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

[dog]

Good points Larry. What my plan is, is to get the other 390A going and 
measure the 2nd osc in that one and compare/swap xtals from one to the 
other. That may help define the xtals. I went through #2 and I had 
outputs ranging from a low of 2.6 @ 8MC to a high of 9.8 @31MC. But the 
trend was from lower at the low end to high at the higher end with some 
variations in there, like 2.7 at 27 and 5.3 at 18. Without known good 
components (crystals/transformers) the only place to go is the bypass 
and coupling caps as well as any resistors. Not discounting the tube. I 
haven't dug into the 30V on #1 yet, that sure seems like an anomaly.

On 7/25/2019 01:13, Larry H 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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