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75A-4 Black Philips Screws

Discussion in 'Amplitude Modulation - AM Fans' started by K4KYV, Apr 16, 2018.

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  1. K4KYV

    K4KYV Subscriber QRZ Page

    KA9JLM likes this.
  2. KM3F

    KM3F Ham Member QRZ Page

    I have seen them at SEARS and ACE Hardware.
  3. K5UJ

    K5UJ Ham Member QRZ Page

    This is good to know; thanks Don. I wonder if the A-4 used the same rack panel pitch and diameter of the A-1,2,3. The April ER has a couple of articles about the A line receivers. Collins used a completely different PTO for the S line because it only had to track 200 kcs. There was an interview with the Collins engineer who designed the PTO or A-4? can't remember now, and he told Ray Osterwald that the A-4 was better than any of the S line sets.
    N5DMC likes this.
  4. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    Several things:

    First of all, the 75A-4 is NOT a "rack mount" like the 75A-1, 75A-2, 75A-2A, and 75A-3. The 75A-4 has an unusual cabinet design that is held together with 6-32 machine screws. All standard rack mounts use 10-32 machine screws and those are what is used with the remainder of the A-Line receivers as well as the 51J- / R-388- series receivers.

    Having been employed by the Collins Radio Company, and having worked on a LOT of A-Line receivers and S-Line receivers, I have to disagree with whomever said that the 75A-4 is better than any of the S-Line receivers. Yes, it is better than the 75S-1 / 75S-2. However, not so with the 75S-3- series. The AVC / AGC in the 75A-4 is definitely lacking especially when compared with the 75S-3- series. The 75A-4 does cover 1000 kHz with each band position and the 75S- series only covers 200 kHz in each segment and the 75A-4 does cover 160-meters. That is an advantage in favor of the 75A-4. But, the 75S- series can cover from 3.2 MHz all the way to 30.0 MHz (with the proper heterodyne oscillator crystals) which the 75A-4 cannot cover.

    There have been many modifications published for the 75A-4 to "correct" a lot of problems with the receiver (Electric Radio has a fairly thick book, called a compendium, that has copies of most of those modifications that have appeared in the various amateur radio magazines). In comparison, there are very few modifications for the S-Line that have appeared in the magazines and most of those were concerning the 75S-1 / 75S-2. The other modifications are generally for adding features to the S-Line receivers and not to correct things that are wrong with the design.

    I do own a 75A-1, 75A-2, 75A-3, 75A-4, 2-each 75S-1, 75S-3A, 51J-2, and an R-388 (51J-3) and have owned a 75S-3. In addition, I have worked on a lot of the A-Line receivers and a lot of 75S-3B / 75S-3C receivers as well as the 75S-1, 75S-2, 75S-3 and 75S-3A receivers. As such, I am pretty familiar with all of those Collins Receivers. Of all of the Collins receivers, I rate the 75S-3A the highest as do a fair number of other owners of Collins receivers.

    The 75A-4 is a very good receiver and certain modifications do improve on the original design. However, at least in my opinion, for general amateur radio use the 75S-3A is better.

    Now, for AM operation, many AM operators prefer the 75A-1. I prefer the 75A-2 with the 75A-1 being 2nd and the 75A-3 being in 3rd place. My 75A-4 does have an 8 kHz i.f. filter that does improve the AM operation. However, for AM, the 75A-4 is not all that good!

    Glen, K9STH

    technical adviser-1b.jpeg
  5. K4KYV

    K4KYV Subscriber QRZ Page

    Except for its lack of 160m coverage.

    BTW, I picked up a 75A-1 a couple of weeks ago, rough shape. The dial cord needs re-stringing, the top cover that goes over the PTO and alignment trimmers, and the xtal filter cover is missing. A lot of corrosion on the tops of the transformers, but the set looks restorable. I have the original dial pointer. The main reason I don't plan to restore it myself is its lack of 160m coverage. If anyone would be interested in it, I could bring it to Dayton, which I plan to attend next month. I'd prefer to trade than to sell, but all options are open.
  6. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page


    The 75A-1 actually does cover 160-meters. The basic receiver, just like in the 75A-2, 75A-2A, 75A-3 and 75A-4 covers 1500 kHz to 2500 kHz. However, there is no band switch position to access the 160-meter band. It is pretty easy to add a switch that disconnects the crystal controlled converters that add the other bands to connect the antenna directly to the basic receiver.

    Just like in the other A-Line receivers, the i.f. tunes "backwards" from the other bands. One can add a dial scale to indicate the "backwards" tuning.

    Glen, K9STH
  7. K5UJ

    K5UJ Ham Member QRZ Page

    There was an article in ER at one time on adding 160 to the 75A-1. It was issue number 34 Feb. 1992 on page 18, Putting the Collins 75A-1 Receiver on 160 meters by Howard Mills.
  8. W8KHK

    W8KHK Ham Member QRZ Page

    Glen, I have a 75A-3, 75A-4, and a 51J-4, among other communications receivers. I realize the 75S-3 will provide continuous coverage with the proper heterodyne crystals, as does the 51J series. I also realize the audio amplifier section of the 75A-4 is lacking, thus I added a cathode follower where the 5Y3 used to live, feeding an external audio amplifier.

    I am curious as to why you feel the 75A-4 is inferior to the 75S series? You mention modifications published that correct a lot of problems with the 75A-4. I am not interested in starting a debate, but, since I own this receiver, could you be specific as to some of the problems and potential mods to improve the performance of this receiver? I have been generally pleased with its performance, and I have been pleasantly surprised at how effective the noise limiter is with my EMI environment. Thank you for any suggestions you may offer.
  9. K5UJ

    K5UJ Ham Member QRZ Page

    Before you come on with all guns blazing do some reading:

    Asking if the A-4 used the same kind of screws isn't claiming it is a rack and panel set. Although it was sad that Collins began the cheap table top sheet metal shell cabinet style with that set, following EFJ's Ranger, etc. The 1950s saw the end of the commercially manufactured rack mount gear for hams. I don't run anything that is that kind of construction in which the front panel fits over the cabinet. I think that kind of construction sucks.

    The guy quoted in ER was the designer of the 75A-4. I guess he was biased.
  10. W7TFO

    W7TFO Subscriber QRZ Page

    Screws, screws, screws.



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