[R-390] hum

[Bill Hawkins]

Natan,

If you get hum immediately, before the tubes warm up and not from the
speaker,
then the power transformer and anything influenced by its magnetic fields
are the only possible causes.

The transformer may be overloaded by an internal short or something fed by
a winding. Have you checked the AC fuse? Does it have the right rating?
Should be about 3 amps, not 20 amps. Perhaps your beginner's mistake was
not checking the fuse first, before looking for an outlet to plug it in.

If the fuse is wrong, or shorted inside at the holder, then use your
troubleshooting skills to see if a winding is overloaded.

Then again, it could be something completely different.

Bill Hawkins


-----Original Message-----
From: Natan Huffman
Sent: Friday, July 24, 2009 10:40 PM

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