Started my 75A-4 Work

Discussion in 'Amplitude Modulation' started by KA4KOE, Oct 11, 2019.

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  1. KA4KOE

    KA4KOE Ham Member QRZ Page

    I purchased my 75A-4 from Liz K4GHT about a year and a half ago. At the time I did not realize she was dying and in retrospect, I think she liked me as a friend and wanted to do something nice for me, given the low price of this largely unrestored radio. It had all the filters and worked as well and looks nice. We'll, I've started on the capacitor recap job....about 26 to install. Also going to do the filter shunt mod to protect those little cans.

    All of this is significant because I'm getting back in the groove after a lot of health problems. Next I have to repair my R390 Collins and finish restoration of a HQ-140X. Finally, restring a Zenith B600.

    Then.....the Bauer 707 repair.

    This week I removed some things from my upstairs shack and relocated the work bench so I can actually have room to do things in here. The futon simply had to go.

    Be happy for me!

    my75A4.jpg bench.jpg
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    You are going to replace the silver mica coupling capacitors? Those capacitors are just as bad as the "black beauty" capacitors. If you have not found the list (probably have found it), the list can be found at

    Of course, the 2-wire power cord has to go being replaced with a 3-wire power cord.

    The "stock" AM filter is a little too narrow for most AM operators tastes. It does work well but a wider filter is better. The 8 kHz Collins mechanical filter, for the R-390- series, works very well. They do have to have a 9-pin miniature plug adapted. Unfortunately, these filters are getting fairly hard to find. Fair Radio used to sell them but then stopped this practice. Now, at least the last time I looked, Fair Radio is only selling these filters to persons purchasing an R-390- series from them.

    I do have one of the 8 kHz filters in my 75A-4. Since I don't use the receiver for CW, I also have a 2.1 kHz filter, as well as the "stock" 3.1 kHz filter, for SSB. The 2.1 kHz filter, at least in my experience, works much better.

    Glen, K9STH
  3. KA4KOE

    KA4KOE Ham Member QRZ Page

    All of the 7 deadly caps except C-52 which was not in my stash of parts. I am going to do the B+ filter mod next. All black beauties gone. New multisection.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    You don't have to replace the coupling capacitors with new silver-mica or dipped mica capacitors. The primary reason that Collins used sliver-mica capacitors was that, in the time frame when the receiver was designed, small value ceramic capacitors were not readily available but silver-mica capacitors were available. I just use ceramic capacitors when replacing those coupling / bypass capacitors.

    Now, in certain tuned circuits, where excellent stability is required, ceramic capacitors are not usually a good choice. Otherwise, modern ceramic capacitors work fine where the smaller values are concerned.

    Glen, K9STH

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