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

[Alan Victor]

Another approach in lieu of replacing the crystal, particularly if you do
not have one on hand. Replace the xtal with its equivalent Z value at its
operating frequency. For series resonate xtals about 20 ohms R where the
xtal was located. A blocking C may be required depending on the circuit
configuration. The LO should run and at least you can get a sense that the
xtal is the culprit or not. I have done this with overtone as well parallel
mode xtal oscillators. It would be nice to find the Collins spec on the
xtals used in the R390A and non A units.

Alan

On Wed, Jul 24, 2019 at 10:18 PM Larry H <larry41gm at gmail.com> wrote:

> 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
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