Re: {Collins} 75A-1 trimmer capacitor repair/replacement



> On Oct 14, 2017, at 3:13 PM, VK2WD via Collins <collins@xxxxxxxxxxxxx> wrote:
> 
> Hi,
> 
> I am in the process of getting a 75A-1 back on the air and found a number of
> trimmer capacitors in the tuneable IF and front end are either jammed or
> broken (will turn but no change in capacitance or intermittent with
> pressure).
> 
> 
> 
> I have spent a few hours searching the reflector and the Collins site but I
> haven't found exactly what I am looking for.  I am hoping someone can
> assist.
> 
> 
> 
> 1.       Can the trimmer caps that no longer tune be repaired or do they
> need replacing? - Sometimes. Often they won’t tune because the contact between the spring and the shaft is bad due to dirt. After you get the shields off the tuning section, you will see that you can remove the spring by sliding it away from its slotted end. Most of the time you can do this without even unsoldering the wire connected but remove the wire if you experience any difficulty.

Once you get the spring contact slid away, the rotating part will pull out from the top side. Clean this gently in methanol or other very mild cleaner and look at it to see if there is still a half circle of metal on the side that faces the other half of the trimmer. If so, take an ohmmeter and ensure that this metal is making contact with the center shaft. 

There should be a soft plastic insulator in there and it should be clean, flat and soft.

Clean the spring contact with methanol as well as the part of the trimmer that is still attached to the chassis.

Do NOT try to clean these in place as this will likely leave you needing to buy some from me (I have a few dozen).
> Once you are done cleaning everything, reverse your steps to reassemble and test. If they still don’t work, you will need to replace them..


> 
> 2.       Is there a source of replacement caps (sorry I had to ask) or ones
> that fit the mounting holes and chassis opening? I have about a dozen of them. At this point in the game, I have to charge $20 each for them because they are work to remove and I am not really looking for more work to do.
> 
> 3.       It looks like the various front end "modules" and variable IF
> stages are attached by 3 screws (each) through the main chassis.  Removing
> the screws, and wires going to the tubes plus the rotary switch shaft should
> enable the modules to be removed for repair.  If there is a simpler way I
> would appreciate knowing what this is. You do not even need to remove that shaft our any switch wafers to repair the trimmers or even to replace them.
> 
> 4.       If 3 above is roughly correct, I assume the rotary switch shaft is
> removed by removing the front panel, removing the two nuts holding the front
> switch mechanism to the chassis and pulling the shaft out the front of the
> receiver?  If this is not the way to do it, could someone let me know the
> correct procedure. Good luck removing the front panel! While the gray fascia panel can be removed with about an hour’s work with a heat gun that will serve you no benefit because the front panel that is part of the chassis is spot welded to the rest of the chassis. If you find that you DO need to remove the fiber shaft, just loosen the set screws on the coupler on the from of the shaft and pull the shaft (carefully) out the back through the hole in the chassis intended for that purpose. Be VERY careful to not break a switch wafer if you do this and be even more careful to not rotate the bandswitch or any of the wafers while the shaft is out of the receiver.

Best of luck to you! This is not as hard to do as it first looks…

Gary



> 
> 
> 
> Thanks and regards,
> 
> Brad
> 
> VK2WD
> 
> 
> 
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