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 Post subject: Zenith 40A speaker
PostPosted: Sep Thu 16, 2021 3:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1714
Location: Mt. Prospect, IL USA
Hello all,

I managed to pick up the two chassis for a Zenith 40A. A previous owner had cut the umbilical cables at the back of the tuner/amp chassis, and I had to splice them back together as I was doing the recap. The only other fault found was an open grid resistor in one of the RF amp cans. The set plays well, but I need to have a proper field coil, unfortunately not having the original speaker. Does anyone know what the field coil resistance is for this set? I also have a 42A console with its original speaker. Could this be the same speaker as the 40A uses? The 40A uses a type 10 output tube, but the 42A uses the 50. (The fact that the tube came with the set is a small miracle...) If anyone can help, it would be appreciated...


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 Post subject: Re: Zenith 40A speaker
PostPosted: Sep Fri 17, 2021 2:35 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8157
Location: Near Brandon, Iowa
Chuck Braun wrote:
Hello all,

I managed to pick up the two chassis for a Zenith 40A. A previous owner had cut the umbilical cables at the back of the tuner/amp chassis, and I had to splice them back together as I was doing the recap. The only other fault found was an open grid resistor in one of the RF amp cans. The set plays well, but I need to have a proper field coil, unfortunately not having the original speaker. Does anyone know what the field coil resistance is for this set? I also have a 42A console with its original speaker. Could this be the same speaker as the 40A uses? The 40A uses a type 10 output tube, but the 42A uses the 50. (The fact that the tube came with the set is a small miracle...) If anyone can help, it would be appreciated...

40A schematics:
http://www.nostalgiaair.org/pagesbymode ... 040885.pdf

Looking at the 40A info, I don't see that the loudspeaker field coil is an integral part of either chassis' circuitry. Most loudspeaker-compatible sets of this era gave users the option of using a moving-iron permanent magnet speaker or an externally-powered electrodynamic speaker. The latter speaker had no connection to the radio other than the audio signal wires; the speaker field coil was powered by household line AC via a copper sulfide rectifier contained within the speaker cabinet.

So, you can use any speaker you like (even a modern permanent-magnet dynamic type) provided that you can match the low impedance of its voice coil to the high impedance of the radio's output tube plate circuit.

If your Model 42's speaker has no AC power cord, it is probably of the moving-iron type and can be used with your Model 40.


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 Post subject: Re: Zenith 40A speaker
PostPosted: Sep Fri 17, 2021 1:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1714
Location: Mt. Prospect, IL USA
Okay, thanks! The 40A power supply I have has a two-prong socket for the speaker field plug. There is a filter choke on the power supply, the output of which supplies B+ voltage to the type 10 output tube. Downstream from that is the field, which supplies B+ , through the voltage divider, to the RF and AF tubes. So at least I can be sure that not all of the B+ current flows through the field, so its resistance value would be less likely to cause overvoltage to the whole system...


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 Post subject: Re: Zenith 40A speaker
PostPosted: Sep Tue 21, 2021 11:16 am 
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Joined: Jul Mon 21, 2008 1:00 pm
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Location: Grantham, New Hampshire
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