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

    Welcome back to AR there Mr. Steve, good to have you back on board the AR wagon. I was QRT for a long time myself and got back into this great hobby. Lots of interesting aspects that encompass AR these days, along with some great people in it's ranks. Hope to work you on the bands once you get back up and running from 7J6 land. :)
  2. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I have two 75A4s, purchased in the early 1980s, one for $200 and the second for $100 (that one had all 3 filters).

    The audio section in the A4 is an afterthought at best. I ended up fattening up the audio coupling caps in the audio section and cutting out the 510 pf micas Collins added in later serial number editions designed to severely limit high frequency response, and was able to get relatively flat audio response 80-5000 Hz. The 6AQ5 and minuscule audio output transformer, comparable to the one in a cheap 5-tube AC/DC BC receiver, is a disaster. I ended up using an outboard audio amplifier (a 10-watt monaural "hi-fi" amplifier of about the same vintage as the receiver) plus a 15" studio monitor speaker from a BC station, and was able to get reasonably good audio. I pulled out the 6AQ5 since it was no longer in use and just drawing power and generating heat.

    A few years ago I acquired a Sherwood Engineering SE-3 synchronous detector, which made me realise just how atrocious the stock AM diode detector and CW/SSB product detector really are. With the SE-3 I discovered how good a SSB signal can sound (although most of the ones you actually hear on the ham bands are worse than the stock 75A-4 audio). I still use the 10w outboard amplifier and studio monitor speaker with the SE-3.

    I added another outboard box when I first got the receiver, containing additional mechanical filters. It plugs in, in place of one of the stock filters, with cables running through the ventilation louvres on the side of the cabinet, mounted on a bracket that hooks onto the louvres for support. The whole receiver could easily be returned to stock, but I can't see why anyone would ever want to do that. I now have a choice of 300 Hz, 3.1, 4, 6, 8 and 16 kHz selectivity using both stock and outboard filters. I would never go back to only 3 filters.

    Since I work mainly 160, 80 and 40m, I have lived with the noisy 6BA7 mixer stages without too much of a problem. I use a homebrew 20 dB attenuator between antenna and receiver to take care of overloading which occurs in the presence of excessively strong signals. Over time the PTO tuning range has spread out beyond the correction range of the PTO alignment slug, so I have to re-caliblate as I tune from one end of the and to the other.

    The filter blow-by is quite noticeable, especially with the CW filter. I suspect the problem is the single-ended filters and the close proximity of the input and output terminals and circuitry. The R-390A corrected that problem by using double-ended filters with the input well isolated from the output.

    I have not noticed any deterioration of the stock mechanical filters in mine. A few years ago I purchased one of the 6 kHz Dave Curry mechanical filters advertised in Electric Radio, and found it to be identical to the original in every respect. No better nor any worse. I could plug the two filters into two slots in the same receiver, and tell absolutely no difference in skirt selectivity or insertion loss with either one. The Curry filter is a modern-day Rockwell filter stuffed into an original size can, along with a pair of matching transformers.

    I read somewhere that Rockwell is discontinuing their line of "modern" mechanical filters, because commercial/military equipment have now almost completely converted to DSP technology to replace analogue selectivity filters. The newer Rockwell filters, designed for solid state receivers, have much lower input/output impedance than the originals, although like my xtal filter, this can be corrected using input and output matching transformers. My 300 Hz CW filter is a Collins crystal filter with 1000 ohms in/out impedance. I used a pair of Radio Shack transistor radio i.f. transformers I purchased in the 1970s, and they work perfectly.
  3. W5INC

    W5INC Ham Member QRZ Page

    About to start replacing the caps all 25 of them in the 75A-4 this morning and 1 problem. That is finding the braid or wire shielding for the SB #1 mod. The wire braid I have is way to large in size for the wires which looks like to me it is in the 20 to 22 ga. size. I got the braid off of a mic cord and was going to bundle all of the wires inside of it but that leaves a lot to be desired. Checking with the electrical parts outlets, these folks only sell the braid in 100' lengths and the cost is over 100.00 for the roll. I only need about 2' of the braid to accomplish the task. Any suggestions on a source of wire braid that can be used for the SB mod? :)
  4. KM1H

    KM1H Ham Member QRZ Page

    Small coax such as RG-58 or 59
  5. W5INC

    W5INC Ham Member QRZ Page

    Going along slowly & not by choice :) in changing out the caps. I have run across this piece here that my manual is calling C-62 a plate decoupling cap for a 6BA7 which is the 2nd mixer in the rig. Manual calls for a paper .1 mmf cap. I am stumped here, as I have never seen a cap that looks like this guy here in the center of the photo, so really not sure how to proceed on this. It looks like a big resistor to me, but it is in the correct place according to the schematic. Just want to make sure, so nothing gets FUBAR in the radio.


    Then looking at C-71 which is supposed to be plate blocking Mica cap, which I am guessing to be the round brown guy with 2 red dots at each end with a black dot in the middle. Is this correct, hopefully so. Are the red rectangular pieces in this photo a capacitor also, as I have never run across these before. I guess that is what happens when you work on a radio that is older then you are, for the 1st time. This tube that has C-71 attached gets changed out in a SB to a 12AX7 instead of the 12AT7 that is now residing in that place. The old saying is proven true 1 more time, in that the is no real progress without a struggle. :) Thanks for the help from the more knowledgeable folks in the BA Forum.

  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    What mod or drawing are you going by ?

    I think you may have the wrong location of the caps. That looks like a Black Beauty.

    If you need some small braid, I have some I can send you.

    Good Luck.
  7. KM1H

    KM1H Ham Member QRZ Page

    Those are the infamous Sprague Bumble Bee caps, a cheaper version of their Black Beauty which wasnt much better.....all are bad

    That round part is L-25, a 2 mh RF choke.

  8. W5INC

    W5INC Ham Member QRZ Page

    TNX Mr. Carl, I was looking for the famous Black Beauty guys, no wonder why I couldn't find them. I wanted to double check + make sure and not create any more work for myself. TNX agn. :)
  9. W5INC

    W5INC Ham Member QRZ Page

    Going by the 75A-4 manual itself Mr. Don. That cap is running off the plate of V5 the 2nd mixer and goes to ground. I have the Revised March 1957 copy of the manual. I do need some of that small braid and be more then glad to pay for the piece along with postage, plus your time in doing so Mr. Don. I sure do appreciate the help in getting these SB done correctly. :)
  10. K9STH

    K9STH Ham Member QRZ Page

    The red, rectangular, items are the mica capacitors. Although not in the 75A-4, there are other rectangular capacitors, usually brown, but can be black or other colors, which are actually paper capacitors. Those are often called "micamold" which was a trademark for that type of capacitor. Those capacitors definitely need to be replaced immediately!

    If you read the color code on the large capacitor, you will find that it does indicate 0.1 mfd. Some people do call those "bumblebee" and others just call them "black beauty". In the 75A-2 and 75A-3, there are similar, virtually always leaky, paper capacitors that have a brown body and not a black body. When working on either receiver model, those capacitors need to be immediately replaced.

    The 75A-1, like many military surplus receivers from World War II, uses "bathtub" paper capacitors. Although those capacitors generally last a fair amount of time, they are starting to go bad on a regular basis and definitely need to be replaced.

    Glen, K9STH
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