[R-390] SW R-390a Second Pass

[Roger Ruszkowski]

Ken,

You wrote, 

I am getting sensitivity variations between bands, 
not octaves but bands within an octave. The only module untouched so far is the 
Crystal Oscillator deck and I'm expecting a number of bad micas in there too 
causing the variations. 

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Each band has a crystal in the second mixer oscillator deck.
Ok so some of the bands are harmonics of a crystal.
Which is even less wonderful.

Every one of those crystals are going to have a different level of output
going into the mixer. Thus a different level of on every band.

Do deoxit the crystal oscillator deck switch and all the crystal sockets.

Getting the trim cap circuit board out of the deck to disassemble the trim
caps may be more aggravation than you want to engage in.
It can be done if you have to.

As you change bands you bring a different crystal on line.
It needs to warm up and stabilize.

We usually just dial through the bands and peak the cold crystals
and move on to the next band. get it all done it 4 minutes.

Not necessarily the best method.

Changing out crystal for better performance can help.
Selecting crystals by trial and measurement is a lot like 
selecting tubes for best signal to noise performance.

If a band you use is down, then by all means try a crystal swap.

But after a while get back to using the receiver as a radio and not
an educational device to hone repair skills on.

If you are going to swap caps in the crystal deck you may want to
sub in a variable cap and see what the optimum value should be.

Then think about the stray capacitance your testing has associated with it.

And then put in a new fixed cap with an ideal value.

You can measure where you would like to be.
Put that value in and see how close you are.
go up or down a bit in value to see if the fixed cap is closer to the peak you found with 
the variable cap. You will find a difference between the variable cap and the fixed cap values that 
represent the stray capacitance of the test set up.


If you are adjusting the trim caps on the deck and only get one peak on the trimmer
then the trimmer either does not have enough capacitance of has way to much.
This will be a clue to you that that cap and crystal needs some changes.

We expect two peaks on the trimmers as they go on each side from max cap to min cap.

Ken,
Keep having fun with your receiver.
Roger.