[R-390] hum

[Barry]

> I'm a very new subscriber and just received my first R390A today although 
> I
> became very familiar with this receiver during the 60's courtesy of Uncle
> Sam.  After checking tube seats I applied 120 VAC to the radio for the 
> first
> time today and was greeted by a nasty audible hum as soon as it was turned
> on and before the tubes had emission.  The hum is heard even with no 
> speaker
> attached and turning controls do not change the amplitude or frequency of
> the hum.  Tomorrow, I'll look at the output of the power supply board with
> my scope to verify any presence of ripple.
>
>
> Have I made an old beginner's mistake?  I do not remember this design 
> having
> a pronounced electro mechanical hum that is persistent after being turned
> on.  My R390A is a late model 1967/68 contract EAC model.
>
>
> Thank you,
>
> Natan W6XR

Hi Natan,

No, the radio should not hum.  Probably cause is filter capacitors in the AF 
deck, but it could be a lot of things.  The output of the power supply board 
should show nothing but rectified AC.  You need to get to the "other side" 
of the filter capacitors to see if they're not doing their job.

73,
Barry - N4BUQ