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

[Jon Schlegel]

Roger

Since I only want to see the effect if the IF module, I've been 
looking only at the Diode Load audio signal rather than the normal 
outputs of the audio module.  With the IF module firmly mounted in 
the receiver in its normal configuration, here's what I'm measuring.

1)    With the receiver's existing AC heater supply, I see 60 Hz 
levels between 33 to 44 dB below a 30% modulated 1 kHz tone depending 
on bandwidth selection of 4, 8 or 16 kHz.  This is an 11 dB variation 
depending on mech filter selection.

2)    With an external DC heater supply, the 60 Hz level I'm seeing 
is about 38 to 39 dB below the same tone when for bandwidth settings 
of 4, 8 and 16 kHz.  Only a 1 dB variation depending on mech filter selection.

REMARKS: Since with DC heaters there is almost no variation in hum 
level across mech filter selection, I will assume for the time being 
that filter blowby isn't a problem.  Roy pointed out this issue and 
reminded me that I do need to check the module's out-of-band 
responses now that I've reinstalled the filters into the IF module.

So assuming for the moment that filter blowby is not a problem, why 
does the 60 Hz level change so drastically only when running on AC 
heaters and mech filter selection is changed?  I'm starting to think 
about things like the fact that the power connector suppling heater 
power among everything else is located at the same end of the module 
that the lowest level/most sensitive stages are located.

Regards,
Jon


At 07:23 PM 5/20/2009 -0400, Flowertime01 at wmconnect.com wrote:
>Jon,
>
>We are running out of easy things real quick.
>I missed the mail where you did isolate the filter cans and checked them
>all.
>
>Time to pull the back shells off the IF and Audio deck connectors and
>inspect for crud and good bonds on all the shields on all the shielded wires.
>
>You may want to go over the front panel and check the shielded wire bonds
>around all the switches also.
>
>I forget if you have an alternate receiver and have been able to isolate
>the problem to within a sub assembly or not.
>
>Roger AI4NI