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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

    I been wanting a receiver and after polling the zed collective here at various times in the past I purchased a 75A-4 from a very reputable source, pretty much sight unseen. It is a stock unit in every way except for some reason I was told, a tube socket was added to the back of the rig some time in the past. I was told this tube socket is no longer in use in the radio, any guesses to why this might have been added? :eek: It does have the 4:1 front VFO knob setup and I am very sure the rig will be in good shape once it arrives here this week. It will need some TLC I am sure to get back up and running in top shape.

    I will be putting in the new cap kit available from KE9PQ as it is 1 stop shopping for the most part. I have looked at 1 mod for the AVC circuit which includes changing out tubes and removing a resistor, which is suppose to improve the performance of the rig. Some folks say it works well and another school of thought is that all it does is just raise the noise level without gaining that much performance in weak signal work from the mod. I always like to ask the Zed collective here their opinions on these matters, as practical real World experience is very hard to beat. I don't want to reinvent the wheel in this project and that is why I am asking the Zed brain trust for their opinions on which mods to this receiver are worthwhile and which 1s aren't. I see new filters are available from the KE9PQ website, has anyone here on the Zed tried out these new filters as of yet in their 75A-4 radio? 75A-4s have been around close to 60 years now, so I am guessing here there aren't many new mods that could come along over the horizon and improve on what has already been established in upgrades for this rig. Thanks again to the Zed collective for their time & knowledge in this area. :)
  2. K9STH

    K9STH Ham Member QRZ Page

    For information on just about every modification ever published for the 75A-4, the "75A-4 Modification Compendium", available from Electric Radio Magazine, is well worth the $20.00 (plus shipping) that it costs.

    Definitely replace ALL of the "black beauty" capacitors in the receiver. Also, there are a number of silver-mica capacitors that are very troublesome. Replace those!

    There are also several Collins Service Bulletins on the 75A-4 which need to be incorporated in the receiver. Of course, depending on the serial number of the receiver, 1, or more, of these bulletins may have been made before the receiver was shipped from the factory.

    There are various filters available, for the 75A-4, from several sources that work very well. For AM, the 8 kHz filter for the R-390 / R-390A can be adapted with just a 9-pin miniature plug and some wire. The 2 KHz filter also works very well for SSB, much better than the original 3.1 kHz filter that came with the 75A-4. I have 1 of the 8 kHz filters in my 75A-4 along with a 2.1 kHz filter that was originally used with the S-Line.

    Glen, K9STH
    Technical Adviser
    Collins Collectors' Association
  3. KM1H

    KM1H Ham Member QRZ Page

    The A4 was not a good AM radio and a bit marginal on SSB as Collins didnt understand ham style AGC until the 75S3C.

    It overloaded easy, the 6DC6 RF stage is a dog on 10-15M, the filter circuits have blowby and 60 year old filters have deteriorated to the point of poor performance. Toss in marginal audio, bad caps inside the PTO plus the paper and mica caps already mentioned and you have an old radio well past its prime.

    That said and done Ive had mine since 1965 when they were just an obsolete old radio and dirt cheap even with 3 filters.
    Ive made many mods to it in the 60's mostly and it is my go to radio when it comes to digging into the noise on CW, especially on 160. No overload, no phase noise, no discernible mixer crud as in modern gear, great audio for hours of listening, real effective AGC. OTOH the died in the wool Collins freaks on the CCA hate me but I could care less. Most werent even born when I bought it and I did those mods while working at National Radio with all the R&D test equipment available to verify each step.

    What you do to yours determines on what you want to use it for. Start by telling us your intentions.

  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    Denigrating a COLLINS radio! You old iconoclast you!
  5. K9STH

    K9STH Ham Member QRZ Page

    Actually, the AVC/AGC circuits in the 75S-3, 75S-3A, 75S-3B, and 75S-3C receivers are basically the same. Not so in the 75S-1 / 75S-2. Those receivers suffer from quite a few problems and are definitely inferior to the Heath SB-300 and SB-301 receivers, let alone the 75S-3- series.

    For AM, of the Collins 75A- Line receivers, many consider the 75A-1 to be the best, followed by the 75A-2 and then the 75A-3. The 75A-4 comes in dead last! For CW, at least in my opinion, the 75A-2 is the best of the lot followed by the 75A-3, 75A-1, and then the 75A-4. For SSB, the 75A-4 "stock" is the best of the lot. However, a 75A-2 or 75A-3, with a product detector installed in the NBFM socket, is definitely not a "bad" receiver. But, a goodly number of receivers, manufactured by other companies, that were originally designed for SSB, generally perform better on SSB.

    I have all 4 of the 75A- Line receivers and have worked on quite a number of those receivers for others.

    Glen, K9STH
  6. W5INC

    W5INC Ham Member QRZ Page

    TNX to K9STH, yourself Mr. Carl and to Mr. Eric, The Renaissance Man up in AK. I will mostly be using the A4 on the CW side of the spectrum, some SSB and no AM. I haven't become a AM aficionado as of yet and have no real plans to get on that side of the spectrum. But who knows. It seems the N0DMG AGC mod is a must along with changing the wax paper Black beauty caps. Changing the 470 pf mica caps in the IF amps and AGC section looks to be another useful mod. Then I have seen 1 school of thought suggesting changing out the resistors not exposed to RF with new metal film 1/2 watt units, as this will supposedly make the A4 noise floor even less pronounced. Dave Curry is producing new filters for these rigs and are supposedly better designed and manufactured then the old Collins units. Here is a link to Electric Radio's review of the new filters now available.

    A copy of ER's mod book will be ordered today so I won't be reinventing the wheel in getting this old BA back up and running, better then new hopefully. TNX again to all of the educated Zed members in this field here, for sharing their knowledge freely 1 more time. :)
  7. KD2AZI

    KD2AZI XML Subscriber QRZ Page

    I just finished a 75A-3 and now spending a lot of time just fine tuning, testing, etc. I recommend just getting yours to work Stock first, then evaluate the need for mods. In my case I had my hands full reversing out some badly implemented product detector mod, and PTO repair + endpoint and linearity correction. Lots of resources out there on the 'net for PTO including a great article at RadioMuseum. Here was my adventure. Good luck!
    N8ZBA likes this.
  8. K9STH

    K9STH Ham Member QRZ Page

    Frankly, there is no way that the "new" filters can have zero insertion loss since they are passive devices only. A lower insertion loss than the originals is definitely possible. But, "zero" insertion loss? No way!

    Now, in the 60-years since the original mechanical filters were built, there have been advances in the production of those devices plus the fact that the older filters do deteriorate with age. The "new" filters, according to the cited article, still use a Rockwell-Collins mechanical filter. As such, these "new" filters benefit from 60-years worth of experience in the manufacturing of mechanical filters as well as being "brand new".

    Glen, K9STH
  9. KM1H

    KM1H Ham Member QRZ Page

    I dont have that ER mod info so cant comment there; Id appreciate a copy when you get it. You have to take that magazine with a grain of salt as there is no verification or peer review on most of what gets published. Ive come across a few great ones as well as many dogs on many subjects.

    Some of my mods:

    6GM6 RF amp, its as simple as just a swap or customize some resistor values for best performance using 10M as your platform.

    7360 beam deflection mixers in place of the poorly implemented 6BA7's. Those tubes are now very expensive and a dual triode mixer, especially a Pullen circuit, would work as well or better. Since I have a couple NIB spares Im not about to change the A4 but Ive considered doing it to my 75A3 as I like that radio a lot after a few AM mods but its deaf on 15 and 10M compared to a very mildly souped up NC-300 or HRO-60. Since I also have a rebuilt 32V2 (mostly stock) getting the A3 to where I want it is a priority...just not sure what number!

    SS Hang AGC

    Gain equalization and modifying the AGC for best results per stage and not one voltage and time constant fits all. This took a lot of work and time.

    Cascaded 500, 800, and 2.1 filters with a unity gain SS IF amp between stages to make up for the loss and act as both an impedance match and isolator between filters.

    Cleaned up the audio distortion. And added a passive + tuneable active audio filter to get rid of a lot of the hiss, also added a pair of audio notch filters. This is all external and some day Id like to implement an audio AGC that complements the other one.

    Externally and from the top the radio appears stock, all the dirty deeds are all underneath (-;

    Last edited: Jan 27, 2015
  10. K9STH

    K9STH Ham Member QRZ Page

    The Electric Radio Compendium is just copies of virtually all of the modifications, that were published in various amateur radio magazines, in one volume. There are some comments, by Osterwald, the owner/editor of ER, about his experiences with various modifications. However, the modification information is reprinted without any editing, etc.

    Basically, the book just saves a lot of time, and frustration, trying to locate the articles.

    Glen, K9STH
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