The AM Forum Receiver Suite: Post your photos and stories here!

Discussion in 'Amplitude Modulation' started by N6YW, Aug 15, 2016.

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  1. K4KYV

    K4KYV Subscriber QRZ Page

    The front-end, frequency stability, dial accuracy and re-setability and selectivity are excellent, but the receiver is pure crap from the detector onwards. At first I modified the first stages of the audio section and noise limiter in mine and got fairly decent response using an outboard audio amp, but the product detector still produced highly distorted audio. I hadn't realised how good some SSB signals could sound until I added the SE-3. The tiny audio output transformer in the A-4 is about the same thing you would see in a cheap 5-tube AC/DC broadcast receiver. In later models they made some changes at the factory that made the audio even worse. Art Collins must have been a fan of that narrow, pinched, scratchy, tinny distorted audio you hear with most "vintage" SSB transmitters and transceivers made more than just a few years ago. I suspect it was the influence of the "return" of AM to the ham bands and the subsequent advent of ESSB, that finally got the manufactures of ham equipment to pay attention to audio quality.

    All my 75A-4s are in too good shape to use, but if I ever run across a real basket-case A-4 beyond redemption, I plan to use the basic front-end, and work that into a switchable i.f. amplifier with the choice of either mechanical filters or the extra i.f. strip from a R-392 that I have, and feed the output of that into the SE-3.

    I'm supposed to pick up an RCA AR-88 at Dayton. I hope it's repairable if not in working condition, and I'm eager to see how well I like that in comparison to my other receivers.
  2. KM1H

    KM1H Ham Member QRZ Page

    Tests that were made in R&D at National said my 75A4 front end was pure junk for any serious use and put truth to the comment "If you want a frequency meter buy a Collins and if you want to hear them buy a National". In those days of 1965 I didnt have much use for Nationals and only old men used them.

    Both mixers were changed to 7360's and the deaf 6DC6 RF amp was replaced by a 6GM6 along with some other changes. It was also recapped using the Mylar film caps used in National production since around 1959 and are still fine.

    With headphones and low audio drive it was OK; the only time I used an old Hallicrafters 10" speaker was when I wasnt operating. For the BCB and SW a HQ-129X and another Halli speaker were used.

    OTOH the receivers I use these days have all been audio tweaked for AM without going overboard with changing transformers; good wide range headphones and low drive work fine for me.
    AF6LJ likes this.
  3. K4KYV

    K4KYV Subscriber QRZ Page

    I haven't had any problem with my A-4. I can pull signals out of the mud that many others with modern gear say they can't hear. I rarely use it on higher frequencies than 40m though. I think the higher bands are where performance falls off. It does have cross-modulation when I listen to one strong signal and there is another strong signal somewhere else in the band, but an outboard switchable 20 dB attenuator between the antenna and the input terminal on the receiver cures that.

    My 1935-era HRO performed almost as well on 160 thru 20 as the A-4 sensitivity and s/n-wise, but didn't have the stability or dial resetability of the A-4. Stock selectivity wasn't as good, but I rigged up a couple of outboard mechanical filters and with that, I didn't see a tremendous difference between what I could hear with either receiver. The HRO with its two cascaded RF stages suffered from cross-modulation too.
    AF6LJ likes this.
  4. K5UJ

    K5UJ Ham Member QRZ Page

    The AR and CR88s at 16 and 10 kc passband are filtered by IF transformers only. The envelope detector can produce some noticeable distortion though, usually when the signal is strong and over modulated.

    You can take an A-3 and switch out the xtal filter and feed the audio out to an external amp. There are articles out there on homebrewing filters for the A-3. One is ER#64 Aug. 1994 by Petrich on p.20. I think that's the one where he writes about building a little LC IF filter. With that you'd avoid the mechanical filter distortion. I've built filters using ceramic filters mostly due to laziness.
    KM1H and AF6LJ like this.
  5. K4KYV

    K4KYV Subscriber QRZ Page

    Both my R-390 non-As have flaky intermittent problems that I haven't got round to fixing, plus they still have the selectivity gap between 4 and 8 kc.
  6. KM1H

    KM1H Ham Member QRZ Page

    While my HRO-60 has a gaggle of IF transformers that sharpen the passband beyond what is considered too sharp these days it sounds absolutely wunnerful wunnerful on 75 and 40 since there is no signal distortion and the audio recovery is about the best ever according to Jay, W1VD. The Tone control can provide a nice balance. Up on the wide open higher bands the HRO-50 or NC-300 gets the nod; both are red hot on 10 with minimal tweaks.
  7. K5UJ

    K5UJ Ham Member QRZ Page

    The '88s are hot on the high bands (a decent antenna is needed nowadays) due to the polystyrene used on the high band front end coil forms. The NC300 is a sleeper--I've seen them at hamfests for a little over $100 and these aren't basket cases but nice clean sets in good shape. A great receiver for a ham with no spondulix for the Collins, the super pros, R390s etc. The only red flag with the 300 and 303 is (I'm told--never attempted it myself) that restringing the dial cord is a nightmare.
  8. KM1H

    KM1H Ham Member QRZ Page

    It is Dial cordsS plus a chain. Yes, it is a bit hard on most first timers but Ive been doing it since the 60's:rolleyes: And it becomes easy after a few; I dont remove the panel either.
  9. KM1H

    KM1H Ham Member QRZ Page

    The A4 becomes a bit deaf on 20 and a lot more above that. Collins didnt design anything for hams, it was all a rehash of commercial/military gear until years later. The A4 overloads easy even during band openings and falls flat on its face with contests and DX pileups. Thankfully the mods work well.

    Most anything works well on 160/80, sometimes 40, and above that separates the men from the also rans.
    My early second run 1935 HRO has marginal AM AVC and the crystal filter is barely second generation. But after a full overhaul it is useable under non stringent conditions of QRM. The only mods were to zener the HFO and BFO and to leave the cabinet top open. National improved the cooling a few runs later by increasing the ventilation hole size.
    The pair of 58's as RF amps go back to a June 1932 release and were state of the art at the time. The much improved 6K7 was released in August 35 but National plodded along until WW2 when they were forced to use the metal tubes. The 6D6 is just a 6V 58.

    National used two RF tubes primarily to improve image rejection and they made no provision to reduce gain with the lower frequency coil sets so using the RF gain control was mandatory. On 10M the 58's are gasping for air and generate a lot more noise than external but they are useable when the band is wide open. The competing RME 69 is more of a slug, even on 20, with that rats nest bandswitching arrangement.

    I use the HRO, an early NC-101X, and a 1934 FB-XA when running HB or period correct CW gear mostly at 5 or 50W.
  10. N2DTS

    N2DTS Ham Member QRZ Page

    I never liked anything Collins for an AM receiver, although the R390a was my main RX long ago.
    75a4 looked great, so did the R388, 75s1, 51J4, I don't want any of them, although I always thought you could redesign a 75s1 into something decent for AM.

    SX17, quite good, good audio and selectivity choices, who knew what actual frequency you were listening to, very odd bandspread.
    About the same for the Scott SLRM.
    NC 300 and 303, not bad, kind of funky looking.
    Drake, all too narrow.
    SX101, narrow?
    Heathkit? Don't even think about it.
    Modern radios like the Icom 735, ts440, 756 pro series, all junk for AM fidelity.
    Even a cheap sdr blows away any receiver you can think of.
    20 to 20,000 Hz audio, any filter you want, no distortion, even on excessivly modulated signals, ecss, sync AM, selectible sideband, notch filters, etc.

    I never had one but hear an NC183 series was very good.

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