[R-390] R-390A Conversion Oscillator Low output

[Larry H]

Sometimes our conversion oscillators will produce low or high output and we
would like to correct the situation.  The approach I take is to find the
failing component and repair it.  I certainly do know from past experience
that this can seem like a daunting and hopeless situation.  When I was in
the service, I worked on a surface search radar transmitter for 8 days.
The thing is that it was located in a very small tight 'closet' with
minimum A/C and it was 110 degrees.  After replacing all of the parts
twice, it still did not work.  This lead to a major part replacement - the
103 foot 2" coax from the transmitter to the antenna.  Well, that did fix
it.  I guess what I'm getting at is you just need to figure out what part
is bad and get a good replacement.

Now I know that it seems like modifying the circuit seems like a good
answer, but this usually leads to other issues and sometimes serious ones.
There are cases where a mod is justified, but I think this is where the
science shows that the circuit was not designed correctly in the first
place.  I *don't* think this is the case with the conversion oscillators in
the R-390A.

The reason that it is important to find the bad component and replace it is
that since it is causing incorrect operation, it will most likely continue
to get worse.  This is where the 'R390 list' or friends come in, we will
help you find the bad component or problem in the circuit.  I know that
most of the R-390A users are excellent technicians, so will not need any
assistance.

So, for the 2nd crystal oscillator, any component can cause low output.
Sometimes measuring components, voltages or scoping the wave forms will
divulge the culprit, but not always.  However, I think replacing the 'easy'
to replace parts should be the first step.  And don't forget to check the
tube and crystal connections.  This quickly gets us to the crystals - IMHO
the only way to test a crystal is to put it into a known good 2nd crystal
oscillator and see what it does.  Now in the case where most of the 1 mh
bands output is low from the oscillator, it's most likely that the crystals
are not the problem.  I could see up to 7 going bad, but not likely.
Alternatively, put a know good crystal in the problem oscillator and see
what it does.  This test will help you decide if crystals are the problem
or not.  Once you determine that the tube and crystals are good, then you
can do the measurements and switch testing and ground checking.  I think
the next step is to check the 2 bypass caps, C410 and C411.  If they are
OK, replace caps C408 and C409.  Or you could check the switch for chemical
contamination first.  A resistance check might give you the answer, but it
might act differently at RF.  If you get to this point, there's not much
left, the mini coax and T401.  I think I'd inspect the coax and measure it
and see.  You might try sending a signal through the coax from a generator.

Regards, Larry