[R-390] Humm problem,

[Bob Camp]

Hi

Well it depends a little on just how loud the hum is. Lots of hum 
probably means a blown filter capacitor. A blown filter cap probably 
would not take out the rest of the radio.

One quick "assumption" is that the audio gain still works. In other 
words the hum gets louder when you turn the gain up. Assuming this is 
true then:

The first thing you need to check before anything else is the magic 
blocking capacitor in series with the mechanical filters. DO NOT rotate 
the filter select switch until you are sure the cap is ok. If you have 
re-caped the radio then disregard this ....

Best bet would be one of the tubes later in the IF strip. When you 
loose one the gain of the stuff that's left isn't enough to give you 
noise on the output. All you get is the background hum that was there 
all along. Depending on how you have the IF gain set up you may get the 
same set of symptoms when one of the tubes in series with the current 
regulator goes open filament. There is also a marvelous piece of coax 
that goes from the detector over to the audio chassis that is known to 
do pretty much the same thing.

If the assumption above is not correct then you have lost one of the 
tubes on the audio deck past the gain control.

Either way an open filament is likely to be the way the tube went out. 
A quick check to see if they are glowing or not may be the fastest way 
to find the problem.

	Enjoy!

		Bob Camp
		KB8TQ




On Oct 4, 2004, at 7:28 PM, Charles B wrote:

>
> Hi all,
>
> Any ideas on the following problem.  When I turn the R-390A on and 
> wait until it warms up, I get nothing but a Humming sound in all three 
> positions: AGC, MGC, CAL?  There are no signals.  This happened 
> unexpectedly when I turned the receiver on one morning.
>
> Thanks
>
> Chuck
>
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