The Collins 75A-4 Thread.

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

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  1. N2EY

    N2EY Premium Subscriber QRZ Page

    How many were actually sold....and for how much?

    That's what really matters, IMHO. Asking price doesn't matter, if nobody buys.

    ----

    $600 today is about $80 in 1967 dollars.

    73 de Jim, N2EY
     
  2. K4KYV

    K4KYV Subscriber QRZ Page

    I remember back in the mid 60s I had always wanted a 75A-4 but it was way out of my price range - typically about $300, but still a fraction of its original selling price. In the early 80s they had steadily held at about that same price, although the dollar had devalued considerably. I thought the one I bought for $200 in 1982 was a bargain, then I found another, all 3 filters included, for $100. The spinner knob and 4:1 mechanism had been ruined by a previous owner, but I found a n.o.s. repacement, unopened in the original Collins box, for $35 from the old Ham Trader Yellow Sheets.

    Most of the ones I saw at Hamvention Friday morning were still there Saturday afternoon just before the thunderstorm threat closed the flea market down early.

    I was asking $75 for my 75A-1, poor aesthetic condition, non-working but repairable. It came back home with me Sunday.
     
  3. N2DTS

    N2DTS Ham Member QRZ Page

    I got one cheap, in like new shape, and did not think much of it as an AM receiver.
    It was a good ssb receiver and very nice looking.

    I think you could modify a 75s1 into a nice AM receiver.
     
  4. KA4KOE

    KA4KOE Ham Member QRZ Page

    Mine was cheaper than that. Like I said, it was from a friend.
     
  5. K5UJ

    K5UJ Ham Member QRZ Page

    A4s are still up there in price but I don't know if they sell for as much as they list for. The A3 is often overlooked as a lower cost option, often listing for half what people want for A4s. But AM ops usually take a pass because they want either an A4 or the earlier A1 or A2. The reasoning for this is that it is thought that the A3 was engineered for air traffic control audio, but any width of filter can be put in the mechanical filter sockets to open it up. But the big thing you get with an A4 is pass band tuning. Wasn't that the purpose of Ashtabula's broadcast band Q5er with his BC348?

    p.s. I'm leery of forking out close to $1K for anything at a hamfest. All my rigs and receivers came to me by other means. I just remembered I bought a Knight T-50 at a fest for $50 once. I felt a bit foolish for spending $175 last weekend on that AM processor.
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    UJ:

    The problem with the 75A-2A and 75A-3 is that the mechanical filter is in the circuit all the time. You can get narrower bandwidth using the crystal filter but no wider than what the mechanical filter sets.

    There is a way to compensate for this. However, the vast majority of amateur radio operators are not familiar with how to accomplish the effective elimination of the mechanical filter. The solution is very simple, in the 2nd mechanical filter position (it is "blank" from the factory) install a circuit consisting of 2-each capacitors and resistors. This broadens the bandwidth to out to as much a 10 kHz (depending on the values of the capacitors and resistors) which improves the AM reception immensely.

    Since the "network" is only used when the filter switch is in the "B" position, "normal" operation of the receiver is not affected.

    Such a network was installed in my 75A-3 when I obtained the receiver. Wires are just plugged into the mechanical filter connections with no solder, etc. As such, the network can be removed at any time with nothing remaining behind to indicate that it had ever been installed.

    Glen, K9STH
     
  7. K4KYV

    K4KYV Subscriber QRZ Page

    Have you tried it out yet?
     
  8. K5UJ

    K5UJ Ham Member QRZ Page

    Not yet. So far, I replaced some missing terminal strip screws on the back panel, looked inside, reglued a LED cover on the front panel and fired it up on 115 v. It looks normal, but I want to get the manual before I try putting any signal through it. A fellow in Maine is sending me the manual (I hope).
     
  9. K5UJ

    K5UJ Ham Member QRZ Page

    I find that hard to believe. All you need is a pair of loading resistors and a DC blocking cap in series. I've tried it but then the set is wide as a barn door. I think the IF cans in the A3 were adjusted with the assumption the mech. filter would do most of the filtering. With that out, the set is around 15 kc wide. I've effected a compromise using Murata ceramic filters, but I don't have the Z matched to the homebrew filter boards I've concocted, so there's passband ripple.

    Rob
    K5UJ
     
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    UJ:

    Have you looked at the "technical" questions asked here on QRZ.com? Unfortunately, the number of amateur radio operators, with any real technical ability, is shrinking every day and, also unfortunately, it "seems" that there are more than a few operators who don't even know which end of a soldering iron is hot!

    It helps to "tighten up" the bandwidth a bit more if you put series capacitors on both sides of the filter connection. Neither side of the mechanical filter goes to ground.

    The crystal filter can be used to also "tighten up" the bandwidth.

    In addition, the bandwidth can definitely be reduced by the proper alignment of the 455 kHz i.f. cans. However, this procedure is NOT in the 75A-3 manual but is in the 75A-2 manual. Since the primary difference between the receivers is the additional mechanical filter(s) (the 75A-2A is a modified 75A-2 that has had the mechanical filter circuitry added which turns it into a 75A-3), the alignment procedure outlined in the 75A-2 manual can definitely be used with the 75A-3.

    Glen, K9STH
     

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