same view ??
Every bare connection on UK radios of that type had to conform to regulations (at the time ) even dial knob grub screws had to have wax inserted to insulate the metal to metal contact from the user .
Legislation was introduced banning this form of radio receiver in the UK from being sold through business outlets .
Make no mistake those radios were sold cheap to people who could not afford the full price of a transformer isolated radio ----- profit "before " life .
Done --I can assure you for cheapness /more profit --outlawed in the UK .
110 volts AC across your heart might not kill you ( unless you have a heart problem) BUT 240 AC certainly will.
The automatic continuous heart beat is interrupted in many cases -- other cases just killed the users .
I am with Chris ---- NO information on converting or using NONE mains transformer isolated radios or TV,s of that era should be given out and I hope DIY Audio holds the same view ??
Every bare connection on UK radios of that type had to conform to regulations (at the time ) even dial knob grub screws had to have wax inserted to insulate the metal to metal contact from the user .
Legislation was introduced banning this form of radio receiver in the UK from being sold through business outlets .
Make no mistake those radios were sold cheap to people who could not afford the full price of a transformer isolated radio ----- profit "before " life .
Certainly meant no harm to myself or others. If this thread offends or breaks rules, please delete.
My intention was to make it safer. If no such thing, I will reassemble into it’s wooden cocoon and enjoy it while it lasts.
Thanks,
Jeff
My intention was to make it safer. If no such thing, I will reassemble into it’s wooden cocoon and enjoy it while it lasts.
Thanks,
Jeff
There seems to be adequate awareness of safety implications and plenty of good advice given. The OP is not advocating doing anything hazardous with the set so the discussion can continue as long as that continues to be the case.
In that case, if the antenna is disconnected from the AC line and isolated ("galvanically") from AC, or mounted entirely within the outer shell, can a GFI be incorporated into the shell?
Second question, could an external GFI provide modern levels of safety?
All good fortune,
Chris
Second question, could an external GFI provide modern levels of safety?
All good fortune,
Chris
If the case is entirely enclosed, there is very little danger. (Remember, we still had smallpox, world wars, leaded gasoline.) UL and CSA were pretty effective about checking for exposed metal. If totally enclosed it is no more dangerous than any other electrical appliance. Even including the GFI, which is live metal parts inside a thin plastic shell.
FWIW, these radios mostly used an internal loop antenna, though some also had a capacitor for a long-wire outdoor antenna.
Danger comes when we tap into old radios for hi-fi or guitar amps. Exposed electrical audio connections. That's dangerous.
FWIW, DIYAudio's rules changed a couple years back, after I was red-tagged for posting an old RCA schematic and not applying Wite-Out around the power input.
FWIW, these radios mostly used an internal loop antenna, though some also had a capacitor for a long-wire outdoor antenna.
Danger comes when we tap into old radios for hi-fi or guitar amps. Exposed electrical audio connections. That's dangerous.
FWIW, DIYAudio's rules changed a couple years back, after I was red-tagged for posting an old RCA schematic and not applying Wite-Out around the power input.
Buy an isolation transformer, put this inside a separate box. Keep all the safety rules.
Power your old radio via isolation transformer.
Power your old radio via isolation transformer.
Some time ago, there was a thread that described what could be called a "high end" power transformerless unit. It had an O/P transformer which contained a separate NFB winding and had "floating" speaker connections. I speculated that the device may have been intended for 1 of the remaining DC house current zones. Yes, parts of NYC's Greenwich Village had DC house current well within living memory.
Tom Edison did his incandescent lighting demonstration in Washington Square Park, with DC and his generating equipment remained in service for quite some time.
Tom Edison did his incandescent lighting demonstration in Washington Square Park, with DC and his generating equipment remained in service for quite some time.
Use an EI core transformer, not a toroid. I spy a half wave rectifier in there.
Didn't Thomas A. Edison eventually lose the current war against George Westinghouse?
Btw, nowadays we return to DC when it comes to large power transmissions over great distances again.
Best regards!
Btw, nowadays we return to DC when it comes to large power transmissions over great distances again.
Best regards!
I expected this movie would get low reviews from the general public. I thought it was fascinating movie:
The Current War
The Current War (2017) - IMDb
Thanks for seeing that! Torroids are small and easy to mount was all I was thinking of.
Here is a small isolation box, about the size of two laptop docking station bricks.
https://bkpmedia.s3.amazonaws.com/downloads/datasheets/en-us/1604A_datasheet.pdf