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Zenith H725 - Printable Version

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Zenith H725 - EdHolland - 03-17-2025

Hi Folks,

It has been quite a while since I had a genuine antique radio project, but a new one is set to come this way soon. A friend of Mrs H wants me to take a look at a radio that belonged to her Grandma. It is a Zenith H725, AM/FM receiver Circa 1952, and looks quite interesting. This will be the first series heater string radio I've worked on, and also the first without a transformer, so I'll have to brush up on the safety equipment and procedures around here. 

I am expecting to get a first look at it some time this week, and will update here as things progress.

Ed

RE: Zenith H725 - MrFixr55 - 03-17-2025

Ed, I have one of these and the performance is rather surprising considering the 1.0 - 1.5W output stage.  Having an 8" speaker and a solid state rectifier instead of a 35W4 certainly helps the performance of this unit.

The schematic is available on Nostalgia Air.  Most of the caps in the set may be ceramic disks.  The electrolytic on my et is still good.

Do be careful of these issues:
Hope this helps.

RE: Zenith H725 - EdHolland - 03-18-2025

Thanks, Mr Fix,

I found the schematic, and noted the hot chassis configuration and the selenium rectifier. Didn't know about the possibility of different O/P tubes so that is very helpful to know.
You anticipated my next question which was regarding changes to the power switch wiring for improved safety - I'll look into these mods once things are working as originally designed. I may also pre-empt rectifier problems by substituting a modern diode + whatever series resistor may be required to ensure operating voltages are correct.

Cheers,

Ed

RE: Zenith H725 - David - 03-18-2025

Ed Have fun with the set have 2. The Selenium drops the voltage app 7 volts the 1n4007 drops voltage app 7/10 of a volt. Make sure to install a polarized plug, and check the outlet were the lady will be playing the radio. David

RE: Zenith H725 - EdHolland - 03-18-2025

Thank you David - very helpful info.

RE: Zenith H725 - Arran - 03-18-2025

Ed:
I'm not sure how much work you want to put into this but you could convert the set from being a hot chassis to a floating chassis set. Someone from the U.K on the A.R.F mentioned that they like to do this with British radios over there, since as he said "getting shocked by 240 volts hurts a lot". IN fact I was looking over mt Bush DAC 90A and I recon it would be easy to do it with that set since most of the caps are mounted on a terminal board. The H725 might be more difficult, I don't know, but then again you could just copy a floating chassis set, like a Philco FM model.
Regards
Arran

RE: Zenith H725 - RodB - 03-19-2025

It's easier to attach a polarized plug ensuring the wide connector of the plug is connected to the striped wire and the striped wire connected to the B- of the radio. Then connect the other plug prong to the power switch. When the set is off, the only power in the chassis is at the switch.

RE: Zenith H725 - EdHolland - 03-19-2025

I'll take a look at all that once the set arrives on the bench. Hopefully this will be tomorrow, and I can then do a proper assessment. First things first of course.

Assuming the insulating back is still intact and there are no exposed metal areas, I suppose the hot chassis is not technically a problem, but it can be improved, if polarized plugs and house wiring can be trusted.

The floating chassis idea is interesting.

Yes. 240V hurts (I am from the UK)

RE: Zenith H725 - morzh - 03-19-2025

We had 220V (I am from the USSR) and I am sure at that point one would not be able to tell the difference between 220V and 240V and judge what hurts more Icon_lol
I had my fair share of zaps. 120V does not come close.

RE: Zenith H725 - RodB - 03-19-2025

So, what does an AB5 (All British 5) do with the remaining 120 volts or so. They must have higher heater voltages.

RE: Zenith H725 - morzh - 03-19-2025

They probably did.
You could look up my Telefunken restoration 2 years or so ago. (has both the sch with tubes and my mod for 120V).
They still have a bucking resistor, but when converted to 120V, there was not enough voltage left for the rectifier (50V required) so I used solid state doubler.
The summary voltage of the restr of the tubes is 100V; with the rectifier it would be 150V, so you still had to buck 70V.
But for conversion to 120V it is not enough.

RE: Zenith H725 - David - 03-19-2025

The AA5 is American set, made in EU sets have power transformers. Hallicrafters did sell some of the S-38 series in Europe by adding a step down transformer. The reason for making the statement about checking the house wiring if the place was built before 1978 no ground wire. The amount of creative wiring out their is amazing. The nice thing about this set is the plastic cabinet and plastic knobs. David

RE: Zenith H725 - Arran - 03-19-2025

Well the Bush radio I mentioned, as well as the Phillips "Philleta" I have, had a large wire wound resistor with taps on it, the Phillips was designed to run on anything from 250 volts, down to 120 volts, the Bush is more like Mike's Telefunken, the set was designed to run off of voltages ranging from 200-250 volts. Murphy was another British company that made a lot of hot chassis AC/DC sets, they also liked using rubber/gutta percha covered wire apparently, which likes to disintegrate like the stuff used in 1939-42 Philcos.
Because of the electrical code in Canada all of the AC/DC sets had a floating chassis, and they didn't even allow those until around 1940 or so. So the only hot chassis AC/DC sets I see are imports from elsewhere, usually the U.S, but it seemed to be the prerogative of the manufacturer, Brand Z did this, as did Fada, but Philco built floating chassis sets regardless of whether they were US or Canadian made. I have an Airline 94BR-1526 from around 1949, and they ran the B- in a rather interesting way, they used a length of 14 awg wire and ran it from the power switch, to another fixed point, below the tube sockets, and then tied all of the electrical grounds onto it.
Regards
Arran

RE: Zenith H725 - Paul Philco322 - 03-19-2025

[Image: https://radioattic.com/soldradios/sold_radios_2008/Kaetz_Zenith_H725_(1951).jpg]
Nice looking and large Bakelite set. Ed Romney in his book Fixing up Nice Old Radios covers this one extensively. I have a copy and may be able to scan you the pages covering this set. LMK. PM me Paul B

RE: Zenith H725 - EdHolland - 03-20-2025

Thanks, Paul - I'll let you know.

...This is a wonderful community of friends Icon_smile

To answer the AB5 question, I have a couple of UK sets, in particular, a Ferranti 125 very much like this example:

https://www.snellingsmuseum.co.uk/artefact/audio/ferranti-125

Series string heaters (100mA), and a large dropper resistor take care of the 240V. This radio uses the U series valves with the B8A base - something of a short-lived offshoot, for example the output valve is the UL41 heater 45 Volts at 100 mA http://www.r-type.org/exhib/abc0004.htm.

You do get quite a bit more HT, which is a benefit for the audio output, but things do get warm, even on the large chassis. Mine is unrestored (but worked well when I got it). I'm sure it will get some attention eventually.

Mrs H should be collecting the Zenith during a lunch meet-up with our friend today.