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model 41-280 noisy static - madsowry - 12-25-2018

Hi all
     Working on this model 41-280.  Changed all caps and resistors out of spec.  Cleaned tuning cap real good.  Powered it up and got pretty good reception right away but so noisy can hardly make out what they are saying.  It's hard to even align it because of the noisy static.  Hard to make out the tone from sig. gen.  Maybe I got a noisy resistor in the works somewhere.  I even tried a suggestion from a post on ARF putting a .1uf cap between the ant. and radio.  Helped a little but still really noisy.  Any help would be greatly appreciated..

www.nostalgiaair.org/pagesbymodel/415/M0013415.pdf

Thanks

Dick

RE: model 41-280 noisy static - Radioroslyn - 12-25-2018

You might try using your .1 cap w/one end connected to the chassis and the other go thru and connect it to the control grid of each tube. In the rf and IF stages it will significantly cut the signal. At audio it will lower the tone quite a bit. This maybe enough to determine what stage is the problem.

Just as a wild guess it could be the output transformer.

Happy troubleshooting!

RE: model 41-280 noisy static - madsowry - 12-25-2018

Thanks Terry  I'm assuming I should connect the .1uf to the control grids one at a time.  Should I hear no static that would be the stage in trouble?  I ohmed out the output trans. and it was right on.  I suppose underload it could be weak or something.  Also my voltages are all within 10v. of what the schematic says except the XXL tubes.  The 1st Det. is supposed to be 95v, mine measured 65v.  The Osc XXL measured 42 and was supposed to be 40v.  I can wiggle all the tubes and don't get any popping so I think the pins are okay.

RE: model 41-280 noisy static - radiodale - 12-25-2018

I file all loctal tube pins before I turn the radio on.

RE: model 41-280 noisy static - Radioroslyn - 12-26-2018

> Should I hear no static that would be the stage in trouble? Yes but check the stage before and after to be sure. If you have a tracer sometimes you can find the faulty stage but if the noise is coming from the hv (like arcing)sometimes it will show up everywhere w/the tracer. In that case you can disconnect the hv feed stage by stage to find the fault.

Have a look at 26,28, and 30 these could cause low voltage on the XXL's. Here's a better diagram: http://www.audiophool.com/Philco.html

RE: model 41-280 noisy static - madsowry - 12-26-2018

Terry    I tested like you said and on the 2 7B7s it cut the static out and I could hear the tone from the sig. gen. good.  On the 1st Det XXL it cut the static out just like the other two.  The OSC XXL didn't cut the static out just dropped in intensity a little bit.  Could the Osc XXL be a little faulty?  It measured good on my Jackson tube tester.  Or am I thinking about this in the wrong way.

     Also I did find a mistake on my part on part 30.  47k instead of 4.7k.  DUH.  But the voltage only increased 12 volts to 77,  should be 95v.

     Thanks for the tip Radiodale.  First time working with loctal tubes so that is good to know.  I filed them shiny but still have the noisy static problem.


Dick

RE: model 41-280 noisy static - Radioroslyn - 12-26-2018

So what happens if you remove the XXL osc tube? Static gone?

RE: model 41-280 noisy static - madsowry - 12-26-2018

No still there just no reception.

RE: model 41-280 noisy static - Radioroslyn - 12-26-2018

I understand but is there noisy static??

RE: model 41-280 noisy static - madsowry - 12-26-2018

Yes      the same noisy static as when the tube is in.

RE: model 41-280 noisy static - madsowry - 12-26-2018

I'm wondering if maybe one of the three mica caps that are in that area could be leaking.  I checked the resistors- they seem good.  When I switch band to police it gets a little quieter and then to SW it gets real quiet.  Seems like its something in the area of the osc tube.

RE: model 41-280 noisy static - Radioroslyn - 12-26-2018

Best guess is 25 the 250mmfd cap. To double check the osc for noise disconnect one end of 26 (33K resistor org,org,org). This will remove the high voltage from the 250mmfd cap and osc.

RE: model 41-280 noisy static - madsowry - 12-26-2018

Terry     
      I removed one end of 26 and the noise was still there.  Changed out the 250mica part 25 with a 330 pf and still the same.  I cleaned the band switch before and I'm thinking if it was that it would pop when you switched bands.  I don't know.  I'm at a loss.  I'll walk away and give it a couple days to fix itself. HAHA

Thanks for all your help

Dick

RE: model 41-280 noisy static - Radioroslyn - 12-26-2018

Ok try disconnecting the right side of 28. This will kill the hv to the mixer and osc tubes and sg voltage to the if's.

RE: model 41-280 noisy static - madsowry - 12-26-2018

OK  I separated 28 on the right side and I got a slight hum out of the speaker with the volume at max.  No noisy static.  I put a clip cord on 28 and touched back where it was and got the noisy static.  Took it back off and  connected my sig gen up and got a nice tone.  So what is this telling you cause I'm still lost.  It seems to me that you cut the radio in half so now you know the problem is either in the IFs or the XXLs.  Is that anywhere near right?