AM RECEIVER SUGGESTIONS & QUESTIONS

[k4kyv]

The stock 75A4 may not be the greatest AM receiver out there for to-day's money, but it performs superbly under piss-beat QRM condx, and I  like the freq stability, dial readout accuracy/resetability, i.f. notch filter and passband tuning feature.  I was able to solve the front-end overload problem by inserting an outboard homebrew switchable 20 dB attenuator pad in the antenna line.  With some antennas I run the attenuator in line all the time and still have plenty of sensitivity.  I think that's the problem - Collins tried to build in too much front-end sensitivity.

Like the R-390A, the shortcomings in the audio can be overcome with an external audio amp and good speaker, but a  couple of simple mods must be performed unless you pick the audio right off the diode  load.  There is a test point on the the chassis you can use for access to the diode load if you wish.

To me, what places the 'A4 low on the  list of AM receivers is the present-day inflated price, driven up by the trophy collectors who pay big bux and never even plan to actually use the receiver.

I would say it is an excellent choice if you can find one in the <$400 range, and you are willing to bridge additional capacitance across a couple of coupling caps and clip out a couple of "hyellofying" caps.  I put (appropriately reversible) mods in almost almost all my stuff anyway, so to me the 'A4 mods were no big deal and pretty much routine for my station.

I have two 'A4's.  Bought both back in the early 1980's.  Paid $200 for one, and $100 for the second.  The second even had a full array of filters, including  the stock Collins 6 kc/s one.  The downside to that one was that a previous owner had completely wrecked the spinner knob and vernier tuning mechanism.  But I was lucky to find a nos, in unopened box, Collins conversion kit complete with mechanism and new spinner knob,  plus instructions on how to install it - the whole thing for $35!

To-day that kit would go to a trophy collector for more than I paid for both receivers combined (even with price corrected for inflation), and like the unbuilt Heathkits, the buyer would likely never dare even remove the first staple from the factory sealed plastic bag it was packed in.

As for Super-Pro's, with the pushpull audio stage, they are great if you can keep them working properly, and plan to use them strictly for AM, and never attempt to listen in cw or ssb modes.  The main drawback is that the frequency stability sucks!  The reason is that the basic mechanical design of the main tuning capacitor assembly makes it prone to thermal expansion/contraction.  The second drawback is that some parts of the circuitry are extremely hard to work on - particularly the rf amplifier and mixer subassembly.  It is practically impossible to gain access to some of the components to replace them - particularly those plastic moulded paper caps that you will usually find leaking an oily residue.

The problem I have with the R-388/51J series is the 500 kc/s i.f. stage.  It is extremely difficult to find replacement filters for even the stock selectivities, let alone wider bandpass such as 8 kc/s or higher. So you are pretty well stuck with restricted, 6 kc/s selectivity in the mechanical filtered models.

[Vortex Joe - N3IBX]

From what I can dig up, the differences between the SP-10/100/200 models seem to be little more than all glass tubes (SP-10), metal tubes in the front end (SP-100), and all metal tubes (SP-200).  I suspect that there were more modifications/upgrades over the life of this series, but I can't find much on it.



Can anyone point out some impressions about what makes your favorite Hammarlund SP model stand out above the others?

Dave,
       The reasons why the Super-Pro 10 is my favorite in the SP line:
  
       1) The high fidelity PP 42 audio outpoot is fantastic
       2) Very Oldbuzzardly and kewl looking (The SP-10 dates to late 1935,early 1936. Some like mine, even had an 8 ohm audio output)
       3) Good adjustable selectivity
      
A drawback: The "R" Meter reads backwards and is only a general indicator of received signal strength.

As a point of reference: Don't quote me on this, but I believe there were something like 26 "JX-" variants of the SP-600. Regardless of which one you have, they're all quite good, though very different from the SP-10,100 etc series; and have a built in power supply. I have one of the very first SP-600's, a low serial# "JX-1" built in August, 1951. It's performance seems identical to a "JX-17" I owned 20 years ago.

Regards,
          Joe N3IBX

[Steve - WB3HUZ]

Hey wait a minute you picked the SP600 just cuz someone posted a picture of it. Take a lookit this SP200. It's a virtual "black crackle festival", for gawd's sake....

Yep. That black-crackle SP-200 is one of the prettiest RXs ever made, IMHO. And the audio on AM is killer. Wish I still had one. Instead I have a 400, with tan crinkle and less cool looking knobs. Still killer audio though.

[John K5PRO]

I was guilty of pulling up this old subject here and adding my comments yesterday, about the NC-300 I am about to get. I have the 75A3 now. The previous postings were Sept 2005, so a lot of you are replying back to that information I fear. I should have just opened a new topic, but this one seemed appropiate to ask my questions. thanks for all the comments.

[w3jn]

Well, some new questions have been asked.

Don, at $400 a 75A-4 is an outstanding buy for one of the best battle-mode radios ever made.  Keep the front end attenuator and external audio amp handy though!  As others have pointed out the thing I have against the 75A-4 the most is that it's a very poor value for a receiver nowadays.

The SP200/400 series can be made more stable by (a) centering the variable cap plates (loosen the locknut on the end of each cap, and carefully adjust in or out with a screwdriver for a peak or dip when BFOed against a carrier) and (b) cleaning the wipers on the oscillator section of the variable capacitor.

The older SP-series isn't nearly as hateful to work on as the SP-600.  The caps in the RF compartment are a bit of a bother, but they're at least accessible if you have a small dikes and a soldering iron with a long tip.  More than I can say for an SX-28....

The PS on a SP-400 is a bit smaller than the BC-779/BC1004 power supplies.  It's a "cat coffin" design rather like a larger HP23, rather than a huge rack mount affair like the BC-779/BC1004 PS.  The military PS has a bunch of oil caps for filters vs electrolytics in the SP-400.

The SP series is very easy to align despite the variable IF selectivity (watch for B+ on the trimmer caps in the IF cans!!) and once set on the proper IF frequency the readout is dead nuts on, in my experience.  Many hams love to align them on 455 KC for some reason instead of the required 465 KC.

The things I don't like about the pre-600 SPs:

1)  Tuning not as smooth as a SP-600
2)  A UL nightmare... high voltage exposed everywhere on the rear panel, and high voltage on the trimmer cap screws
3)  If the on/off switch carps out you gotta remove the RF deck to remove it.

73 John

[nq5t]

I have been offered a NC-300 for nothing. ...  And yet another gripe I read about  is image rejection on the higher bands, like 20 meters.

 

The real issue on 20M is feedthrough of an image of one of the SWBC bands.  Daytime no problem, but when that band is open at night you hear a lot of it all over 20M.  There is a simple adjustable 9.7 Mhz mixer trap circuit, but it isn't all that effective.  This is true on both the 300 and 303, although it seems to be worse on my 303 than on a 300 I once had.  The radio really needs a band-reject or high-pass filter in front of it.

Oddly, this trap circuit isn't even mentioned in the 303 manual, although it is present in both radios.

Except for this image issue, the 303 is one of my favorite radios, especially with the audio tapped off the detector output and fed to an external amp.  Mostly because I love that big slide-rule dial 

[David, K3TUE]

The things I don't like about the pre-600 SPs:

[...]
2)  A UL nightmare... high voltage exposed everywhere on the rear panel, and high voltage on the trimmer cap screws
[...]

I was planing on making a power panel screw cover plate of hard plastic tubing,
but which trimmer caps are you referring to - where?

[w3jn]

Jeez, nice necro 

The IF trimmers on the plate side are hot with B+.  Use an insulated screwdriver here.

[WQ9E]

Originally there were covers over the exposed terminals on the back of the SP-200/400 series radio and PS but these were lost in most cases.  Of the 4 that I have only two came with their proper covers; 1 was covered with electrical tape and the 4'th was bare.  Still not as bad as the Hallicrafters HT-19 (NBFM and CW with a 4-65 final) which brings the plate and screen to voltages to open binding posts on the back for easy hookup of an external modulator.  I built Plexiglas covers for mine since having a set of 1,750 volt plate supply terminals uncovered on the panel didn't seem like a great idea.

The SP-200/400 do sound nice on AM.  My favorite all around AM receivers are the from the Hallicrafters SX-100 family (100,101,115,117).  They have enough bandwidth to sound good when conditions permit and narrower choices are available when needed and my favorite feature is the selectable sideband on AM which often allows SSB QRM to be removed by switching sidebands.  The HQ-170/180 also have this ability to select sidebands but the audio isn't as good and unlike many others I actually like the Heathkit RX-1 which also provides selectable sideband.  I have a couple of RX-1's and as long as the IF gain is reduced to a reasonable level it sounds pretty good.   

Over the weekend I realized how much I missed being able to easily choose the best sideband to avoid QRM.  Right now my main AM rigs are a Ranger/Desk KW with an SX-88 and a Viking 500 paired with a Pierson KP-81.  The Pierson has good selectivity from its multi-stage 455 Khz IF and the SX-88 provides very nice selectivity from its low 50 Khz. final IF but neither provides the ease of switch selectable sideband and both require retuning to provide this performance.

[K6JEK]

Do you love it?  Did you have one when you were younger?  Did you want one but couldn't afford it?  Did your buddy across town have one and you wished you did?  Was it the receiver at the high school radio club?

Those are as important as the specifications.  Almost anything is good enough for rag chewing.  For awhile I was using an NC-270 because it was the receiver at the Benner Junior High School station and as cute as a button. I used an NC-125 because it was my novice receiver and a 75S-3C because I used to drool over the pictures in the handbook, etc.

One more important consideration.  Do you really want to lift it?  I have an SX-28 which is heavy enough but I've managed to resist getting an AR-88 because I figured I'd end up in a back brace.

-- Jon

PS: Receivers I actually use the most?  74A-4, SX-28, R4-C and (horrors) Ten-Tec Omni VII. The R4-C with Sherwood audio & PS, no 6 KC filter (straight through)  is a killer receiver for AM, CW & SSB even though it doesn't weigh much.

[W1GFH]

I'm still a fan of the SP-200, although mine has developed some weird warm-up symptoms lately, it is terrific if you like the classic AM hi-fi sound. Yes, exposed power terminals are a bit sporty. If anyone has some plexi covers for sale, let me know.

(Hiya Jon/JEK. Yup, the "2nd best sounding Ranger on the West Coast" has moved to a Boston suburb. Check out pix of the new W1GFH slimmed-down, "Johnny Novice In The Basement" shack - attached)

[K6JEK]

I'm still a fan of the SP-200, although mine has developed some weird warm-up symptoms lately, it is terrific if you like the classic AM hi-fi sound. Yes, exposed power terminals are a bit sporty. If anyone has some plexi covers for sale, let me know.

(Hiya Jon/JEK. Yup, the "2nd best sounding Ranger on the West Coast" has moved to a Boston suburb. Check out pix of the new W1GFH slimmed-down, "Johnny Novice In The Basement" shack - attached)

Good looking essential shack.  Now that you're out of the neighborhood I'm willing to concede that you actually had the best sounding Ranger on the West Coast even though Bob Heil gave mine the title.  He was drinking champagne at No Money's place.  But weren't you using some fancy schmanz microphone and audio set up?

[W1GFH]

Good looking essential shack.  Now that you're out of the neighborhood I'm willing to concede that you actually had the best sounding Ranger on the West Coast even though Bob Heil gave me the title.  He was drinking -- champagne at No Money's place. 

You got it right, that was the night Bob Heil reportedly snatched KO6NM's RE-27 off the stand and threw it into the swimming pool. From then on, all the So Cal "big guns" switched over to Heil PR 40's.

But weren't you using some fancy schmanz microphone and audio set up?

Yup, big voodoo: a cheapo Marshall condenser and ART EQ into the mic input.

[Fred k2dx]

Let me chime in regarding the 75A4 on AM, even though it's a bit late. Mine came with an old homebrew IF filter substitute constructed long ago. It sounds terrific. I have made the few audio changes Don suggests, well worthwhile IMHO if you use the internal audio stages.

But about the IF filters... I have had both the original Collins and the current aftermarket. They are OK, good for battlemode, but under quiet band conditions the old LC filter is way better sounding. The one I have uses a miniature IF transformer. It seems to be about 12 Khz or so... never measured it. There was another version using two miniature IF cans I saw somewhere. It may have been in an old CQ magazine. Both of these were published long ago but seem to have been forgotten about?

I ended up using this filter lineup: Collins 3Khz, aftermarket 6Khz, and the homebrewed 'wide' LC.

The LC filter sounds way cleaner... not extended in response as much as just cleaner. Maybe this is what is referred to as ringing in the R390a vs R390. But if anyone attached to their 75A4 is curious I strongly suggest giving it a try - whether you use the internal AF stages or external amp.

On the selectable sideband QRM dodging technique: The Hallicrafters SX 115 offered this as the only AM receiving mode, probably due to the low last IF. Normal AM reception is carrier and either sideband, out to 5 KHz (for a 10 KHz equivalent mode). Not a shabby receiver at all, for either AM, SSB, or CW. It was supposed to compete with 75A4.

 

[Vortex Joe - N3IBX]

Let me chime in regarding the 75A4 on AM, even though it's a bit late. Mine came with an old homebrew IF filter substitute constructed long ago. It sounds terrific. I have made the few audio changes Don suggests, well worthwhile IMHO if you use the internal audio stages.

But about the IF filters... I have had both the original Collins and the current aftermarket. They are OK, good for battlemode, but under quiet band conditions the old LC filter is way better sounding. The one I have uses a miniature IF transformer. It seems to be about 12 Khz or so... never measured it. There was another version using two miniature IF cans I saw somewhere. It may have been in an old CQ magazine. Both of these were published long ago but seem to have been forgotten about?

I ended up using this filter lineup: Collins 3Khz, aftermarket 6Khz, and the homebrewed 'wide' LC.

The LC filter sounds way cleaner... not extended in response as much as just cleaner. Maybe this is what is referred to as ringing in the R390a vs R390. But if anyone attached to their 75A4 is curious I strongly suggest giving it a try - whether you use the internal AF stages or external amp.


 

Fred,
      Is the 6KHZ filter in your A4 a aftermarket mechanical or a xtal job? I'm curious. Also, how would you compare the aftermarket mechanical filters to the original Collins branded one? I'd think they're both about equal, but maybe not due to advances in technology since the original Collins filter was designed.

I have a 75A-4A that I'm very much in love with. It's the very best "battle conditions" receiver I own. I mated it to a 32V-2, and use the pair on the aether once in a while, especially when the going gets tough.

Mod-U-Later,
                Joe Cro N3IBX

[W3RSW]

I had a SP-200 some years ago, complete with the rear term.strip covers, but traded it off.  ...sigh...  If I recall correctly the power cable was cloth covered.  Neat thing about the 200 was the sliding eyeshutter for each band.  way cool.
About the audio, the driver transformer was the same size as the output transformer.  How could you go wrong?... 

Tim mentioned -

...I KNOW THIS MAY APPEAR STRANGE,BUT FER SUM REASON MY HQ100 HAS INCREDIBLE AM AUDIO...I USE EQ,ETC ON THE OTHER RECEIVERS HERE,BUT THAT LITTLE '100 SOUNDS JUST GREAT...UNDERSTAND,THAT IS WHEN THE BANDS ARE NOT CROWDED,AND CANT COMPETE WITH QRM FROM OTHER STATIONS,BUT I'M SOLD ON THE STOCK AUDIO FROM THIS THING...MAKES ALL THE AM GANG SOUND GREAT...TIM..SK..

I guess good sound is in the ear of the beholder...  .  We've probably beat it to death in other threads, but that's gotta be the effect of those cool couplets and feedback circuit Hammarlund used.  The higher the volume the tighter the audio bandwidth. 

At strong BC shortwave station level, one has more than enough audio to enjoy full audio bandwidth (vol. control at min. levels.)   I wonder if a better AVC would negate the 'variable' audio bandwidth characteristics.  Almost have to if all the audio is presented to the AF amps at the same level.   But Hammarlund put that circuit in the HQ100 and 110 and from my experience with a 110 it worked well enough to be noticable.

[K3ZS]

I have an HQ-110.    For AM operation, the narrowing of the audio bandwidth will never happen at normal audio levels.    For SSB or CW, you can turn the audio all the way up and use the RF gain control,  you will then notice the decrease in audio bandwidth that is more useful for those modes.   I also have an HQ-140X, which I believe is a better receiver for AM when signals are closer together.   Also have an NC-183, which sounds the best for AM when connected to a HIFI speaker, due to its 10W push-pull audio output stage.    On 10M I like the HQ-110 because of its dual conversion and no images.