The Collins 75A-4 Thread.

Discussion in 'Amplitude Modulation' started by N6YW, May 16, 2017.

ad: L-HROutlet
ad: l-rl
ad: MessiPaoloni-1
ad: Left-2
ad: L-MFJ
ad: HRDLLC-2
ad: L-Geochron
ad: abrind-2
ad: Left-3
  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    IMG_5083.JPG Greetings to the group.
    After owning several 75A-4's over the years, and of course going through the sometimes
    troublesome modifications on certain units throughout the production run, I thought it
    would be fun to exercise our collective minds in discussing the various attributes to this fine
    receiver, both pro and con. This stems from recent comments made by a couple of our
    members here regarding comparisons between the A-4 and National variants.
    I intend to make this an A-4 only thread but comparisons if worthy are welcomed.

    Presently I have serial #1524 in my shop. It may become mine as the owner expresses no
    real desire to keep it as it won't see much use. As we go through the discussions, this receiver
    will be used as an example of current condition, performance grading, and repair/service
    bulletin application, and of course accepted modifications to insure the highest performance.

    I hope to use this thread as a tool for understanding to everyone interested in this fine
    design, but most importantly, for those not so well acquainted with the design and it's place
    in radio history.

    As a starter resource, the Collins Collectors Association maintains a deep historically correct
    database of information on the 75A-4. For those of you not so well informed on the subject,
    I advise you take time and read up on the background of it's design and production.
    This is a requisite study in the history of radio design advancement and should be respected as such
    in the premise of this discussion.

    Have fun, and let it rip! I look forward to the comments.
    K0UO, AE7LP, WQ5Q and 1 other person like this.
  2. W6ELH

    W6ELH Ham Member QRZ Page

    I'm rather new to the boat anchor world, and can offer nothing scholarly, but I do love my 75A-4!

    I began this little journey working on a series of Hallicrafters receivers, and thought them very nice. I then rebuilt a National NC-173 and had to admit to myself that its quality of construction really shamed the Hallicrafters. Then I acquired my 75A-4 and "found religion" as some might say. LOVELY in its meticulous construction, and pleasing to work on and to work with. I just re-read my notebook pages on the Collins rebuild. After putting the Collins into service, I did some "A / B" comparisons between it and my FT-991. Over several evenings, I came to the conclusion that the 75A-4 heard 90% to 95% as well as the modern Yaesu, and with much more pleasing audio on good signals. Pretty amazing to me considering the 60-year span in technology!

    I don't have the serial number at hand, but was pleased to learn that my 75A-4 came off the assembly line in 1956 - just like I did!

    I look forward to hearing what you more experienced folks have to say about this delightful instrument. Cheers... Jim W6ELH

    KA4KOE, K0ELB, WQ5Q and 2 others like this.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    I like my 75A-4. I have a set of gold dust twins. My KWS-1 gets great audio reports.

    Mine are original. I had no good reason for modification, Yet.

    Too bad they don't make real boat anchors anymore.
    K9ASE and AF6LJ like this.
  4. VE3AJM

    VE3AJM Ham Member QRZ Page

    I'm in the middle of going through my 75A4 serial # 1636. I upgraded the S meter circuit and pots as per the Collins bulletins, and have replaced most of the mica coupling caps and black beauties that are known to go bad. I haven't done any other mods to it as of now. I am using a 8kc R390a mechanical filter in one of the filter slots. So I have the 3.1, 6 and 8kc for filters in there. I found the alignment, especially of the AVC circuit makes a huge difference on how the rx performs. I put a simple high pass filter on the Collins 270G speaker to improve it. The audio on this rx is another discussion.

    I'll be curious with what you find with yours, and how things progress.

    Al VE3AJM
    Last edited: May 16, 2017
    WQ5Q and AF6LJ like this.
  5. N2EY

    N2EY XML Subscriber QRZ Page

    Way back in the late 1980s.....when there was a hamfest at the old Route 611 Drive In.....a ham was selling his 75A-4. It had the book, three filters, and the reduction knob. It was in decent working shape - not mint - and unmodified.

    He wanted $250.....and I had the money. Said I'd think about it. Somebody else got it.

    The marks from kicking myself are almost healed.

    73 and more pix pse de Jim, N2EY
    KE4OH likes this.
  6. K9STH

    K9STH Ham Member QRZ Page

    The R-390A 8 kHz filter makes a world of difference in how the receiver performs on AM. They are again available from Fair Radio:

    For improved SSB operation, a narrower filter than the stock 3.1 kHz bandwidth is preferred. I use a 2.1 kHz filter that is used in the S-Line in my 75A-4. However, Fair Radio has the 2.0 kHz filter from the R-390A available at a price much less than the "proper" filter or even the one for the S-Line.

    It does require a 9-pin miniature "header" (plug), some hookup wire, and a piece of shrink tubing to use the R-390A filters with the 75A-4.

    My 75A-4 is right in the middle of production (s/n 25XX) but it has the 15-meter calibration of the later units and not the earlier calibration in which the serial number occurs. It does not have the 4:1 reduction knob. Finding an original 4:1 knob assembly is an exercise in frustration although at least one is available for $299.99 from KE9PQ. A reproduction knob is available, again from KE9PQ, for $39.99.



    I have all the parts to add reduction tuning to my 75A-4. However, the reduction will be 6:1 and not 4:1. Also, it will not have the "fly wheel" that is present in the original 4:1 drive assembly. The modification will use one of the reproduction knobs and a 6:1 vernier drive. When I get it done, the modification will be completely reversible as well as being fairly easy to accomplish. An article, on how to do the modification, will be put on my website.

    Replacing all the "black beauty" or "bumblebee" capacitors is a must! There is like a 99% chance that they are leaky. Then, there are the "deadly" capacitors that also need to be changed.

    I have had no problems replacing the silver-mica coupling capacitors with "normal" disc ceramic capacitors. I really believe that Collins used silver mica capacitors in those circuits because, in the time period when the 75A-4 was being manufactured, reliable, small value, disc ceramic capacitors were not really available.

    The number of modification articles, that have been published on the 75A-4, is numerous. Most of those modifications really do not improve the performance of the receiver and some even hurt the performance. There is a compendium available from Electric Radio Magazine that has quite a number of these articles contained therein. Making any modifications should be carefully considered as to not compromise the performance of the receiver.

    There were several modifications that were supposed to be from Collins Radio and that Service Bulletins would be forthcoming. But, such was not the case and Collins even put out a letter about those modifications:

    There were several Service Bulletins issued by the Collins Radio Company for the 75A-4. These Service Bulletins, as well as one of the manuals, as well as other information on the 75A-4, are available, for free, as downloads, at the following URL:

    All that said and done, most AM operators believe that the 75A-1 is the best of the A-Line receivers for AM operation followed by the 75A-2. The 75A-3, or 75A-2A, if a wider filter is put in the 2nd filter position, is about the same as the 75A-2.

    I do have all 4 of the A-Line receivers, 75A-1, 75A-2, 75A-3, and 75A-4 and concur with the ratings of the receivers for AM reception. Now, for SSB, the order is reversed even if a product detector has been added.

    Glen, K9STH
    WQ5Q and N6YW like this.
  7. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Thanks for chiming in. The links are very much appreciated.
    I own all of the materials pertaining to the 75A-4 including the 75A-4 Compendium.
    I use that for reference only and I completely agree about the numerous modifications
    that were invented back in the day, many of which were no doubt frowned upon by
    Collins Radio. In fact, many articles written by well intentioned "Hams" in CQ
    magazine did much to ruin many radios, not just Collins. Fortunately, most of the
    modifications are reversible.
    I am in the process of determining what has been done to this one. The photos I post here
    illustrate some wiring changes which I plan to unravel. IMG_5087.JPG IMG_5086.JPG IMG_5085.JPG IMG_5084.JPG
  8. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Here is a view of a major wiring change with an apparent pot added. Not sure yet what it's use is for.
    Check it out. IMG_5088.JPG
    Well duh...
    All I had to do was flip it over and see that it's the S meter adjustment. What is curious are
    the two wires left un-terminated. I'll investigate.
    New .1 600 volt bypass and coupling caps going in today, along with replacement bias supply
    electrolytics. I'll have to wait on the B+ filters until it arrives.
    Last edited: May 16, 2017
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    One caveat with non-standard filters: The stock A-4 filters use a fixed 100pf mica capacitor to resonate the input and output coils in the filters. All the non-standard filters I have ever tried in mine showed insertion loss and excessive passband ripple. Replacing the 100pf trimmers with a set of variable compression mica trimmers allowed coils in the filters to be tuned to resonance, which brought the insertion loss to the same as the standard filter and got rid of the passband ripple. The filters in the R-390A use approximately 350 pf for the resonating caps, if I recall correctly without consulting the 390A schematic in my service manual.

    I think this may be the same reason that non-standard mechanical filters are said to be problematic with the 75A-3. Collins made different filter types that are physically the same size and will drop into the slot, but it's said that only the specific filters made for the A-3 will work properly. I have never owned or worked on an A-3, so I can't say from experience; that's just my theory.

    I put my non-standard filters in a separate outboard box, and each filter has its own set of trimmers. I clipped one lead on the 100 pf micas inside the receiver (they could quickly be soldered back if I wanted to return it to stock). The trimmer adjustments take into account the capacitance of the 1/8" diameter coax I run between the main chassis and the exterior box. I have the choice of 0.3, 3.1, 4, 6, 8 and 16 kHz selectivity using various non-standard filters, including the 4 and 16 from the R-390A. Another 8 kHz single-ended filter plugs into the 9-pin socket, but I had to use a variable trimmer with it since 100 pf wouldn't resonate the filter. Since the filter box already has a 8 kc filter, I may move that one to another receiver. The photos below show mounting details. I used the ventilation louvres on the side of the receiver to hang a mounting bracket and for the coax leads to exit. The plastic tab on the front panel holds the box securely in place.

    mounting detail.JPG

    Front view.JPG

    Filterslot adaptor.JPG
  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Don't throw them away after you remove them. Put them on ePay. Audiophools will pay a fortune for them.
    W5MW likes this.

Share This Page