The Collins 75A-4 Thread.

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

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  1. WA3VJB

    WA3VJB Ham Member QRZ Page

    The 75A4 is not a very good receiver for AM when left stock, as many owners do who have an eye toward maximum collector value unmodified.

    The high distortion and poor response of the audio stages, and the inadequate passband with stock mechanical filters, make this an unpleasant receiver for most AM reception. Aesthetically, the receiver is somewhat the orphan, mismatched in size against the senior Collins AM tabletop transmitter, the 32V series, yet not really a good optical fit with the junior Collins tabletop transmitters that were not capable of AM. Although marketed alongside the KWS-1, this transmitter itself falls short on AM, with only one sideband and a carrier, and an unsavory number of compromises against audio quality.

    But, as you already have seen with the posts about modification and re-working, the 75A4 can be made to sound good on AM. If you buy into one at below market prices, the idea of modification is easier to consider.

    Or, you can procure the older 75A series that were oriented toward quality AM. By the time the A4 came along, Collins had begun to shortchange AM in favor of other, incomplete voice modes.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    That may be true, But I have never had a bad audio report.

    All of the power on 1 sideband was a smart move, And Art was proud of it.

    I like the pictures being posted, But they are blowing out my dialup ISP connection. :(
    AF6LJ likes this.
  3. K5UJ

    K5UJ Ham Member QRZ Page

    Collins changed the A-3 filter slot capacitance in mid-production from 330 pF to 130 pF further complicating things.

    Not everything Collins did was wonderful. The ham gear had cost cutting measures like any other manufacturer. They had to offer a certain level of quality while still selling enough product to generate some amount of profit. It's obvious the 32Vs were designed to visually match the 75As in cabinet appearance and size but that meant the 32V cabinets were crammed, making them a PITA to work on, in my opinion. The big thing with the A-lines, the A-3 and A-4 at least, were the PTOs and mechanical filters. Not bashing Collins--the broadcast and gov't gear was pretty good, but the Collins fanatics need to realize Collins was a radio company in business to make profits and not everything was the ne plus ultra.

    I know what you mean. Folks on need to downsize their photos. Not everyone has a 25 Mb download speed.
    KM1H, AF6LJ and KA9JLM like this.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    The maximum size photograph that will accept is 1 MB. I do like to keep photographs around 500 kB for the best detail and not be that overloading of those that still have a relatively slow download feed.

    Glen, K9STH
    AF6LJ likes this.
  5. K4KYV

    K4KYV Premium Subscriber QRZ Page

    Sometimes I take snapshots with my cheap point-and-shoot digital camera for quick posting. When I upload them on, the "full size" option results in a picture that is way too big. The thumbnail option is way too small. I need something about midway in between. I haven't dicked around with jpg files to try to reduce them either in file size or the physical size displayed on the screen. If there's a quick and easy way to do that with a couple of clicks, I'd like to be able to reduce mine. But if it's a complex 15-step procedure with multiple menus that I won't remember from one time to the next and will have to re-learn each time, I'm not gonna bother. I have better things to do with my time.
    AF6LJ likes this.
  6. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Are you guys really complaining about the quality of the pictures I uploaded? They were done in a manner that provides
    detail in order to present the topic.
    KM1H and AF6LJ like this.
  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi Don,

    I really like Ken Ward's Makeup for exactly this issue. It's very easy to use and a couple clicks puts the pictures at 250k (or whatever you want).

    Shareware (give what you will...)

    Having posted this, I still think it's nice to post Thumbnails so you don't have to slog through large photos on your iPhone or dial-up for instance... and download the biguns when you get home or have time to tie up the phone line.
    Last edited: May 17, 2017
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    I love those pictures, then again I am not trying to see the Zed on a smart phone or an undersized tablet.
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    Do you have the Microsoft Office suite or something similar on your computer? If so, then there is an easy way to reduce the size of photographs.

    Glen, K9STH
  10. K4KYV

    K4KYV Premium Subscriber QRZ Page

    I got my first one circa 1981. 3.1 filter only, needed work. $200. It's the same one I use to-day. Successfully refurbished it and installed additional mechanical filters.

    Got a real deal on another one a year or so later. All 3 filters (CW, SSB, AM), $100. I was running the service shop at a 2-way radio sales & service at the time; they also dealt in some ham gear and ran monthly display ads in QST. If you took out a display ad, you got your issue of QST about 10 days early, in time to allow you to proofread your ad (not sure what that was supposed to accomplish since it would have been too late to correct any errors). Whenever the latest QST came, I always dropped everything and grabbed it to scan through the ham-ads to see if there was anything that might have interested me, since I had more than a week's head start over regular subscribers.

    One month I spotted an ad for a 75A-4, all 3 filters, vernier spinner knob, best offer. I gave the guy a call and talked briefly with him. He said he was a lawyer by profession and had been briefly interested in ham radio and bought the receiver, but then lost interest and just wanted to get rid of it. He said the receiver was in good shape except that the tuning knob was cracked. I decided that was a minor issue and started out with a low-ball first offer, intending to work up to a mutually agreeable price if possible. He immediately accepted my $100 offer and seemed anxious to seal the deal before I backed down. I sent him a USPS m.o. for $100 and about a week later the receiver arrived. It was as he said, except that the cracked tuning knob was a mosaic of bakelite; it must have been broken in 10 pieces and glued back together with epoxy, like a jigsaw puzzle. The receiver needed some minor work but I got it going and it performed just as well as my other one and although the glued-together knob was wobbly, it functioned OK.

    Later on, the dial cord broke and to replace it you have to remove the front panel. To remove the front panel you have to take off the spinner knob and vernier dial mechanism. Then to get to the dial cord pulleys you have to remove the kilocycle dial disc. That's where the fun began. Someone had evidently tried to remove those pieces, whose set-screws take a "Bristo" spline driver, using an Allen driver. The allen driver breaks off the splines and leaves you with a pivot/bearing assembly, nearly impossible to remove. To add insult to injury, someone had apparently tried to use diagonal cutters to grab the wallowed-out screw in the vernier dial bushing, and managed to cut it off cleanly right at the surface of the bushing. I finally gave up trying to remove it non-destructively, and filed through the brass bushing to remove the cog-wheel. I had a regular non-vernier knob and bushing I could use, so I decided to sacrifice the vernier mechanism to get the receiver working again. The kilocycle dial disc was an even greater challenge, since the set-screw is difficult to reach, and I didn't want to sacrifice the kilocycle dial. I ended up using a #49 reverse-direction drill bit, which snugly gripped the rounded-out hole in the screw head, to manually turn the screw. But Collins used green stuff on all the sets-crews that resembles Loc-Tite, and the screw wouldn't budge. I decided the only way to loosen it was to heat it, but how to heat it without damaging the kilocycle dial? I had a teeny diameter slim-barrelled soldering iron that would slip into the space, so I made a heat shield out of cardboard and aluminium foil, that allowed me to reach the hot soldering iron in behind he plastic disc without damaging it. But the soldering iron tip would not transfer enough heat to the screw to melt the Loc-tite. Then another idea flashed in my head: heat sink compound. I didn't have any fresh compound, but pulled an old solid state audio amplifier out of the junkbox, removed the power transistors, and managed to scrape off a blob of heat sink compound, which I carefully placed on the soldering iron tip and inserted it in the hole. It worked! I was able to insert the reverse drill bit into the hole and back out the hot screw.

    After replacing the dial cord, I looked in my screw collection and found one screw in the whole box the proper size with the proper threads, that would serve as a set-screw. It has a slotted head instead of Bristo, but at least no-one will ever round it out with an Allen driver. The receiver worked great, except for the 1:1 ratio tuning dial. I figured the reason the guy was so eager to accept my offer was that he thought he was ripping me off, by downplaying the mangled dial mechanism. But again luck was with me. Shortly afterwards, I spotted an ad in the old Ham Trader Yellow Sheets for a 75A-4 vernier tuning mechanism and spinner knob for $35. I immediately called the guy and he said I was his first caller and he sold it to me. When it arrived, I saw that it was a virgin Collins vernier knob assembly kit, still in the sealed box, with each tiny piece of microscopic hardware enclosed separately in its own postage-stamp size manilla envelope with the Collins part number stamped on the outside, each still unopened and fastened with the original staple. I installed the new vernier dial per the Collins instruction sheet and naturally, it worked perfectly.

    I made out like a bandit; a working 75A-4 with all 3 filters and a brand new vernier dial mechanism/spinner knob, all for $135.
    WA3VJB and N2EY like this.

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