75A-4 mods: 1s that work well

Discussion in '"Boat Anchor" & Classic Equipment' started by W5INC, Jan 25, 2015.

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  1. W5INC

    W5INC Ham Member QRZ Page

    TNX for the answer Mr. Glenn, boy I am sure glad just a few of the red mica caps have to be replaced and not the lot of them. Sure would be awhile taking care of that. The person whom wired this rig at the factory made 1 complete turn of the wire on each terminal for each component, sometimes 2. Soldered very well also. With the layout as it is the receiver, it had to have been assembled in pieces that were already wired and then put together for final assembly. I'm guessing this is how it is done, even today.

    While replacing 1 of the Black Beautys I found a resistor blown apart, that was right under the big cap. R86 @ 39K that was in the AVC 455 IF coil, pin C along with the Black Beauty being changed. I'm sure the AVC and Noise Clipper wasn't working correctly and the problem might have been hiding from a previous owner in the past. The big cap hid it very well to say the least.


    Looking forward to aligning this unit, the manual says use 100 Mv reading for S9 signals. So I am guessing this should be the benchmark used for this radio instead of today's 50 Mv used for more modern radios. TNX to Glenn, Mr. Carl and the rest of the Forum members for their help in getting this BA back on the bands. :)
  2. K9STH

    K9STH Ham Member QRZ Page

    The 100 microvolt reading included a 6 dB pad which made the actual input voltage 50 microvolts. The reason for the pad was that, in the "goode olde dayes", most signal generators did not have a true 50-ohm output. Therefore, a "matching" 6 dB pad was specified to get a true reading.

    A modern signal generator, with a true 50-ohm output, doesn't need the matching pad and, therefore, the output from the signal generator should be 50 microvolts for an S-9 reading.

    Glen, K9STH
  3. KM1H

    KM1H Ham Member QRZ Page

    Those are not a trademark of those caps, Micamold was a company name that made mica caps as well as paper caps encased in a bakelite housing; theirs were usually in a lozenge shape package.
    It is usually a reasonably safe bet if the cap is over 1000pf it is paper. Several other companies built similar looking ones in the 30's and thru WW2; experienced restorers are wise to them for decades now but newbies are still in learning mode (-;

    The Black Beauty was a Sprague trademark and looked nothing like the Bumblebee (not trademarked but named decades ago by service techs) which was usually the volume OEM choice when price was important. The Black Beauty is an all black case...surprised there?....and the graphics for paper caps is yellow. Film caps in the same package are red graphics and likely still good; HP used a lot of them in the 50's.

  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I will get it in the mail today, Sent to your QRZ address.

    You should get it Thursday or Friday, Lord willing and the creek don't rise.

    Good Luck on your project.
  5. W5INC

    W5INC Ham Member QRZ Page

    Many Thanks there Mr. Don for the help/pieces. Should have every thing ready to go as I have 20 of the caps now in place. Been working on it in my free time and almost done. Needle nose pliers come in very handy along with a good small diagonal wire cutter. Now all I need is a good external speaker for the guy, been trying to find 1 but no Luck as of yet. I seen a very nice Hallicrafter speaker, 1 with the big chrome H on the front. That got snapped up very quickly on the other for sale website, I guess there just aren't many left around anymore. :)
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I have the original Collins speaker that came with my twins. They are a bit hard to come by.

    Really any good quality 8 inch speaker should do just as well.

    Have Fun, I bet you are.
  7. W5INC

    W5INC Ham Member QRZ Page

    Yes I am enjoying getting this guy back in shape and the beauty about these radios is they are constructed very well. Collins must have hired some very good folks to assemble their products. Which is another reason a lot of these rigs are still around and even being used on the bands today. Plus the American work ethic to produce a quality product was a lot different back in the day, then it is today. Nice, neat and a lot better to work on the a birds nest of wiring I have seen in other rigs.

    I am guessing here Mr. Don you have a 270G-1 speaker, which I think actually looks better then the G-2 model. I'm sure the room gets filled with sound from that combo. I can't believe what the price is for an A Line OP console, with speaker, antenna selector, rotor control and I'm thinking watt meter also. That piece (price) is like the Golden Chalice, to certain Collins collectors and the CRM. :)
  8. K9STH

    K9STH Ham Member QRZ Page

    The 312A-1 speaker was the original speaker available for the 75A-4.


    30L-1 4a.JPG

    There also was the SC-101 "station control" available which was based on the 312A-1. In fact, the 312A-1 came with a panel to which various accessories could be added.


    Glen, K9STH
  9. W5INC

    W5INC Ham Member QRZ Page

    That SC-101 must be a very rare bird Mr. Glenn because the price they sell for is rather high. That sure looks like the full blown model right there in the pic with light and all. These SC-101s must have been an expensive add on back in the day, especially the 1 with the rotor control installed in the unit. Not many AR OPs during this time frame had beams to turn. 1 of these guys sold for 6K on the Ebay, according to this guy here! :)

  10. K9STH

    K9STH Ham Member QRZ Page

    The URL you listed is that of Dave Meitzen, AA9TT. He lives a few miles from me and is a very good friend of mine as well as being a client. He "dabbles" in buying and selling amateur radio equipment and usually has me check out the items before selling them.

    The 312A-1 came with a blank panel which could be drilled, punched, etc., to add accessory items. The 312A-1 was foundation for the SC-101.

    Glen, K9STH
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