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 Post subject: Zenith c835 rectifier change
PostPosted: Nov Fri 15, 2019 8:18 pm 
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Location: Aurora Colorado
Well, I am working on this Zenith C835 , with the 8C01 chassis . I got it with no tubes,

and no tube shields, so I shelved it for a while. I ran into it again, and found that I had gotten

most of the parts, so dove in !

The recap went fine, and I changed a few resistors along the way. This has a selenium rectifier

in it , which I did not test. I went with a 1N5408 diode, and put a 72 ohm 10 watt resistor in front

of that , to limit the inrush current. I figured I could adjust it later, if needed.

My problem is that I get 166 volts at the first cap, which is the 82 mfd. The schematic says

I should have 120 volts there, So, I want to drop 46 volts. I have a .15 amp tube string, so, I used

44/ .15 and 293 ohms for the resistor, I used the dropper cap spreadsheet, and it came out the same.

So,I grabbed a 280 ohm resistor, put it inline with the diode, and I still have 166 volts at the cap !

So I decided to put the other rectifier back in the circuit, and I got a solid 120 volts !

If 1.141 gives r.m.s. voltage, that would be what I am seeing, so, once I add a load, will it change ?

I did do some reading about doing this on the forums, and it seemed like that was all I needed to

do. It is an 8 tube set, rated at 90 watts.

What Am I missing guys? Thanks for looking.

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 Post subject: Re: Zenith c835 rectifier change
PostPosted: Nov Fri 15, 2019 9:04 pm 
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Joined: Nov Tue 14, 2017 5:09 am
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Location: Austin, Texas
You will not get any significant voltage reduction until you have a load on the supply.
With no current, the voltage drop is zero across the resistors you added.
You will need to make the resistor value adjustment after you have the supply loaded.

Jay


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 Post subject: Re: Zenith c835 rectifier change
PostPosted: Nov Fri 15, 2019 10:08 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
The 0.15-amp rating of the tubes is the current needed to light their heaters. But they are not powered by the rectifier, so that current does not load the power supply. Consequently the filter capacitor charges up to nearly the full peak voltage of the AC line. The radio has to draw normal B+ current, then the voltage will come down to normal. Putting more resistors in series with the rectifier won't change anything.

Now why isn't the radio drawing B+ current? Well, if the tubes aren't lit, or if there is an open resistor in the B+ line, bad solder joint, or an open audio output transformer, that would result in what you are seeing. Once you get the B+ flowing again you'll probably find you don't need much resistance in series with the diode, but a small resistor to limit the peak current is a good idea.

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 Post subject: Re: Zenith c835 rectifier change
PostPosted: Nov Sat 16, 2019 12:32 am 
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Also agree that your set is missing the B+ load. If the tubes are all lit, then isolate the B+ nodes that are not conducting. Open output transformer primaries, open IF transformer primaries, open B+ resistors, any of these could result in low B+ load on the supply.

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 Post subject: Re: Zenith c835 rectifier change
PostPosted: Nov Sat 16, 2019 4:48 am 
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Right, I would suggest get it playing with the selenium rectifier, then decide if you want to bypass it with the new diode.
Hope your volume control is OK. Be careful not to use a strong solvent cleaner on those.

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 Post subject: Re: Zenith c835 rectifier change
PostPosted: Nov Sat 16, 2019 2:30 pm 
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Thanks to all of you for the explanation of the how the circuit works, and why it is not working.I am going to
add the tubes, and see how things shake out.
And thanks for the tip on the vol. control, :)

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