04-18-2021, 11:51 PM
The correct polarity of a diaphragm speaker when no (+) sign is noted and the speaker is used in the plate circuit of the last audio tube is such when the plate current is flowing, the diaphragm is drawn inward. This polarity re-reinforces the magnets and increases the sensitivity of the speaker. Time, drains away the strength of the magnets that even the correct polarity cannot restore. In such a speaker it is possible to re-magnetize the magnets by the use of a neodymium magnet. Careful observation using a compass will determine the the N & S poles of the internal magnet. By shunting the OEM magnet with the neodymium the raping the OEM magnet with a small tack hammer then a stronger magnetic field will be set.
If the original tinsel cord is still in place for the speaker it may have a red thread in the cord, this is the positive thread and should connect to the tip of a phone plug if it is used. When inserted into a radio that has a speaker jack once the connection is made the frame of the jack becomes connected to B+ via the speaker but the shell of the plug makes shock contact difficult.
This same polarity should be applied to high impedance magnetic headphones as well.
The Antique Wireless Association has recently made available a circuit board and a parts list for a power supply that could power this radio. A part list and sources are included, cost estimate should not exceed $100, fabricate ones own cabinet.
https://www.antiquewireless.org/homepage/pcb-item-5/
You may find more battery set parts available at the Antique Radio Forum Classified ads... There have been recent posts in regard to A-K 20. Google with Antique Radio Forum in the search terms.
The ARBE-III folks have suitable high resistance audio transformers that would be more appropriate for this radio. The Hammond Radiola III replacement transformer is also O.K.
When the radio is completely restored a short and direct earth ground in place and an antenna of 35 to 50ft. used, if, the reception is still weak, consider lowering the value of the grid resistors from the 600 ohms to 500 or even lower. That will increase the gain of the RF stages. This value can only be lower until the radio oscillates uncontrollably. These resistors can be 1/4 watt carbon and do not have to be wire wound.
A major issue with 20's era radios are the connections both soldered and mechanical. They must all be re-done even if they check O.K. Do keep the wire size at least 18ga for the "A" battery for minimum voltage drop...
Chas
If the original tinsel cord is still in place for the speaker it may have a red thread in the cord, this is the positive thread and should connect to the tip of a phone plug if it is used. When inserted into a radio that has a speaker jack once the connection is made the frame of the jack becomes connected to B+ via the speaker but the shell of the plug makes shock contact difficult.
This same polarity should be applied to high impedance magnetic headphones as well.
The Antique Wireless Association has recently made available a circuit board and a parts list for a power supply that could power this radio. A part list and sources are included, cost estimate should not exceed $100, fabricate ones own cabinet.
https://www.antiquewireless.org/homepage/pcb-item-5/
You may find more battery set parts available at the Antique Radio Forum Classified ads... There have been recent posts in regard to A-K 20. Google with Antique Radio Forum in the search terms.
The ARBE-III folks have suitable high resistance audio transformers that would be more appropriate for this radio. The Hammond Radiola III replacement transformer is also O.K.
When the radio is completely restored a short and direct earth ground in place and an antenna of 35 to 50ft. used, if, the reception is still weak, consider lowering the value of the grid resistors from the 600 ohms to 500 or even lower. That will increase the gain of the RF stages. This value can only be lower until the radio oscillates uncontrollably. These resistors can be 1/4 watt carbon and do not have to be wire wound.
A major issue with 20's era radios are the connections both soldered and mechanical. They must all be re-done even if they check O.K. Do keep the wire size at least 18ga for the "A" battery for minimum voltage drop...
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
