Zenith 70/71 - Printable Version

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+--- Thread: Zenith 70/71 (/showthread.php?tid=26098)

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RE: Zenith 70/71 - RodB - 11-24-2025

Hey guys, logic tells me that if the transformer primary resistance measured 1300 ohms then the center tap to each end wire will measure about 650 ohms. If you don't measure any resistance on one of the leads then it's an open coil. It's possible that the wire lead has disconnected from the coil wire and can be easily repaired.

You don't have to match the resistance of the primary to a replacement transformer. A replacement transformer primary will need to have an impedance close to the plate impedance of the output tube and a secondary impedance to match the speaker. You will also have to use ohms law to calculate the power requirement of a replacement transformer. You can find charts online that match the tube with opt.

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RE: Zenith 70/71 - dconant - 12-13-2025

Hello, I have decided to continue on with this radio. I am going to scrap the original speaker and output transformer. I don't know what to get from Hammond for the output transformer. I will be using a permanent magnet speaker. I hope to be able to get the field coil frame out enough to use it. I would appreciate it if someone would recommend an appropriate transformer. Would one of the listed below work?

Also MrFixr55 says I can use a 10K pot across the antenna terminals. I assume he is saying a single winding pot. If so what do I do about the other winding? Not sure how to wire the pot so advice on this would be appreciated.

Thank you,
Dan

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RE: Zenith 70/71 - Arran - 12-15-2025

I don't understand why the speaker has to be replaced, if the audio output transformer is defective. The mechanical design of the original transformer is a bit unique, but that doesn't stop one from using a new one with the original speaker. I seem to recall that the plate load impedance of #45 tubes is similar to #42s/6F6s/6V6s that one that works with a push pull pair of those will work with the #45s. Anyhow the one you want out of that selection of Hammond transformers would be the 125D or 125E. Antique Electronic Supply has a generic audio output transformer that is less expensive, but I don't think it has the tapped secondary, you could also see if a NOS one of the Hammond, or Stancor multi tap ones shows up on fleabay. Another option is to try a line matching transformer for a P.A system, there are a lot of those around, new and used, cheap, what's more they have a big core in them so should sound good once you figure out the wiring.
Even if you end up using a new speaker don't toss the original, a new output transformer can mount on the frame of a new speaker, and so can a filter choke, if needed.
Regards
Arran

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RE: Zenith 70/71 - dconant - 12-15-2025

Thanks Arran, You are right so no harm just getting the transformer and trying it with the original speaker. If I decide the speaker is not going to cut it I can then do a perm mag speaker.

I still need to suggestions on how to deal with the volume control. There is no fixing the original one.

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