[R-390] 60 Hz Hum Output from R-390A IF Module

[Jon Schlegel]

Hi Roger,

Thanks for the good ideas on grounding.  Some efforts to improve 
grounding on the extender have dropped the hum to about 33 dB below 
30% modulation.  Not too awful bad.  My next step will be to put the 
module back into the chassis as you suggest and I expect hum level to 
improve from that.  Guess I wasn't ready for the sensitivity to the 
extent that I'm seeing.  I can only guess that it's mainly all that 
heater current finding it's way back to home plate any which way it can.

I think the extender cable effort is worthwhile since I have been 
able to identify some sensitive hum entry points and this my lead to 
possibly doing some things to help desensitize the module to the hum.

I'm guessing that the bypass caps are ok as I replaced all the usual 
suspects with orange drops.  Ground point corrosion could still be an 
issue so I'll check into that.

Thanks again and regards,
Jon


At 05:58 PM 5/5/2009 -0400, Flowertime01 at wmconnect.com wrote:
>Jon Schlegel,
>
>Are you having any luck with your hum in the If deck?
>
>I am first going to suggest you drop the extender cable and get the IF deck
>back into the receiver. Back when, any test that was not all things in
>place was just a distraction. When being defined as 1968 to 1975. If 
>you have to
>work on the IF deck live (yes you some times must do this) then set the
>receiver up on end and swing the IF deck out on the bench. Stick the 
>knobs back
>on the IF deck shafts so you can twist the bandwidth and BFO. Get off the
>chair or get the receiver on the right height of bench that lets you look
>into the deck where you need to work.
>
>If this still gets you 60 cycle hum (it may) then start swapping tube out.
>Always tubes were the first line problem. Second is clean the tube pins and
>shields. Third is all the ground points need to get unbolted and rebolted to
>get the crud out of them. Do not remove the bolt. Just loosen, deoxit bath,
>and retighten. Fourth is a leaky cap. One of the bypass caps have gone
>resistive and not doing a good filter job.
>
>These are not easy to resolve. The deck forms a big loop and finding source
>point is not an easy task.
>
>Let us know how the project is going.
>
>Roger Ruszkowski
>
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