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

    The 75A-4 was definitely NOT designed for hi-fi audio! It was designed for SSB audio or "communications quality" audio.

    Glen, K9STH
    WA3VJB likes this.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    My 75A-4 sounds good using the matching Collins speaker.

    I think it is a 8 inch speaker.
  3. W8KHK

    W8KHK Ham Member QRZ Page

    At the Atlanta Hamfestival on June 2nd, a gentleman had for sale a pristeen 75A4, including the matching speaker. It was lubricated, recapped, and fully aligned. He said it did not need any other restoration. It looked to be in showroom mint condition, so I just had to take a couple photos. He was asking 850 for the package. CAM03782.jpg CAM03782.jpg

    Attached Files:

  4. W8KHK

    W8KHK Ham Member QRZ Page

    I know it is not an A4, but the same gentleman had a mint ART13, clean except for a bit of hangar rash on the handles. Thought some of you might like to see it. He was asking 300, with the original shipping container. CAM03779.jpg CAM03780.jpg
    WA3VJB, W3KMP and W2VW like this.
  5. KA4KOE

    KA4KOE Ham Member QRZ Page

    Wow. I would've snapped that up in an instant.
  6. K4KYV

    K4KYV Subscriber QRZ Page

    A (reversible) modification I did on mine, was to remove the part of the dial drag assembly that grabs the kilocycle dial disc, which is not needed with the 4:1 reduction drive. I left the piece with the shaft and knob in place to avoid a blank hole in the panel. Collins simply re-named it "dial lock" after they made the 4:1 drive standard issue, but I have never found the dial lock feature useful for anything.

    Removing the drag/lock allows you to loosen the kilocycle dial and move it forwards just a little (but make sure you don't re-position it far enough to cause it to rub against anything when it turns). This places it closer to the red pointer, reducing the visual parallax I find very annoying with the stock dial configuration.

    PS: I have thought of a couple of useful purposes for that hole, if I could only find a potentiometer or a switch thin enough to fit in place and not jam against the kilocycle disc. One might be to add a front-panel fine adjustment to the S-meter zero pot, since every receiver I have encountered has had some s-meter zero drift, whether with the original circuit or (lesser so) with the 500Ω pot upgrade. Another idea would be to replace the dial drag knob with a push-button switch to turn on the 100 kc marker. This would be very useful for quickly re-setting the kilocycle dial to zero when changing bands or tuning to the opposite end of the frequency range, since few PTOs these days are perfectly linear, and the different crystals are usually not right on with each other for all the bands, and a button next to the tuning knob would alleviate having to reach up to the function switch to manually turn on the calibrate oscillator.
    N6YW and N2EY like this.
  7. WB6YZZ

    WB6YZZ Ham Member QRZ Page

    I just acquired a 75A-4 with a sticky S-Meter. Anyone have experience fixing a meter? TNX
  8. K5UJ

    K5UJ Ham Member QRZ Page

  9. KA4KOE

    KA4KOE Ham Member QRZ Page

    My A4 is an April/May 1956 unit. Aside from recapping, is the AGC mod worth doing? I notice some distortion on SSB especially with loud signals. Not much, but it is there.

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