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 zenith 7C05 keeps blowing resistior......

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29lowboy



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Join date : 2010-10-15

PostSubject: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 6:05 pm

in attempt to hook up a dial light i did something terrible and now when i switch on, i smoke the 22 ohm resistor on the + side of the rectifier. I replaced the rectifier with a diode and series resistor but no luck...... the 3rd new 22ohm lasted about 5 seconds longer with the diode but still blows aparts in a smoking fit Crying or Very sad i dont know where to go from here, please help!
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Tube Radio



Posts : 31
Join date : 2010-10-12

PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 7:10 pm

If the radio did not come with a dial lamp originally, remove it. Your resistor will then stop blowing. of course if you really want the dial lamp just increase the wattage rating of the 22 ohm resistor. What you do is when it is working measure the voltage across the resistor. Then use ohms law (I-V/R then P=I times V) to figure the wattage the resistor is dissipating. Ideally you want a resistor rated at twice the wattage, but anything over the wattage dissipated will work.
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jack shirley



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 7:12 pm

Sounds like a B+ short and not necessarily related to the dial light. Do you have fresh filter caps and are they installed correctly? And since you're popping the resistor you might want to check to see if the new diode has gone to a short.

That resistor is basically a surge limiter to protect the diode. Any sort of B+ overload will cook it.
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 8:53 pm

well the dial lamp is not in there at all now, it was when i had temporarily hooked it up i had some static that was'nt there before, so i unhooked the lamp and the static went out with the light. And i thought i had the wrong terminal so i moved it and on power up smoked a 22 ohm resistor underneath the chassis.....i knew i messed up so i completely removed the lamp and replaced the resistor only to see it burst open and smolder again on power up. i was told it could be the rectifier and so i replaced it(and the resistor) and on power up POOF! smoked it again, just not as fast this time. I recapped this radio and its was working great with no issues at all until i tried to hook up the dial lamp..... i hope i did'nt fry something big.....
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tubes4life



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Location : northern Florida

PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 9:37 pm

I'm thinking that when you moved the terminal and burnt the first resistor, you probably smoked the filter caps as well, and now they are shorted. Try some new filters and see what happens.

One more thing; are you working from a schematic? If so, make sure your connections match the schematic.
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 9:42 pm

I was working from a schematic of a similar chassis but i found the right one now, do i have to recap the whole radio? or just the electrolytics?
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tubes4life



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 9:57 pm

Just the electrolytics.
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Johnnysan



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Fri Oct 15, 2010 10:40 pm

Did you completely remove the lamp socket and all wiring to it? After that there is only the 22 ohm resistor and the rectifier. It sounds like there is still a direct short to ground after the 22 ohm resistor.
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 5:38 am

everything was removed- lamp and wiring.....there are only two electrolytic caps in this radio
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pixellany



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 6:19 am

Can you post a link to the schematic?
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 5:49 pm

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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 5:50 pm

its resistor r40
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 6:43 pm

okay, good news - and bad news, i replaced both electro-caps and the resistor, powered up and did'nt smoke the resistor!! Very Happy i even tuned in a couple station but quickly smelled something burning......my 35C5 output tube was glowing cherry red on top so i yanked the cord. well now i fell like im getting somewhere, should i replace the rest of the caps and the 35C5 and see where im at?!?
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Tube Radio



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 7:11 pm

Yes if there are any paper caps replace them all. Also check the ceramic disc caps. If any are cracked replace them. Wouldn't hurt to replace all the resistors as well or if you would rather not do that then at least all that are out of tolerance.
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sat Oct 16, 2010 8:10 pm

all the paper caps were replaced abotu two months prior to this whole situation
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smeezekitty

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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sun Oct 17, 2010 12:05 am

Check resistors.
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sun Oct 17, 2010 4:39 pm

as im checking the resistors i found something strange, i have the schematics with pics and im checking them one at a time moving down the line, im underneath the chassis and i find R38 is not there.......or even evidence that it ever was. the terminal is clean of solder. the resistor is supposed to bridge two wires on the terminal strip, one leading to the M5 and one to A17, instead of being on seperate terminals divided by resistor they meet on the same terminal with a 01 disc capacitor to ground. is this a mistake that was missed by the inspector or was this this a change of design in later year model?
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sun Oct 17, 2010 6:16 pm

the more i look at the schematic and the picture and my radio the more things im finding that dont match.........this is a 7C05 chassis stamped right on there.
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29lowboy



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PostSubject: Re: zenith 7C05 keeps blowing resistior......   Sun Oct 17, 2010 6:22 pm

so if the electrolytic caps were shorted is it possible that the fixed caps that i replaced for the paper ones could be shorted too, and what about the transformer?
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