[R-390] No reception below 8 mhz

[Bill Riches]

When I receive an R-390a for repair the first thing I do is check all of the
tubes, mainly for shorts and emission. We all know that a tube that tests
good maybe is not.  The only real test is to substitute  a "known good tube"
for the questionable

one.  My Heathkit TT-1a is one of the better tube testers and does well.  

 

A recent 390a came in with intermittent operation below 8 mhz - above was
fine.  Oh - simple fix 100pf cap across 17 mhz xformer.  Replace that and
the two mica caps in the grid circuit of the osc tube. Still was
intermittent.  Replaced 17 mhz xtal - still intermittent.

 

It is a simple circuit - not much in it.  How about the 6AK5 osc tube - it
checked ok - replaced tube and it has been playing for a week!   That proves
that tube testing lets you know if the filament is ok and is it shorted or
leakage maybe.  I retested the tube again - fil lights and no shorts or
leakage with good emission!  The TT1a has a "grid emission" test that I
rarely use.  Turned the magic knob and Voila - tons of grid emission.  So
the problem was the tube!

 

In the past we have all noticed that tubes that don't read well when tested
work just fine in some circuits.

 

In this repair I could have just replaced the 6AK5 tube without removing the
rf deck and see how it worked.  We seem to become programmed - no rx below 8
mhz - change the 100pf cap!  In hindsight I would have just replaced the osc
tube since it was a local SWL's radio and he delivered it to me.  If it
acted up again I would then have pulled the RF deck and replaced or second
most famous cap!  (On refurbs it is always changed since RF deck is out for
other work.

 

73,

 

Bill, WA2DVU

Cape May, NJ