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 burning scorched filter resistor

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devildog63



Join date : 2016-05-04

PostSubject: burning scorched filter resistor   2016-05-04, 5:00 pm

I built a emerson 547A radio from scratch using a schematic. It uses a 35w4 rectifier tube. I have a 1k ohm 1 watt resistor that connects across the two filter capacitors on the B+ side that is just scorching hot. It's so hot I swear it's gonna melt. It was blue now it's a dull brown due to the incredible heat it is having to dissipate. What in the world is causing this? Is it a a schematic error ? I though about going to a higher wattage resistor but the schematic cal,s for a 1 watt. BTW this is the first radio I have built. Any help with this is greatly appriciated.
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Cliff Jones
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Join date : 2010-11-22

PostSubject: Re: burning scorched filter resistor   2016-05-04, 5:16 pm

That resistor is getting hot because of too much current, a short, and that leads to a bad filter cap(s) one of the most common problems. If the rectifier glows cherry red on its plates, turn it off imeadiatly to prevent damage.
Ps: you may have your filter caps backwards, so check for correct polarity wiring, and correct voltage of those caps!

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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-06, 6:07 pm

Thanks for the response. I checked polarity of the caps and they are correct. The rectifier tube is normal. Plate is not red. The only thing that is downstream from the resistor is the output transformer and then it goes to the plate of the 50b5. Plate voltages are normal. Also has good reception and performs pretty well. I have a 33 uf and 47 uf for the filters. If one of these went leaky would that cause the high current thru the resistor ? I'm guessing it would since the other side of the cap connects to ground. BTW all my components are new besides the ones that are no longer manufactured in if transformers etc. Thanks alot!!
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Cliff Jones
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PostSubject: Re: burning scorched filter resistor   2016-05-06, 8:47 pm

I mentioned also to check the actual voltage that the capacitors themselves were to be operated at, not the voltage of the radio. What is the maximum operating voltage the capacitors can tolerate? Should be printed on the capacitors. They must be higher that the DC operating voltage of the receiver.

And yes leaking capacitors can cause this issue, the voltage only see's  the eisiest path to take and a leaky capacitor looks like nothing more than a low resistance path to ground. Now AC VOLTAGE is meant to pass to ground, that's part of the filtering needed, but it's suppose to block DC VOLTAGE from going to ground. So the DC VOLTAGE sees a low resistance, which is called leakage in this case will go to ground causing excessive current and that overheated resistor goes into thermal meltdown.

Or the resistor has already failed from a previous short, and it's resistance, has dropped, and allowing more current to pass, being a hotter running resistor. You may also have an incorrectly wired part, or a shorted transformer, power, audio, IF, or tube or resistor. Test the out of circuit values of suspected parts.

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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-07, 5:53 am

Rodger that Cliff. I know my filter caps are 500 volt, I always go above the voltage called for. I'll check the other items soon. Mother's day weekend. I do have a issue with the IF can between the detector and output. A loud hum comes up unless I press down on one of the variable caps with a screw driver on top of the if can to get it to stop. I aligned the radio with a rf generator and everything trimmed out good. It's a shame they don't manufacturer those anymore. Thanks
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Cliff Jones
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PostSubject: Re: burning scorched filter resistor   2016-05-07, 6:01 pm

Look also at the external cap, if I remember it's a 0.02 mfd's designated as 8 or 9 on the schematic. If that's shorted or is leaky you would get hum and it's connected from the IF TO B+, lift one lead and see if that filter resistor still gets hot.

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FrankB
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PostSubject: Re: burning scorched filter resistor   2016-05-14, 3:44 pm

Most likely bad filter cap. replace them both, and check for a leaky/shorted screen bypass cap too.
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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-14, 6:07 pm

Thanks for the reply. As I have mentioned I built this set from scratch with all new parts with the exception of the ones no longer manufactured. I did replace the filter caps just for fun but it still gets hot. CLIFF, I still need to check cap between the IF and B+. Is it common to get bad new parts? I usally buy from mouser and antique electronic supply. One half of the tunning condenser has a loud deep grinding noise, the lower half like from 540 to 900kc. Above 900 is pretty good. Thanks guys
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FrankB
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PostSubject: Re: burning scorched filter resistor   2016-05-15, 3:23 pm

The grinding noise in the tuning cap is likely the plates touching each other. Use a strong light and a magnifying glass to see the short when you first start hearing the noise. You may even be able to feel the plates rubbing together too.

New parts are sometimes bad. I always test new parts before using them. Over the  years I have found more than one bad one, DOA.
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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-20, 7:14 pm

Ok, I have checked all my caps, no leakage. I did tweak the variable tunning condenser plates, I ohm the sections out but didn't get any continuity . One interesting thing I noticed was that when I turned the tunning cap to a area where low speed grinding noise the dial lamp started to flicker, dimming, is there any good solution for cleaning the tunning condenser? * note after I tweaked the tunning condenser plates it seems to do a little better.
Thanks a lot! !
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Cliff Jones
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PostSubject: Re: burning scorched filter resistor   2016-05-21, 12:09 pm

Did you replace that resistor?
Which part of the capacitor that shorts, the RF section or the oscillator section?
After making the adjustment does the resistor still heat up?
Do you think the problem still needs attention?

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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-21, 2:32 pm

I did change the resistor and it does not heat near as bad as before. Not sure which section it's coming from the oscillator section seems to be working fine. The trimmer cap on the antenna side has very little effect on anything when adjusted, the reciever aligns well with a signal generator . This tunning cap is from a GE circa 1950. So I'm guessing that the dimming dial lamp while turning the cap means the cap is shorting out? Still learning here.
Thanks a lot!
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Cliff Jones
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PostSubject: Re: burning scorched filter resistor   2016-05-21, 6:39 pm

Also look at the capacitor mounting. If the mounting includes rubber grommets, or rubber washers, then when you move the tuner, it may also be a cause because of deteriorating over time. To know which section is the RF it is always larger than the oscillator section. If they are the same size, the distinction would be that number of stator and rotator would be more in the RF section. You can also trace the RF antenna wires trace back to loop or external antenna connection. Also you can look to see which is shorting by looking at the capacitor with the radio on, and in a dark room move the tuner and when you see a spark, then you not only know which section is shorting, but whic plate is shorting.  

Glad to hear of the resolution. 
Also note that the dial lamp must be replaced when it is burned out. The reason is that the pilot lamp balances the current going through the tube rectifier filament. If it burns out your rectifier will have a pronounced reduction in life, because that tube has two filaments, and when the dial lamp burns out more voltage is now on the remaining section that is left, of the rectifier.

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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-22, 9:00 am

I grounded the variable tunning condenser to the chassis using the mounting screws that hold it. Since the schematic shows this . Is this a incorrect way of doing this ? The junk radio I got the variable tunning cap from had isolation gromets. Could the way I connected it be a problem?
Yes my oscillator section is smaller and RF is the larger section.
Thanks for all the help.



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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-22, 4:59 pm

Well guys looks like I found the major problem I was having with all the noise and grinding junk when i turned thru the frequencies.  I had the tunning capacitors frame grounded using the mounting screws. For some reason beyond my understanding it needs to be isolated from the chassis and a separate ground wire attached to the tunning capacitors frame. I used rubber grommets for mounting it . Also the resistor across the filter capacitors is barely warm. Tunning between stations is nice and quite now.  I have found that when things don't go as planned is when you learn. I could not have done it without guys directing me around the radio. 
Thanks alot.
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Cliff Jones
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PostSubject: Re: burning scorched filter resistor   2016-05-23, 1:59 pm

Glad you solved your problem. Cheers. I salute you for you tenacity.

Can you post a picture of your radio?

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devildog63



Join date : 2016-05-04

PostSubject: Re: burning scorched filter resistor   2016-05-23, 5:27 pm

[You must be registered and logged in to see this link.]
By all means. The construction is a emerson 547A.  It's just a chasis and that's all it ever will be. This was my first goal to build a AA5 and I picked the 547 cause the schematic looked simple. It's been a great sense of accomplishment . Next I want to build a short wave or am/fm tube radio. Not sure if I want to build it from scratch or restore one .thanks again!
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Cliff Jones
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PostSubject: Re: burning scorched filter resistor   2016-05-25, 1:53 pm

Congratulations on your nice looking radio!

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