[R-390] Trim Caps on Mechanical Filters

[Flowertime01 at wmconnect.com]

DW Holtman ask,
    
Was the modification that installed variable caps (C-569 through C-571) in 
the input and output of the mechanical filters in the If module applied at Depot 
during rebuilds, or just put on newly manufactured modules? 

There are two flavors of mechanical filter caps in production.
What you see is what you get. The field never went back and "upgraded" the 
caps in the IF deck.

Early production had selected fixed caps.

Later production had 4 caps under the can and 4 more on the side of the deck. 
These units also had 4 more holes in the side of the frame to access the side 
caps.

If you have an IF deck with side caps and no side holes in your frame, you 
have a swapped in IF deck. Conversely if you have holes in the frame and no caps 
you have a swapped out IF deck. This will start some chatter and likely get 
us another dead carcass. So near the new year and such tasks to be dealt with. 

There was much todo about what to do if you had to replace a mechanical 
filter in a deck that did not have trim caps. The last word in 1975 as I was 
getting out of service was still to just put the filter in and ignore the cap 
optimization problems. Wisdom from management powers was that once the caps were 
properly trimmed they needed no further adjustment. If you measured the output 
before you tweaked on the trim caps, you likely found your diddling produced no 
further gain. Experience in the field did not support this wisdom from higher 
management. It was standard procedure to tweak all the filter trim caps on 
every PM event. We wondered why there was no modification work order to at least 
add the top caps to the IF decks. We could see why no one wanted us drilling 
into IF deck and installing caps in the bottom side. 

I think if we look at decks with and without trimmers on the bottom side, we 
will see that the decks have a lot of differences to allow the caps to be 
inserted. There was just not a big blank chassis wall space there waiting for 
additional caps. The extent of these changes may have been why no field 
modification was ever kited and fielded. The idea of providing new decks with caps was 
just a non starter from spending and contract point of view. Letting someone 
drill holes in the frame to trim decks in place was not a popular subject.

We always wondered how much better a deck would have been if it had had 
adjustable caps. Decks without trimmers still passed signal to noise test like 
every other receiver, so we believed the assembly folks did a good job testing and 
selecting the fixed caps.

One myth was the early production was "selected" to closer specification. 
Once Collins had a big pile of good but not good enough filters, a proposal was 
produced and sold to get the trim caps in and the less exact filters in. Myth 
was that a well trimmed deck never needed adjustment. I have no evidence to 
support that statement.

Hope this provides you some insight on the subject.

Roger AI4NI