[ARC5] Another AR-88 problem solved

[Richard Knoppow]

     After fixing the grounding problem I began to think 
about the mis-tracking and need for extra capacitance on the 
second RF trimmers.  I had originally thought the problem 
was on only one band but I realized it was showing up on all 
three top bands.  The only thing in common is the tuning 
capacitor.  I finally inspected the cap section that is used 
on these bands for the second RF and found the plates were 
not quite straight.  The amount of bending was so slight 
that I did not see it in a previous examination.  It takes 
very little to disturb the capacitance law of these caps. I 
straightened the plates as well as I could visually.  That 
seems to have been enough because all three bands then tuned 
correctly. I took the bridged caps off and found the 
trimmers tune at about the center of their range.  I think 
the tracking is fairly good.  I will probably check it 
further but I had about enough of working on the thing for 
one day.  In any case, I probably caused both problems in 
the course of doing other work.  The second RF seems to be 
stable on all frequencies now since adding the extra ground 
wire from the tube shield pin (pin-1) to the wall of the 
internal compartment shield.  I looked at the other RF and 
the mixer; their grounds are arrange somewhat differently. 
I am not sure they are perfect either but they seem to be 
stable.
     BTW, at one point I was considering adjusting the 
bearings on both caps. This was partly because I am not sure 
the rotors are exactly centered.  I am now reluctant to do 
this because I am not sure I can get them set with 
sufficient precision.  I can just see a couple of the clamp 
screws for the stators through the holes in the bottom of 
the tuning unit where the leads feed through. They appear to 
have the same purple paint on them that the bearing 
adjusters and assembly screws on the ends of the frames do. 
I don't think the cap is exactly right but feel like I 
should leave well enough alone, at least for now.
     My suspicion is that if the cap is _perfectly_ centered 
the dial calibration will be exactly on the nose. I've 
learned from other receivers that small deviations in 
centering can make a big difference in calibration and 
tracking.  A project for another day I think.
     Thanks to all who gave these problems some thought and 
tried to help, in fact you did a lot.  I am posting this to 
the RCA list and to the ARC-5 and Boatanchors lists, all of 
whom possess some wonderful members.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com