The Collins 75A-4 Thread.

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

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  1. K4KYV

    K4KYV Subscriber QRZ Page

    If the 75A-3 is like the A-4, the L-C i.f. strip is too broad for congested band condx. Rather than simply using blocking caps and loading resistors in place of the mechanical filter, a better solution would be to find a good 455 kc i.f. transformer, make an adaptor, and wire or plug it into the circuit. That would tighten up the L-C selectivity a little and make a nice wide-band selectivity position for AM.

    I use an 8 kc filter in mine most of the time. 6 is a little too narrow for AM when the band is not crowded or there are no nearby adjacent signals. Often, even under congested conditions, I get better readability with the 8 than with the 6, particularly with the synchronous detector. Despite decades of space-shuttle-audio propaganda to the contrary widely circulated in ham radio literature, the audio frequencies between 3 and 4 kc do carry a lot of the intelligibility in the human voice.

    The outboard sync detector which picks the signal off the final i.f. transformer just before the the diode detector, disables the passband tuning feature, but I don't find that a great disadvantage. I just have to manually tune the signal to one side of the passband, and in CW/SSB mode, manually adjust the BFO in sync with the main tuning, just as you normally do with other receivers.

    I have NEVER seen an A-4 with perfect passband tuning; the heterodyne frequency always wobbles a little and/or shifts as you tune the control back and forth.

    If you have an early 75A-4 with the mechanical filter input wired in series with the B+ line to the plate of the tube, you should immediately do the recommended modification to feed the tube through a 2 mHy r.f. choke resonated with a 68 pf mica cap, feed the filter through a 1000 pf blocking cap, and ground the bottom side of the input winding of the filter. With the original circuit, a shorted plate connection or tube could ruin the mechanical filter by placing the full B+ from the power supply across the winding and burning it out. Collins used a 600v disc ceramic for the blocking cap with the upgrade, but even those can short out; I replaced it with a 3 kV cap in all of mine, even the ones with the factory upgrade, figuring that a 3 kV cap would be less likely to fail with only +230v or so on it, than would one rated for just a little more than double the working voltage.
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    The 75A-3 has 1-more 455 kHz i.f. stages with a transformer which makes 2-each low i.f. transformers plus a tuning coil whereas the 75A-4 has only 1-transformer tuned 455 kHz i.f. stage plus 1-each tuning coil. The basic 455 kHz i.f. bandwidth, in the 75A-3, is a fair bit narrower.

    The 455 kHz i.f. bandwidth, in the 75A-4, can be slightly narrowed by removing R-46, a 22K-ohm resistor across L-27.

    I forgot to mention that with the 75A-3 the basic 455 kHz i.f. bandwidth can be reduced by removing R-83 which is an 82K-ohm resistor across T-6 secondary.

    I have an 8 kHz mechanical filter installed in my 75A-4 for AM use as well as the original 3.1 kHz mechanical filter and a 2.1 kHz mechanical filter which is much better for SSB use than the 3.1 kHz filter.

    Although my 75A-3 does have the R/C filter, I much prefer my 75A-2 or 75A-1 for AM operation. I also have a Collins 51J-2 and a Collins R-388 which is the same as the 51J-3 which also work well for AM.

    Glen, K9STH
  3. K5UJ

    K5UJ Ham Member QRZ Page

    the pin out on the filter slots doesn't matter for the bathtub F filters made for the A3, but if you are sticking in a homebrew perfboard filter, you have to pay attention to where the pin holes are that have B+ on them to make sure the B+ is blocked ahead of whatever filter you employ, and to make sure the filter is in the signal path and not the common side.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    I was making sure that 2-blocking capacitors were used an not just 1 with a jumper on the "low" side. Your previous comment suggested that only a single blocking capacitor was needed. I have found that a capacitor in the "common" side does help.

    I have not had all that good luck using Murata filters in place of a mechanical filter. However, using a 455 kHz i.f. transformer definitely sounds promising. I wonder if using 2 i.f. transformers, in series like in the 75S-1, 75S-2, 75S-3, 75S-3A, and in the early 75S-3B and 75S-3C would work in both the 75A-3 and 75A-4. I have never tried this!

    Glen, K9STH
  5. W8KHK

    W8KHK Ham Member QRZ Page

    I have never used capacitors in place of a mechanical filter, but I have been using a single 455 kHz IF transformer for the wide filter position in my A3 for over 30 years. I attached some solid copper wire to the IF transformer terminals, insulated by plastic sleeve. The other end of each pair of wires is soldered to the terminals of a defunct crystal base. The pin size and spacing perfectly matches the receptacle for the mechanical filter. With the stiff wires, no other mounting is needed. No mods to the A3, just plug and play.
  6. K4KYV

    K4KYV Subscriber QRZ Page

    The 8 kc mechanical filter in my 75A4 is a F455H97. It originally had solder lug connectors, but I discovered that I could straighten out the bent wires that formed the solder lugs, and they are identical gauge to the pins of a 9-pin tube or the stock 75A-4 plug-in filter; they fit right into the 9-pin socket, with correct indexing. I just have to make sure they are properly oriented before plugging in the filter, since the unused pins on the filter are missing. From the type number, the filter should be 9.7 kc nominal bandwidth, but comparing it side-by-side with an 8kc R390A filter, the bandpass characteristics are identical. When I first tried it, it had excessive ripple, which I corrected by replacing the fixed 100 pf mica caps Collins used to resonate the filter's input and output coils with variable trimmers, allowing me to tweak for exact resonance, and that eliminated the excessive ripple and the throughput signal level is the same as with the other filters.
    N2EY likes this.
  7. KM1H

    KM1H Ham Member QRZ Page

    I used Murata 455 kHz CB filters and some broader KIWA filters in several tube RX to get acceptable BW. In the opposite direction Im thinking of swamping the zillion HRO-60 IF's to broaden a bit say for 10 and other low QRM bands. Just cant beat the audio recovery and the PP 6V6 sound from that radio.

  8. N2DTS

    N2DTS Ham Member QRZ Page

    I use Kiwa filters in my homebrew receivers, better then mechanical filters in my book, plus no loss through the filter.
    They used to be $50.00 for the standard filter modules, 6 and 8 KHz would be good, they make a board that holds two filters if you want.
    455 KHz only, they work best with a wide IF chain.
    I come right out of the mixer into the filter, then into the IF chain.
    Seems to perform as well as the sdr's I have had.
  9. K5UJ

    K5UJ Ham Member QRZ Page

    There's an article in ER I found from issue 50 June 1993, Improved Audio for the Collins 75A-4, by Bill Beatty K7CMS. Beatty's method involves replacing T5 (audio output) with a "good 4 watt audio output transformer." The A-4 also has audio feedback which he has modifications for. I imagine changing T5 would send the Collins fanatics into orbit. I still prefer the idea of tapping the detector audio and feeding that out to an external p.p. amp. Speaking of audio, that same issue has an article by Bill KD0HG on his homebrew Williamson EL34 stereo amp.
  10. K4KYV

    K4KYV Subscriber QRZ Page

    Even with the new output transformer, you need to beef up coupling caps in the audio chain. There are several I changed in mine, from .01 mfd to .1. Also, a couple of 510 pf micas designed to by-pass the highs; those were added to later serial #s, since the C- numbering is out of sequence with others around them on the s chematic. I clipped those out of the circuit. That got the audio reasonably flat from below 100~ to around 5 kc. But now I pick my audio off the Sherwood SE-3 through the headphone jack to the outboard audio amp, by-passing the stock audio circuit altogether. I pulled out the 6AQ5 to save on power supply drain and to reduce heating.

    The audio section of the 75A-4 was an afterthought at best. Designed towards space shuttle sound.

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