The Collins 75A-4 Thread.

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

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  1. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    As for the 4:1 tuning knob: At around s/n 3000 the 4:1 started being the standard knob for the 75A-4. Before that, it was an additional priced item that was available as an "add on" kit. Quite a few of these kits were installed on 75A-1, 75A-2, 75A-2A, and 75A-3 receivers as well as on the 75A-4.

    Around s/n 3640 the 15-meter band was changed from 20.8 MHz - 21.8 MHz to 20.5 MHz - 21.5 MHz. My particular 75A-4, s/n 25XX, goes against this since it has the 15-meter band 20.5 MHz to 21.5 MHz but does not have the 4:1 knob.

    Glen, K9STH
    N2EY and AF6LJ like this.
  2. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    I spent much of the day yesterday going through the A-4. I decided to try an alignment first before I shotgunned all
    of the old paper caps and filters. Most tested okay with very little leakage. Man was this thing way out!
    After a full alignment, 20 & 15 meters are absolutely dead with zero crystal oscillator response.
    Could it be that two crystals are bad? Or perhaps the small sandwich mica's in that part of the circuit are bad
    placing DC on the crystals? 160, 80 & 40 responded okay but still not a perky receiver yet. I suspect weak
    tubes and a combination of leaky caps in the early stages.
    Any thoughts?
    AF6LJ likes this.
  3. WA3VJB

    WA3VJB Ham Member QRZ Page

    Wellllll, I dunno how you value your time, but that receiver, to me, would be one of the most expensive in labor cost that I could imagine.

    Great story though, and I sure hope the next owner appreciates all you put into it!
    AF6LJ likes this.
  4. K4KYV

    K4KYV Premium Subscriber QRZ Page

    Maybe so if it had been an obsession to the point of jumping right into it and working non-stop till it was finished. It didn't take all that much time actually; most of the effort went into trying to figure out the steps to do it. Like all my stuff here, it was (is) a WIP that took a while, an hour or two here, 30 minutes there, etc over a period of weeks or even months. It happened to be the subject of tinkering I would have done anyway. One could say the same about my homebrew transmitters or the modified Gates. Probably true for all of us who homebrew our stuff. Just putting in hours of operating without working on the station apparatus gets boring pretty quickly.
    AF6LJ and N2EY like this.
  5. N2EY

    N2EY Premium Subscriber QRZ Page

    Thanks for a GREAT story - and saving a classic from the abuse it had suffered.

    But, as @WA3VJB points out, there was also your time.

    And, I would add, expertise and CARE to do the fix the best way possible.

    And a bit of luck.....but the luck would have not mattered without the expertise and care.

    Thanks again!


    I don't know if I've written this here before, but here goes....

    Consider the case of classic vintage airplanes.

    There are some who think such aircraft should be locked away in museums, restored in cosmetic detail with original parts, but not allowed to fly. Too much risk of damage or loss, and with all-original parts, some things would just be unsafe. (70 year old tires? No modern radios or safety harnesses?)

    There are others who think such aircraft should be kept in flying condition, even if that means some modifications and accommodations to today's requirements. They say that while museums are great, they are just not the same thing as actually flying those aircraft.

    I tend to side with the latter group. Obviously some aircraft should be preserved in museums for purely historical reasons, and there may come a time when it is simply not practical to keep some vintage planes flying. But until that day arrives, I think they should be in the air, not trapped in some museum.

    I think the same thing about classic vintage radios - particularly great ones like the 75A-4. They belong on the air, in use, as long as possible.

    A radio in a "museum" may look nice, but until it is actually used, on the air, it's not really doing what it should be doing.


    73 de Jim, N2EY
    Last edited: May 17, 2017
  6. K5UJ

    K5UJ Ham Member QRZ Page

    I agree. This is why I tell the -everything must be original parts- fanatics that I'm operating a functioning ham station, not a museum. Further, if anyone goes back to ham magazines in the 1950s, they are full of articles on mods to the A line and other gear -- hams were doing non-stock improvements to those old rigs almost from the get-go, so in that sense the sorts of things Don does is more in keeping with tradition than what you have with the all-original-no-non-factory-mods-allowed folks.
    AF6LJ and N2EY like this.
  7. N2EY

    N2EY Premium Subscriber QRZ Page

    I agree! - but with one caveat: All mods should be completely documented. With modern technology, that's really pretty simple.
    AF6LJ likes this.
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Replace ALL of the paper capacitors and the "deadly" capacitors that are in the article that I posted before. Then, realign the receiver. There is a pretty good chance that the 20-meter and 15-meter crystals will oscillate. The usual crystal failure is drift, not refusal to oscillate.

    Any leaky capacitor can definitely affect the performance and such a capacitor can also cause failure of other, considerably more expensive, components.

    Virtually every person who has serviced the 75A-4 recommends replacing all the referenced capacitors and then aligning, etc., the receiver, many even before applying power.

    Glen, K9STH
    AF6LJ likes this.
  9. VE3AJM

    VE3AJM Ham Member QRZ Page

    Shunt feeding the mechanical filters for the earlier A4s is a very important thing to do as per the Collins service bulletins. From the pictures that Billy posted of his serial 15xx, it appears that it needs to be done.

    Al VE3AJM
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  10. KM1H

    KM1H Ham Member QRZ Page

    Ill try not to repeat too much about my A4 since it is a current topic on another thread on here.

    After being completely disgusted with a Drake 2B overload in contest and DX pileup environments I bought the A4 knowing I had work ahead of me to do what I wanted.
    The serial is 1068 and it came with 3 filters, original manual and the A3 speaker for $250 in July 1965 since it was still relatively new and there was still a demand as many hams werent ready yet for downsized packages nor transceivers.
    Working as a Service Dept Tech at National plus ready access to R&D I took on the project with the blessings of management on my own time.

    I wont get into the mod details. All Black Beauty caps were replaced as National already experienced their high failures and every set that came in for service automatically had every paper cap of any brand replaced with modern CDE Mylar film caps. It was there I decided doing things piecemeal on my own gear in later years was totally ridiculous and a waste of time that could lead to many naysayers have found out on various forums.

    The RF amp was changed to a 6GM6 with appropriate voltage and resistor changes. Both mixers became 7360 beam deflection tubes that were designed for commercial SSB service and not TV sets.
    The AGC was modified including a SS Hang AGC circuit.
    After alignment and a few component value tweaks after several back and forth trips to the home station it was ready for prime time in the ARRL CW DX contest of 1966 which was 96 hours spread of two weekends in February and March, 4 weeks apart.

    The selectivity needed work so the AM filter was removed and a 500 cycle filter installed to add a sharper position. Nope, after a week at home it was decided the skirts werent sharp enough. A 6BA6 was added along with sockets for additional 500 and 800 cps cascaded filters using a small aluminum box mounted under the chassis with no extra holes. All signal cables were shielded. Tests revealed much skirt improvement but some passband ripple which was left alone as the next half of the contest was approaching.

    The contest showed the improvement and I thought I was finished BUT the R&D manager was following my progress and suggested bringing it in for an all day Saturday session that coming weekend.
    Using a fancy new GR motorized sweeper he pointed out the problems, the why, and what was needed. I did all the work and tests while he critiqued and incremental changes finally made him satisfied. A year later I was working in R&D.

    With a HB noise source using a Sylvania 5722 noise tube the noise figure appeared to be about 8dB on 10M which was a huge improvement over the stock 15 dB figure. Decades later I bought a HP 8970A NF Meter plus noise source at a company (not mine) belly up auction for $200. After a few tube swaps and alignment tweaks the NF came in at 6.1dB. Contrary to what has been repeated for decades in ham rags and later the Internet, apparently with little thought to verification and accuracy, there are times such a low NF is beneficial. At this time I had moved to the new hilltop location which in itself opened up several more layers of signals on all bands. With YO software designed HB monoband 10-20M 4 el yagis with a F/R (over the full 180 degrees) in the 30-40dB + range I could find holes in the azimuth bearings that were wide open and dead quiet. In one CQWW SSB contest in the 90's (forgot which year) I operated 10M only with a goal of countries and zones only and reached 147/40 which the MM super stations couldnt reach. Another time a DXpedition to the first authorized Vietnam operation in decades came on 10M one early morning grayline long path and nobody in the US was calling. I worked him on the second call and asked when they would be on 80. He came back and said NOW and gave me a SSB frequency and said the op was there listening. That was a one call contact as the grayline peak was on the wane.

    Sorry about the off topic drift. That A4 is still in use and is my go to RX on a noisy 160 CW band and has dug deeper and outheard everything Ive ever owned including a TS-950SD with cascaded 250Hz filters. My 160 DXCC/WAZ is at 300/39.

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