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 Post subject: Zenith H723 and H724
PostPosted: Mar Mon 23, 2020 2:39 pm 
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Joined: Dec Sun 02, 2007 11:59 pm
Posts: 119
Location: Alabaster, Ala.
All the Zenith AM/FM models have pretty much the same circuit with the FM section tuned by cores that go in and out of coils to vary the inductance. EXCEPT the H723 and H724 that used variable capacitors and fixed inductances to tune the FM section.

Was this due to a shortage of components?
Or an experiment to try a different version of their tried and true tuning method?
Maybe a cost saving measure? Which was more costly, the variable capacitors or variable coils?

If anyone owns one of these, how do they perform compared to the usual Zenith circuit?

The Z suffix models also had capacitor tuning. The Z1 and Z2 suffix models went back to inductance tuning.


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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Tue 24, 2020 9:31 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beaver Falls, PA. USA
Likely due to parts shortages during that era, 1950-51.

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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Tue 24, 2020 11:35 pm 
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Joined: Dec Sun 02, 2007 11:59 pm
Posts: 119
Location: Alabaster, Ala.
Tim, I think you are right. This was when FM was just starting to get popular and they were probably selling all they could make and ran short of the variable inductor mechanisms.

Another possibility... perhaps they subcontracted this model to another manufacturer who was more comfortable building and aligning a connventional (variable capacitor) circuit.

Alan
N4MFA


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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Fri 27, 2020 5:28 pm 
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Joined: Jan Sun 06, 2008 3:28 am
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Location: Richmond, VA
I have a H724Z3 I restored. It tends to drift in FM mode. I don’t know if that is a circuit design issue or my electrical restoration was inadequate.

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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Fri 27, 2020 6:36 pm 
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Joined: Dec Sun 02, 2007 11:59 pm
Posts: 119
Location: Alabaster, Ala.
Ed, I have a bunch of AM/FM Zenith radios and every one drifts on FM. After a few minutes they are stable. It's most noticeable on weak stations that need exact tuning to be listenable. Strong stations can be mis-tuned a little and I don't notice it.

That's probably the reason all the later models have AFC (automatic frequency control) to keep them on frequency.

Your 724 looks almost new.


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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Fri 27, 2020 6:57 pm 
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Location: Richmond, VA
Thanks, Alan. I wondered if it might be the early design. One nice feature of the the radio above is that it does have a tone control on the side. Audio quality is decent.

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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Sat 28, 2020 4:53 pm 
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Location: Vancouver Island, Canada
The cure for the drifting FM is a power bar with a switch. No adjustments needed, just a bit more patience. 8)
I like those, Lefty with a handle.

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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Sat 28, 2020 8:26 pm 
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Location: 253 Blanche St. Plymouth, MI USA
My guess is the capacitor tuned ones were cheaper to make.

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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Sat 28, 2020 8:49 pm 
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Joined: Sep Wed 10, 2014 7:13 pm
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Location: Urbana, Illinois
The use of slug tuning for FM was somewhat unique to Zenith.
Zenith's design changed in the late 1950's when they started to use a self-contained "modular" FM front end (12DT8 tube). But it still had slugs for the FM tuning. Just a different mechanical design.

I don't own any Zenith AM/FM sets which have capacitor-type FM tuning. But nearly all of my non-Zenith AM/FM radios use capacitor tuning for FM. This is seen as a couple of main tuning capacitor sections which have only 2 or 3 rotating plates per section.

-EB

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Last edited by electricboyo on Apr Fri 24, 2020 5:21 pm, edited 1 time in total.

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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Sun 29, 2020 6:15 am 
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Joined: Feb Mon 21, 2011 4:19 pm
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Location: San Marcos, CA 92069
I have a Zenith T724 I plan on restoring, but it still plays with all original parts with a slight hum. I don't notice any drift on FM with it. After reading this thread, I tried it turning in on a few days in a row, and the FM station comes in clear when first turned on and doesn't change after over 15 minutes.

I also have a Zenith K725 I restored last year. It had a slight drift the first few minutes of warm up. The FM station would go from slightly distorted to clear over about 3 minutes. Surprisingly changing the old RCA 12AT7 to a NOS Sylvania 12AT7 eliminated any noticeable warm up drift.


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 Post subject: Re: Zenith H723 and H724
PostPosted: Mar Sun 29, 2020 7:49 am 
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Joined: Sep Wed 10, 2014 7:13 pm
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Location: Urbana, Illinois
nash wrote:
Surprisingly changing the old RCA 12AT7 to a NOS Sylvania 12AT7 eliminated any noticeable warm up drift.
One half of the 12AT7 is the local oscillator.The tube itself can be a source of drift. Might need to select from several tubes to get the most stable one.

AFC became a standard feature in FM radios around 1955. It works. If you want to use a vintage radio for daily FM listening it would be best to select a model which has AFC.

-EB

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