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

[Jon Schlegel]

Hi Roger

I installed new Power Supply electrolytics, and recapped the Audio 
and IF modules straight away when I first got the receiver.  Haven't 
done the RF deck yet.

Also I'm not in a position yet to accomplish the complete procedure 
that you've outlined here but from what I've seen of the raw audio 
output of the detector at the Diode Load, I'd say that I'm probably 
within the ballpark of the -30 dB of leftover hum and noise that you 
describe.  Once I deal with the "bursting" oscillation that I've 
previously described I can move on in that direction.

So perhaps hum output level really is "ok".  If that's the case then 
it bothers me that the impressed 60 Hz hum is easily visible on the 
scope as a component riding on top of a recovered 1 kHz tone.  I 
guess my mind set is that if I can see it on a scope, it's really NOT 
acceptable.  So if I can determine the real entry point(s) of the 60 
Hz, perhaps something can be done to improve it.

Regards,
Jon Schlegel



At 07:30 PM 5/8/2009 -0400, Flowertime01 at wmconnect.com wrote:
>Jon,
>
>A thought has come to me that you may want to change the B+ caps for the
>audio and B+ in the audio deck. If you still have the original plug 
>in cans or
>some other old caps, it could get you enough leakage to ripple through the
>IF and audio decks.
>
>How much hum are you getting? Most receivers will give you some hum when
>the RF and Audio are set to max. Run 150 UV of 455 into the IF deck and look
>for -7 volts on the diode load buy setting the Gain pot on the IF deck.
>
>With modulation on you should get 1/2 watt out on the local audio pins of
>the rear terminal board. And 100 Mili watts on the line audio as read with
>the line level meter.
>
>
>Turn the modulation off and you should get a 30 dB drop in the output on
>the local audio pins. Use an AC volt meter with a dB scale and a 600 ohm load
>resistor to measure the half watt of power and 30 dB down from that point
>with the modulation off.
>
>If the receiver will do this for you, what ever level of 60 cycle hum you
>have is within bounds for the receiver. If your hum hold the noise level up
>so you cannot get the 30DB difference, then you do have a problem and do need
>to do some work to find the problem.
>
>Use the procedure in the Y2K manual or a PDF copy of the TM for setting the
>IF gain pot. Start reading there to set up the signal generator and the
>receiver for the test and adjustments.
>
>Roger AI4NI
>
>
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