Repair of Allwave15 speaker field coil and transformer.

[David McGee]

I've been trying to restore for about a year my grandfather's Allwave15.  I've made a lot of progress but hit a wall with the speaker.  When I power up all the tubes seem to work fine but I get zero sound out of the speaker.  As best I can tell, the speaker transformer is not working correctly and the field coil isn't either.  The hum bucker seems to be ok.  I have seen references to installing a  filter choke and power resistor to avoid having to get the field coil rewound.  Is this a good idea for a permanent repair.  I'm not trying to keep the radio original, I just want it to work.  If this is not a good idea, please recommend where I should send the speaker to get it repaired.  Thanks so much. David

[David C. Poland]

FYI - the 2nd field coil is NOT a hum bucking coil. Suggest you carefully study the diagram for the amp and speaker. I have had half dozen AW-15's over the years. The speaker is a special design (by Jensen) for Scott.

The 2175 ohms field coil is in the B+ high voltage circuit - like most 1930's radios. 

The other 760 ohm field is in the negative bias circuit for the output tubes. (never seen it fail)

I had a couple AW-15 speakers with open B+ field coils rewound years ago, but he long since retired. They also needed re-coned as theold paper cone did not age well . 

Check the speaker's output transformer and the amp's interstage audio transformer carefully for continuity.

An old note suggests the voice coil impedence may be 8 ohm - If so, VTVM resistance be just over 6 ohms.

 A replacement output transformer for push-pull  2A3 tube should be rated at least 15 watts.

The field coil powers the magnet for the speaker to operate. To substitute a power resistor for the high voltage circuit, you will also need a suitable permanent magnet type substitute speaker . And also a substitute resistor for the other 760 ohm field coil. So, speaker repair probably the better option.

[David McGee]

Thanks so much for your help.  I really appreciate it.

David C. Poland said:

FYI - the 2nd field coil is NOT a hum bucking coil. Suggest you carefully study the diagram for the amp and speaker. I have had half dozen AW-15's over the years. The speaker is a special design (by Jensen) for Scott.

The 2175 ohms field coil is in the B+ high voltage circuit - like most 1930's radios. 

The other 760 ohm field is in the negative bias circuit for the output tubes. (never seen it fail)

I had a couple AW-15 speakers with open B+ field coils rewound years ago, but he long since retired. They also needed re-coned as theold paper cone did not age well . 

Check the speaker's output transformer and the amp's interstage audio transformer carefully for continuity.

An old note suggests the voice coil impedence may be 8 ohm - If so, VTVM resistance be just over 6 ohms.

 A replacement output transformer for push-pull  2A3 tube should be rated at least 15 watts.

The field coil powers the magnet for the speaker to operate. To substitute a power resistor for the high voltage circuit, you will also need a suitable permanent magnet type substitute speaker . And also a substitute resistor for the other 760 ohm field coil. So, speaker repair probably the better option.