[EdHolland]

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

[MrFixr55]

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:

• The set may have a 35B5 or a 35C5.  They are not interchangeable; the basing diagrams are different.
  
• This is a true "Hot Chassis" set with the B- and low side of the filament string, along with the chassis connects to the power switch.  This means that if the chassis is at neutral potential with the power switch closed, it will be hot with the switch open. and visa versa if the plug is reversed.  The set does have an interlock.  I would recommend installing a polarized plug and move the switch to the Hi side of the filament string and input to the rectifier, to ensure that the chassis is always at neutral potential.
  
• The rectifier is selenium.  these do go bad and stink then they do.
  

Hope this helps.

[EdHolland]

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

[David]

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

[EdHolland]

Thank you David - very helpful info.