75A-4 mods: 1s that work well

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

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  1. W5INC

    W5INC Ham Member QRZ Page

    After a little research on these Twins it looks like the early model 75A-4s had a little different text position on the noise limiter knob then the later models. Was this another Art Collins stickler/anal retentive approach there Mr. Glenn? I only noticed the difference recently, when looking at the guy I have and pics of other A-4s.

  2. K9STH

    K9STH Ham Member QRZ Page

    As far as I can tell, the noise limiter calibration is basically the same on all 75A-4 receivers. However, the rejection tuning calibration definitely varies. Also, depending on the serial number, the left hand small knob, under the main tuning knob, is labeled either "dial drag" or "dial lock".

    Another thing is the 15-meter band coverage. The earlier versions had 20.8 MHz to 21.8 MHz as the coverage and the later versions had 20.5 MHz to 21.5 MHz as the coverage. My particular 75A-4, s/n 2535, is in the production range which should have the 20.8 MHz to 21.8 MHz segment. However, it has the later 20.5 MHz to 21.5 MHz segment! Why? Who knows! The receiver does have the later rejection tuning calibration.

    Your 75A-4 has the earlier rejection tuning calibration.

    Glen, K9STH
  3. K9STH

    K9STH Ham Member QRZ Page

    Forgot to answer the question about Art Collins:

    I really don't know if the changes were a direct result of Art Collins. The 75A-4 was out of production even before I passed my Novice Class license and it wasn't until virtually 8-years after I received my Novice Class license that I went to work for the Collins Radio Company here, in Richardson, Texas, at the "new" corporate headquarters. However, based on my experiences with Art Collins, those changes strongly suggest his doings.

    Glen, K9STH
  4. N2EY

    N2EY XML Subscriber QRZ Page

    Could it have something to do with birdies or other spurs?
  5. K9STH

    K9STH Ham Member QRZ Page


    I doubt it. Almost 2/3rds of the 75A-4 production used the 20.8 MHz to 21.8 MHz segment before going to the 20.5 MHz to 21.5 MHz segment.

    If there had been a problem, the change would have definitely been made considerably sooner. There may have been a need, for some military operation (the Air Force did have a number of 75A-4 receivers and KWS-1 transmitters), to change this range. Then, it might have been due to some "wild hair" changing of his mind and Art Collins made the change. When it comes to equipment modifications, some were done for a specific reason and many were done due to some arbitrary change made by Art Collins.

    Art couldn't keep his hands out of production and would issue arbitrary modifications to be done in equipment and those modifications had to be done before the equipment could ship to the customer. Then, before those changes could be completely implemented, Art would make even more changes. This resulted in equipment not being shipped and no money being received by the company.

    As such, every division had an "Art project" which was something to keep Art occupied, thus keeping him from making modifications to equipment being shipped. For the most part, this worked. Art had something to do, equipment was being shipped to the customer, and, best of all, payments were coming into the company.

    Glen, K9STH
  6. N2EY

    N2EY XML Subscriber QRZ Page

    We may never know. Obviously there was SOME reason for it, because it meant changing a bunch of things besides the heterodyne crystal. The dial drum and the manual, to name just two.

    One thing I find interesting about the A-4 is that the low ends of the various bands don't all line up. 160 and 80 start 300 kc from the lower edge of coverage, 40 and 15 start 200 kc. up (in the original version), 20 and 10 start 0 kc. up.

    It could be as simple as "Art said to change it".

    In any event, the A-4 is a pretty amazing rx for the time. IIRC it's a 1954 design.


    I recall a thread about someone who'd found a Drake 2-B where the calibration was 100 kHz off on either 40 or 20. All the other bands were OK. It turned out that one of the heterodyne crystals had been changed to one exactly 100 kHz lower.

    This seemed an odd thing to do until it is recalled that the 2-B architecture is an 80 meter receiver with a crystal controlled converter in front of it for the other bands, and that the low ends of 40, 20 and 15 all fall at the high end of 75. IOW, when you're listening to, say, 7010, the basic receiver is tuned to 3990 and there's a heterodyne crystal at 11000. Same for 14010 and 18000 heterodyne crystal.

    This works fine UNLESS you have neighbor hams who run high power on 75 phone. Then you may hear them as IF feedthrough. Swapping out the crystal fixed that by putting the low end outside the upper limit of 75. (This could be a real issue on Field Day if the 2-B was used for 40 CW and the same group ran 75 phone....)

    Of course, it could have been fixed by better front-end selectivity, but the crystal swap is easier.

    Maybe something like that prompted the 75A-4 change?
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    I think it may have had something to do with making the Rack Track better.

    the 75A-4 is a Golden Screwdrivers dream.

  8. KM1H

    KM1H Ham Member QRZ Page

    Mine has the earlier range which I preferred at the time since it covers the 13M SW band and I didnt have a general coverage set at the time except for an old WW2 Super Pro which stopped at 20mc....actually it kept drifting right by it and never stopped.

  9. K9STH

    K9STH Ham Member QRZ Page


    The rack tracks on all bands above, and below, the 15-meter band. Therefore, that was not a reason!

    Knowing Art Collins, the decision may have been just because he wanted to make the change or that he wanted to receive something below 20.8 MHz. Frankly, I don't really know, and there may be no one around, these days, who knows just why the change was made.

    Glen, K9STH
  10. AI6KX

    AI6KX Ham Member QRZ Page

    You lucky guys! That receiver was my unattainable dream when I was starting out. My old friend WW7M recently reminded me of how we used to go down to Harrison's on Radio Row just to turn those dials. They were selling used for around $750 back then, which would be ??? now! Then in 1962 I went to Berkeley, where a deceased alumnus had donated a 75A4 and KWS1 to the club station W6BB. I spent far too much time up there in the EE building but it felt like had died and gone to heaven. Now just getting back "into it" with a BA, but it's a riceburner.....

    Steve, ex-K2RDP and W6BMZ, soon to be 7J6???
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