[Collins] 75A-4 / dead V-7 Plate Choke 2.0 Mh L-25

[Dr. Gerald N. Johnson]

On 3/16/2016 8:39 PM, Keith-Radio room wrote:
> Hello Gentlemen,
>
> Recently acquired an additional 75A-4 serial number 3915 and the
> Q-multiplier was not working, upon inspection and a quick voltage check at
> V7 no plate voltage at pin 6 / 12AX7, L-25 (2 Mhy) was open.  Question; has
> any one changed L-25 and what would be the correct current rating and I
> would imagine the DC resistance plays an important role as part of the "T"
> filter in the Q multiplier circuit (R36&  L26) and its calibration.  Is
> there a part number you could suggest out of Mouser or Digikey?  Thanks for
> your assistance.
>
>
Looks like a common Collins part, molded RF choke, probably powdered 
iron core. The 12AX7 only draws 5 ma for both sections, 2.3 volts by the 
voltage chart and 470 ohms to ground from the cathodes. The same choke 
part number is used for the plate feeds ov V-16 and V5 and for the post 
detector low pass filter.

I notice a couple capacitors in the Q-multiplier circuit are listed in 
the attached 75A4 deadly caps file. I converted the .pdf file to .txt so 
it should make the list. They and all the black beauty capacitors need 
to be replaced. A leaky C71 would make the Q multiplier tube draw lots 
of plate current to fry that plate choke.

For a workable part Mouser looks like it should work: 
http://www.mouser.com/ProductDetail/API-Delevan/2500R-42J/?qs=sGAEpiMZZMsg%252by3WlYCkU%2fuy0w9T3GDQJEg0t4BW%252bak%3d
>
> Regards,
>
> Keith
>
>
>
> Keith-KE7GPO
>
> 425-392-8843
>
> RadioRm1 at comcast.net
>
>
>
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

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