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

    Where the sliver mica capacitors are used as coupling capacitors, disc ceramic capacitors work just as well. This is not true where the silver mica capacitors are used in tuned circuits.

    I strongly suspect that the silver mica capacitors were used as coupling capacitors is because, when the 75A-4 was designed, small value disc ceramic capacitors were not generally available whereas silver mica capacitors were in widespread use.

    Unfortunately, "sliver mica disease" is now prevalent in the capacitors used in the 75A-4 and those capacitors definitely need to be replaced.

    There were well over 5000 75A-4 receivers manufactured and, as such, they are definitely not in the "rare" classification. Today, the 75A-4 is a good receiver, but definitely not an excellent receiver! When it was new, it was definitely "head and shoulders" above most of the competition. I have all of the A-Line receivers and several of the S-Line receivers. For SSB and CW, I prefer my 75S-3A and for AM I prefer my 75A-2. Many AMers prefer the 75A-1 but my 75A-2 is easier to use, at least in my opinion!

    Until I built my 160-meter transverter, that I am now using with my 75S-3A / 32S-3 combination, I did use my 75A-4 as my primary 160-meter receiver and it worked very well. However, with the transverter, I do have the ability to transceive as well as the ability to transmit on a separate frequency.

    As for Howard Mills: He does excellent work and his restorations are highly valued. But, I do agree that, in some respects, he does "over" restore doing things like powder coat cabinets instead of the normal "painting". Also, and I do not know if this is still true, the font that he was using on some of his silk-screening of the front panels was not exactly the same as the original.

    My 75A-2 was restored by Howard Mills before I got the receiver. He modified it by adding a product detector that plugs into the NBFM socket and that works very well.

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

    AF6LJ Ham Member QRZ Page

    this is an excellent thread, I have especially enjoyed the articles posted by Carl on the Pullin Mixer.
    Great Stuff. :)
  3. KM1H

    KM1H Ham Member QRZ Page

    Low value as well as many Temperature Compensating ceramic caps were available since WW2 and maybe earlier.
    They came in tubular dog bone style, hollow and solid. Several are inside the PTO and were far better at hollow state RF than discs which I do use often with SS.

    Early larger value discs of say .01 and up were often microphonic and engineers steered away from them until that industry process matured. OTOH some very cost conscious companies forced them upon production.

    All of the old style mica caps, silvered or not, that came in square and rectangular phenolic packages can become leaky as the case cracks at the seams and pollutants creep in. Ive had that problem often in 30's radios across the board.

    AF6LJ likes this.
  4. VE3AJM

    VE3AJM Ham Member QRZ Page

    Most of my ham radio activity is on AM from 160m to 40m. I have fitted a 4kc and 8kc R-390a filters along with a stock 6kc filter on my A4s. The stock audio from this receiver is less than stellar. The audio output transformer is tiny. I pick the signal off the detector output, and feed an external audio amplifier and some better full range speakers, and the sound that can be reproduced is excellent. I'll have to perform the AGC recovery mod so that there isn't the lag in recovery time using PTT. For my limited use of it for SSB receive, its a perfectly acceptable receiver. The rx is stable, you know what frequency you're on and have selectable sideband. I am not a DXer on ham radio. I have the Collins 2.1 and 3.1 kc filters available, but have never had any need to use them.

    I use my A4s with a Datong UC-1 up converter, and the receivers are now general coverage. Cool to be able to tune it on the BC and SW bands. Didn't have to spend a fortune obtaining them either.

    Al VE3AJM
    Last edited: May 21, 2017
    AF6LJ likes this.
  5. K5UJ

    K5UJ Ham Member QRZ Page

    Funny how feelings about looks can differ--I think the post SX-28 Hallicrafter sets are ugly as hell. The SX-73 is the exception.
    AF6LJ likes this.
  6. WZ5Q

    WZ5Q Ham Member QRZ Page

    I agree, the Hallicrafters R274/FRR (aka. SX-73) is a looker.
    I love that Spartan, no nonsense layout.
    My Receiver of choice all the way around for AM.
    With some modifications, it makes a great SSB RX as well.
    AF6LJ likes this.
  7. WA3VJB

    WA3VJB Ham Member QRZ Page

    There's an analogy to the hobby of old cars, and people in that hobby who are known as "polishers." They don't necessarily appreciate or worry about how the car drives, but it must look good, as part of an emphasis on status symbol. Good for them. They enjoy being seen, and bystanders get to see some good-looking cars.

    This thread has to do with the 75A4; for AM it doesn't really drive very well, but if it looks good on a shelf somewhere, that's the sort of customer Howard wants to serve. It's their receiver, and it makes them happy. Thumbs up.

    Howard also excels at the technical end of the restoration, and that's why his broadest audience values his work so highly. Not just anyone has the repair expertise and accumulated knowledge to squeeze all if not more from the radio's performance than likely from the manufacturer's routine level of quality control. Is that "over-restoration," perhaps? I don't think so, and my highly-polished chassis modules in my R390 help testify the radio has reached its highest level of looks & electrical restoration.
    AF6LJ likes this.
  8. K4KYV

    K4KYV Subscriber QRZ Page

    If the converter converts the signal to the 1.5-2.5 mc range with one stage, then the receiver becomes double-conversion for general coverage, just as it already is for the 80 - 10m ham bands. For minimum cross modulation and maximum dynamic range, you want as few conversions as possible. As I recall, the R-390/390A is triple conversion on some ranges.

    One difference between pre-WWII National and Hallicrafters receivers is that National manufactured most of their own parts in-house, the main tuning capacitor and i.f. cans for example, where they could concentrate on quality. Hallicrafters built their receivers, the SX-28 a good example, out of off-the-shelf broadcast receiver components. Hallicrafters used good enough engineering techniques to achieve quality performance with mediocre components, but Hallicrafters receivers are flimsier and less mechanically stable than solidly-built National receivers like the HRO, NC-101 and NC-240 series.
    AF6LJ likes this.
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    Good point.

    Is it the number of conversions that matters, or the number of stages before the real selectivity?

    Good point! How about post-WW2?

    IMLE (In My Limited Experience), the post-war Nationals tended to be more solidly constructed than the Hallicrafters, except for the top-of-the-line receivers such as the SX-101, SX-115, etc.

    73 de Jim, N2EY
  10. K4KYV

    K4KYV Subscriber QRZ Page

    Both. If you could eliminate the RF stages altogether and use an entirely passive pre-selector, that would be an improvement. But that usually doesn't provide enough signal to the 1st converter stage to overcome the internal noise. A mixer stage is worse than a straight-through rf stage in regards to intermodulation products, signal overload and internal noise.

    Ideally, you would have a passive pre-selector directly feeding a low-noise and high dynamic range converter stage, which would then feed directly into the selectivity filter. A 7360 beam-deflection tube (now near-unobtanium) might be made to work as the mixer tube, and modern solid state mixer circuits may possibly be made to perform equally well.

    That's likely true, but I'm not that familiar with many of the post-war receivers. Hallicrafters seem rather flimsy, and the past-war Nationals, like the NC-173 and 183/183D are less solid than the pre-war receivers. I once owned an NC-173 and an SX-28. The NC-173 was more on par with the SX-28. The post-war HROs, HRO-5 all the way to the HRO-60, used the same basic solid design as the pre-war versions with few real changes; they just used more up-to-date components and had more bells and whistles.

    My first real "communications" receiver was the NC-173. Later on, in the late 60s, I acquired a pre-War HRO and used it until the early 80s when I acquired the 75A4. I also owned an SX-28, NC-240D and NC-101XA, but never used them as my main receiver, and eventually sold the NC-240D and gave the SX-28 and NC-173 to ham friends of mine.
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