Zenith M-723

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Zenith M-723

Post by Ed in W. PA on Thu Mar 20, 2014 10:39 pm

Latest project is recapping and cleaning up this Am-Fm table radio. I removed the selenium rectifier and replaced it with a modern diode and my question is by doing this alone, would the raw DC increase due to the lower resistance of the modern diode? I measured the DC at the first filter at 145v at 120vac, Higher than the 139v indicated on the scematic.
Thanks, Ed

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Re: Zenith M-723

Post by 35Z5 on Thu Mar 20, 2014 11:48 pm

Six volts isn't anything to worry about, modern electrolytics will handle a surge of 20-25% higher than the ratings...

If you monitor B+ at turn on, it'll be closer to 165v till the tubes warm up and start drawing current...
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Zenith M-723

Post by Ed in W. PA on Fri Mar 21, 2014 12:01 am

I really wasn't worried about the six volts as this scematic didn't say what the ac input voltage was ( probably 117vac ). Just want to know if there is a difference between the selenium rectifer and the diode voltage.
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Re: Zenith M-723

Post by 35Z5 on Fri Mar 21, 2014 8:50 am

Yes selenium has more voltage drop, silicon will give higher B+ output...

I was thinking there'd more more rise than you have, but singled ended audio output doesn't draw heavy current...
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Zenith M-723

Post by Ed in W. PA on Fri Mar 21, 2014 9:53 am

This being the case would it be wise to increase the value of the current limiting resistor that is in series with the diode? It is presently 22 ohms 1 watt.

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Re: Zenith M-723

Post by Bill Cahill on Fri Mar 21, 2014 11:35 am

You need to raise the resiststance due to the fact that siliconss draw less current.
On todays' line voltage, especially. That set was designed to run on 117 volts ac. Today's is about, with some variations, 124 volts ac.

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Re: Zenith M-723

Post by 35Z5 on Fri Mar 21, 2014 12:45 pm

Ed in W. PA wrote:This being the case would it be wise to increase the value of the current limiting resistor that is in series with the diode? It is presently 22 ohms 1 watt.

You can up the resistor to 33 without any issue but at only a six volt increase, I'd probably just leave as is...
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Zenith M-723

Post by Ed in W. PA on Fri Mar 21, 2014 1:33 pm

Thanks for the advice! I did increase the dropping resistor to 33 ohms and have since buttoned this one up.Oddly enough I checked all of the caps that I changed and they all checked good for value, leakage, and ESR, even the three section electrolytic. I think 48 years speaks well of the quality of something made in the USA.
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Re: Zenith M-723

Post by 35Z5 on Fri Mar 21, 2014 2:31 pm

I wouldn't have even bothered with that new of radio that was performing as expected...

There's at least three around here from 1940/1941 that play just fine, have the orig electrolytics and most of the paper caps... One had a section cut loose and a replacement electrolytic just hanging, well that failed but the main sections of orig are still good...

I don't recommend anyone taking my path without at least checking current draw... Of course if they have hum it's a no brainer...
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Zenith M-723

Post by Ed in W. PA on Fri Mar 21, 2014 2:37 pm

I was mainly concerned about that selenium rectifier. I have had a few of those cut loose over the years and the odor they produce is quite unpleasant.

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