{Collins} Fw: Re: KWM-1 Feed through Capacitor Mod Completed



Please disregard the prior post from me.  I somehow hit "send" too soon.  Sorry.  The complete posting is attached below:


>Gary and List Members,
>
>I have seen many of the posted comments about replacing the feed-thru cap on these older rigs.  However, I have never read a caveat as to the drawback to using a feed-thru insulator and discrete, separate by-pass caps.  So, FWI, here it is:
>
>You are fortunate that you are working at HF (below 30 MHz) with these rigs.  The self-inductance of the leads on the discrete by-pass caps, especially when using 2 or more caps in parallel, is low enough that the caps still act as caps.  It seems that even with 2 caps I series, as noted in Gary's posting, the by-passing is still effective.
>
>You see, there is a real "reason" for the use of feed-thru by-pass caps.  That reason is the fact that such caps have extremely low self-inductance, and thus, the caps continue to act as effective by-passes well into the VHF range.  In effect, tubular feed-thru caps have huge surface areas and very small values of self-inductance for both the center electrode (in contact with the HV) and the outer electrode (in contact with the chassis).  AS you know, Xc decreases with frequency increase and Xl increases with frequency increase.  Real world caps thus have self-resonant frequencies that are determined by their construction, and in order to be effective at by-passing, the frequency applied must be below the self-resonant freq of the cap.

Tubular (also known as "coaxial capacitors") with values of .01 or even 0.1 mfd are commonly used at VHF and higher frequencies because of their unique construction.  I would be interested in knowing if anyone has had a rig problem after replacing one of the feed-thru caps with the insulator-discrete cap combo.  My guess is that there have been no problems, and that's fine.  Should a rig develop instability after such a change, the reason may well be inadequate by-passing from the discrete caps.  If that happens, try shortening the leads.

73, Dale, WA9ENA
CCA Member   
>
>
>-----Original Message-----
>>From: Gary Follett <dukeshifi@xxxxxxxxxxx>
>>Sent: Jul 4, 2013 7:01 PM
>>To: svetanoff@xxxxxxxxxxxxx
>>Cc: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
>>Subject: Re: {Collins} KWM-1 Feed through Capacitor Mod Completed
>>
>>I have a bunch of these feed through caps,new old stock.
>>
>>Gary
>>
>>Sent from my iPad
>>
>>On Jul 4, 2013, at 7:25 PM, William Yates <veniceamplifier@xxxxxxx> wrote:
>>
>>> Greetings and Happy 4th to the list
>>> 
>>> I had my KWM-1 apart for some routine maintenance and figured I should go ahead and replace
>>> the "soon to die at any moment" feed through capacitor by using a simple modification that has
>>> been discussed here previously. I took the chance of using two 600 volt .02uf 715P series Sprague
>>> Orange Drops wired in series. The small ceramic stand off insulator worked perfectly because the
>>> hole in the sub chassis where the old cap was mounted gave sufficient room for the "male" portion
>>> of the insulator to mate through with the "female" portion. Soldered the caps from the anode to ground.
>>> So far, it works great. I noticed not a single problem after testing on air and leaving on for a considerable
>>> amount of time. With 1200 volts rating and .01 capacity measured with my trusty Sencore tester, it measured out
>>> to .012 which seems fine to me... the old cap also measured .012 which is also fine by me.
>>> I would have very much rather used a ceramic type but I could not source one in my stashes of caps,
>>> so I took a chance. I have 100's of the wrong values rated at 2KV but not .01uf.
>>> Bill Carns handed out some very good info regarding the choices and types that are favorable for this
>>> application. I took some Hi resolution photos of the installation to give readers an idea of how I managed
>>> to mount the components should anyone desire to see them.
>>> The beauty of this modification is that it can be done for less than 1 dollar, and about 1 hour of time...
>>> the KWM-1 has a boatload of chassis screws! After 54 years of service, I think Sprague did a remarkable
>>> job of designing and building the feed through capacitors. A real testament to American craftsmanship.
>>> Then again, it's not surprising that Collins Radio chose certain makes and types of materials in which to 
>>> construct these wonderful pieces of equipment.
>>> For future reference, I plan to write an article outlining some techniques for cleaning the tune and load
>>> inductors. This is a very tricky and concise procedure that must be adhered to! Cuz, They Ain't making this
>>> stuff No Mo! I look forward to Sunday's net where the KWM-1 will finally be back on the air.
>>> I simply could not find the guts to check in with non Collins gear., hence my absence.
>>> 73 and Happy 4th!
>>> Billy N6YW
>>>                         
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>> NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
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>>The Collins Reflector reserves the right to refuse access to anyone.
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