75A-4 mods: 1s that work well

Discussion in '"Boat Anchor" & Classic Equipment' started by W5INC, Jan 25, 2015.

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  1. KD2AZI

    KD2AZI XML Subscriber QRZ Page

    I've been reading about various mods found on the 'net. I'm going to try the 6GM6 RF swap.

    About the mod for removing B+ from the filter in the 75A-3 (which I understand Collins implemented in the A-4); I'm wondering what they found that made them change the design. Are there any risks to leaving this alone on my A-3 (I plan to use it frequently). I have the 6kHz and 3.1 kHz filters, if that matters.
  2. K9STH

    K9STH Ham Member QRZ Page

    Removing the B+ from the filter(s) can increase the life span of the filter.

    Here is a circuit that should work for the 75A-3 to remove the B+ from the filters. I haven't tried it. However, the filter connections are changed to what they are in the later 75A-4 receivers.

    Added components are a 2.0 mH to 2.5 mH r.f. choke and a 0.001 mfd (1000 pf) capacitor. The "X" indicates the connections to be broken and then a ground connection, from the "low" side of the mechanical filter, added.

    filter B+ modification.jpeg

    Glen, K9STH
    Technical Adviser
    Collins Radio Collectors' Association
  3. W5INC

    W5INC Ham Member QRZ Page

    Please tell the Forum how the mod turns out for you. :)

    The 75A-4 arrived today and was packaged correctly with foam inserts used for packing material along with sturdy cardboard box. Shipping contents packed nice and tight with nothing moving around inside the box, whichever way you turned it. I bought this guy from W0YVA, Mr. Robert who is a straight up, more then honest person to deal with and took the time/effort to make sure the rig was shipped correctly, without cutting corners in any way. W0YVA is a real credit to the AR ranks and this does indeed go to prove there ares still good, honest folks who sell AR equipment online! I would buy from Mr. Robert again anytime.

    Here are some pics of the rig. It is a very early model with the serial number being #287. In very good shape cosmetically wise for a 60 year old rig and will need a cleaning on the inside chassis to really make it sweet. I haven't plugged it in as of yet but I am sure it will need some TLC to get it ship shape once again. It does indeed have a tube socket installed on the back of the rig for some reason, but no wires look to have ever been soldered on to the terminals. You can see it dead middle in the pic of the inside of the 75A-4. Should make a fine addition to the shack and I sure do like the way the old BA looks. :)


  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Nice. I think I can smell it.

    Now you just need a KWS-1 to go with it.

  5. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Nostalgia - my father had a 75A-3 that he had modified a lot. Still one of the best I've ever used.
  6. W5INC

    W5INC Ham Member QRZ Page

    If there was a KWS-1 in the Houston area, I might just consider getting 1 of those Xmtrs. Maybe you know of a KWS-1 in the Houston area Mr. Don, that could possibly need a new home and is just taking up space, collecting dust bunnies. ;) All you have to do is lift the top cover of the 75A-4 cabinet and you get the BA aroma without even having to plug the receiver in and get the transformer heated up. Big box radios glowing in the dark, what could be better then that? :)
  7. VE7PS

    VE7PS Ham Member QRZ Page

    I just finished up the W1LSB AVC mod from ER #250 (March 2010) on my "new to me" serial #4053 about an hour ago, and in my humble opinion it is very obviously way better than the often touted K7CMS mod which I have in my other A4, #4035. I will be changing that one to the W1LSB mod soon. Yes, there is still filter blowby, but the AVC attack overshoot is gone and STBY to OP recovery is almost instantaneous. Easy to listen to! It will never be a $10K+ modern rig, but for a rig that's almost as old as I am, she sure sounds pretty good!

    Peter, VE7PS
  8. K9STH

    K9STH Ham Member QRZ Page


    Your 75A-4 will definitely require all of the Collins Service Bulletins unless someone has already done some, or all, of them.

    Then, start on modifications.

    Of course, Collins recommended making no modifications to the receiver!


    However, there are certainly SOME modifications that do improve the performance of the 75A-4.

    The W1LSB modification starts on page 89 of the compendium.

    Glen, K9STH
  9. W5INC

    W5INC Ham Member QRZ Page

    TNX Peter for the reply and Welcome aboard to the BA section of the Zed. Good to get new blood in these Forums along with their POVs on subjects. :)

    TNX agn Mr. Glenn for your sage advice and it is dually noted. I will make sure page 89 is in the forefront of mods after checking on and if need be, updating the 75A-4 to the Collins service bulletins 1st. I haven't checked out under the hood as of yet but 1 thing needed for the unit is some feet for the cabinet. I looked around on the web and couldn't find any feet for sale as of yet. (HINT HINT :p) The capacitor project will be another aspect that will be moved to the forefront along with testing the tubes on the TV-7 which could use a workout anyway. I appreciate all of the feedback, advice and opinions from the Zed collective here, as always. :)
  10. KM1H

    KM1H Ham Member QRZ Page

    I'll tell George you liked his mod when I see him at Nearfest in a few months! Since I did mine in 65 and like it Ive not tried any changes but might give it a shot in either the A2 or A3 here, I do have ER complete from about 1994 I believe.
    George also has restored my old CE-200V and graciously allowed me to get my old unrestored CE-100V back. I had several brain farts in the 90's when I thought Id never want to get involved with BA's again!

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