Thank you Phil for reaffirming the concept that got me ejected from the Collins
Folklore reflector.
Drift of circuit alignment NEVER causes 20 dB changes in performance, unless
mis-alignment causes a crystal oscillator to drop out of oscillation.
Alignment of a broken radio causes troubleshooting to be confounded just as you
stated Phil.
The ONLY time alignment should be done on a broken radio is as a reversible
diagnostic when all else fails. By that I mean moving a given adjustment a very
small amount, looking for any increase in signal downstream from that
adjustment, then turning the adjustment exactly back to its original position.
This is analogous to the old technique of using a tuning wand, one end made of
Ferrite, the other end of brass. Moving the brass end near a coil reduces
inductance of the coil while moving the Ferrite end to the coil increases
inductance. A properly aligned circuit should drop in output when you move
either end of the wand nearby the coil in question.
When coils are properly shielded in a can, this becomes difficult to impossible
with the old, large diameter tuning wands. Therefore the use of diagnostic
reversible “alignment” performs the same task.
Caution is essential since many coil cores tend to stick when aged.
A way to overcome this difficulty is to fabricate a similar tuning wand that
has a diameter small enough to enter the opening of a given can. You can
substitute cold steel for the Ferrite (less effective but still useful) and of
course, brass remains the material of choice for the other end and brass of all
diameters is readily available from hardware and hobby stores.
You simply insert each send of the wand into the alignment opening of the coil
in question, trying to prove that the coil is properly aligned by observing
that output drops with insertion of either end.
This method also works very well with Collins/Drake type tuning rack coils
without leaving you with a hopelessly misaligned AND broken radio.
Gary
W0DVN
> On Dec 30, 2018, at 9:33 AM, Peter Bertini <radioconnection@gmail.com> wrote:
>
> Phil
>
> Do NOT attempt to realign those passband filters! Normal aging will not
> give you a 20 dB signal loss. Alignment can mask other problems and can
> compound finding a resolution for your problem. Realignment would problem
> require a sweep generator, or at worst, hours of tedious stagger tuning
> while manually sweeping the response curves.
>
> A high Z scope is fine, since you are tracing 50 ohm signal paths it will
> not load the circuits.
>
> Getting back to your problem: do what Gary suggested. Carefully resolder
> all of the connections for the components in that particular filter. You
> might want to first verify that the associated switching diodes are indeed
> being forward biased when that band range is selected! It could be as
> simple as the diodes not being switched on.
>
> I had a similar passband filter failure in LO system. No fun.
>
> regards
>
> Peter k1zjh
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