The AM Forum

I spent all day Sunday fingering out the de-hnyellification process for this receiver.  Before I write it up (ie I have to take it out of the cabinet to take 8X10 color glossy photos with circles and arrows on the back of each one) I'd like to know if there's any interest at all.

Briefly, though, there are a couple of inherent problems with this radio.  First, the ANL design (which this radio shares with the SX-100 and others) guarantees audio clipping at any modulation index over 70%.  This is easily solved by removing one capacitor with the downside being slightly less effective ANL action.  Second, the AM dicktector output gets fed thru the product detector via a .01 cap in series with a 220K resistor.  WHY they did this I don't know, but resolving this means moving one wire and clipping one jumper on the response switch.  Third, the audio is shaped by various series and shunt capacitors - removing/subbing improves matters quite a bit.  Finally, stagger-tuning the 50 KC IF cans improves high frequency response.

It's still not HIHIFI but it's a vast improvement over what it was.

Let me know and if there's enough interest I'll submit an article to Gary.

73 John

JN,

I'm interested. I've got one I picked up for a price I couldnt resist that's in the Q to get on the bench eventually. Please do tell. Seems like a sweet receiver, this one doesnt function well in lsb mode right now but is very clean on the outside.

Thanks,

Larry

Hi John,

Yep,  sure would be interested in the info on the mods to this fairly FB radio.

73  Thanks for asking,   DE  Vic  KF6RIP

John......I just got mine working about a month ago and I'd like to see the article!!!  It is a pretty cool receiver with good selectivity and OK sensitivity.......It would be FB if de yellified!......Steve

OK guys, you got it!  Guess I gotta dig out the camera and pull the RX apart.  The mods are a LOT simpler to describe via pictures.

73 John

I'm staring at one of these that is in need of a restoration.
Keith

Well get yer dead ass going on that thing, Keith!  It's an outstanding battle-mode receiver but not the best tool to use with FB sounding stations on a empty band on a Sunday afternoon.

Ace Hardware Matte Black is a good match for the cabinet; Metallic Pewter auto touchup is a good match for the silver cover.

73 John

John,
        I have done a lot of work to mine to improve the bottom end response. The improved warmth makes mine sound 100% better, it doesnt have that "rice box" audio sound any more and is quite plesent to listen too. It wouldnt even pass any audio below around 200 Hz. However it has always had a wierd distortion on strong audio peaks. The audio path from the detector through the response switch is hard to follow on the skizmatic, and I was going to pull it out again and do some more experimenting. I wil be very curious to see what you did to eliminate the clipping problem.
                                     The Slab Bacon

Frank- i thought you nailed the distortion problems in the SX101 with the change to the alternate first audio tube and adding a cathode resistor to the first stage, along with a better output xfrmr. i still have not done your mods to mine due to being a bit backed up on the bench with other projects. i want to hash this subject around with you some more on the air this week if we meet up. i like the receiver a lot though, and would like it a lot more with clean audio coming out. right now, although deyellified it sounds a bit like a Comstat 25 on crack. i think 3JN has the later one with product detector though, which is quite different than ours. have fun recaping it John- those dripping bypass caps make a nice greasy pile on the bench after you cut them out....i oughta degrease mine and put them on Epay. "oozing genuine Hallicrafters capacitor oil for that warm pleasing sound and smooth yet subtly bright mid highs"

Slab/Chris - the primary source of clipping distortion comes from the detector.  Remove the .047 cap to ground at the junction of the 180K resistor (from the det) and 1 Meg resistor that goes to the ANL diode.  This will resolve MOST of it.  You can put a bit more resistance in SERIES from the det diode to the R/C filtering network.

THen you need to re-route the audio so it doesn't go thru the damn product detector.  If you trace out the skizmatic you'll see that the audio from the AM det goes to one of the 6BY6 screens thru a .01/220K? R/C series network.  If you pull the 6BY6 prod det the audio goes AWAY - which is ridiculous.

I'll show you when I'm over there for your pharty... and I'll write this up as the clipping distortion pertains to many other Hallicrafters radios as they share ANL circuitry.

73 John

OK-Done
My SX-101A was so ratty it had to have the dishwasher treatment. That being done (Kill or cure) and a good set of tubes installed it seems to be working OK. I notice all of the problems you guys have been pointing out. Has anyone published all of the mods for getting decent AM out of this thing?
Keith
WA1HZK

Keith - I took the thing outta the cabinet, took the pictures I needed, put the radio back together, then when I tried to download the pics onto my computer the disk was bad  :evil:

I gotta take it apart again but now the bench space is taken by an SX-42.  Sometime *real soon now* I promise.

73 John

Pictures are nice but not really necessary. I crawl into the guts of these beasts all the time so if you can do the write up of component and ckt. changes listing the theory of your mods & what you used to modify the ckt./part, that would be a good start. In the mean time there seem to be 4,724 "black beauties" leaking electrons all over the place. I'll start there & get the stock rig re-capped.
Keith

Keith, the mod involves cutting a wire off one of the terminal strips near the front of the radio and moving it to one of the contacts on the response switch.  You need the photo to identify which contact on the switch.  If you've ever tried to trace out what's going on in that switch you'll understand.

Hallicrafters for SOME stupid reason ran the envelope-detected audio thru the 6BY6 product detector.  This mod bypasses that arrangement and feeds the audio directly into the vol control thru the switch (when in AM mode).

Other mods include removing the .047 cap in the ANL circuit, removing a shunt cap in the grid of the first audio which cuts HF response, and you can fatten up the coupling caps thru the audio stages for a bit better bass if you want.

73 John

I'll be waiting for your photos.
When you publish there will be a flurry of activity.
Keith

I wonder if the SX-101A mods will get posted soon?
A lot of us waiting.
Keith

I'm sorry, Keith, been busy with other stuff.  I *think* I have the correct photos.  If not, I just scored another SX-101A at Dayton which will receive "the business" so I can post before/after pics.

I'll try and get it done this weekend.  PM me your email so I can send you the preliminary writeup.

PM Crapped Out? Got some error message

Just send it to
wa1hzk@comcast.net
Thanks
Keith

I got it, Keith... it gives an error msg but DOES send the mail.

And if I had half my faculties I would have noticed that your email addy is in your signature.

73 John

John,

Forgive my jumping in here, buit I would strongly suggest you not perfom the  planned cutting of the leads to/around the Response (function) switch before considering the following:

The product detector is used as a low gain audio amp in AM.  This was, no doubt, an attempt to "level" the audio volume difference between CW-SSB and AM reception.  As to your ANL distortion, first check all the high valkue carbon comps resistotrs that make up the biasing circuitry for the diodes in the ANL and detector circuitry.  It is critical that these be at the right bias for the ANL to work correctly. It is also critical that the AGC be working correctly and providing the right signal level out of the detector.  IF the AGc is letting too high a level get through, ANL clipping will occur with even correctly biased diodes.  Some other radios simply use a pair of diodes to provide simple clipping by shnuting excess signal spikes to ground.  Real crude. IN this circuit, however, the circuit is supposed to have the diodes conducting in normal circumstances, with the audio going throuh the diode. Noise spikes are supposed to cut off the signal and produce a "hole" in the signal, like an audio blanker.  incorrect bias or signal level (AGC problem) can make modulation peaks do the same.  A simple way to check if the AGC is contributing to the problem is to turn off the AGC and use the RF gain. Just cut back the signal and see if at a slightly lower signal level, the distortion clears up.  

As to the IF, this family of Hallicrafters use switchable levels of over coupliing to change the bandwidth  of the 50 KHz IF.  AT the narrowest, it is tuned to 50 KHz. The wider settings spread this bandwidth to one sideband.  The radio is perhaps the only one that provides direct upper or lower sideband selection in AM.  When adjusted correctly, the 5 khz bandwidth will provide 5 KHz of AM audio bandwidth. One has to just get used to tuning in just the one sideband.  The response switch providing Upper or Lower sideband for AM is actually unnecessary and probably confusing.  In either position, as long as the carrier is in the passband, you will tune in the signal, and be able to select either sideband, or both , simply by tuning it in. When correctly tuned in to a DSB AM signal, you will be able to change the switch between USB AM and LSB AM and it should sound the same.  

Another simple check is to tune in the claibrator with the antenna disconnected. Note the dial marking and observe the range of reception as you try the different bandwidths settings, as well as USB and LSB AM.  At the 5KHz setting, you should see a strong signal as you tune up to 5 KHz to the side of the calibrate frequency. It iwll be reveres fot he opposite sideband.

But, in anycae, don't stagger tune the IF cans!  Make sure the switch contacts are clean with deoxit.

Rob, thanks for the note.  My motivation here was to get the most bang in terms of good audio for the least amount of modification.  And those mods must be easily reversible, without wholesale rewiring and replacement of components.

I'm very familiar with the Hallicrafters 50KHz IF system and its use, having owned an SX-101A for 25 years, several other SX-101s, an SX-88 and perhaps a dozen or so SX-100s and SX-96s over the years.

The mods I did were finalized after perhaps 30 hours of fiddling around and experimenting using modern professional-grade test equipment.

I note the following:

1)  Cutting the .047 ANL filter cap out is an EASY and REVERSIBLE mod that increases modulation acceptance to almost 100% without distortion as opposed to the stock 70-80%.  This is probably the easiest and most effective mod you can make on these radios; the fuzz (actually sounds like IMD) disappears like magic at the expense, as I noted, of some loss of effectiveness of the ANL, which isn't all that effective to begin with.  Not too many people use ANLs anymore as with electronic ignition systems the impulse noise that ANLs were meant to correct isn't too common.  It does clip static crashes, etc., to some extent but I'd rather listen to clean audio.  

2)  I did a lot of experimentation with different R/C networks in the AVC circuit and none corrected the modulation acceptance problem nor did they make the AVC more effective.  I just went back to the stock AVC values.

3)  The ANL does not create "holes" in the audio but rather clips the (negative) peaks as it is a series ANL.  See http://n2.net/k6sti/avc.htm and http://n2.net/k6sti/ma.htm  One could probably increase resistance values at the expense of recovered audio level; my efforts toward this end in an SX-100 were not nearly as effective as just removing the ANL filter cap and left the radio with too low recovered audio.

4)  Although the label on the switch reads "5 KC", sweeping the IF shows that 3KC is more like reality.  5KC is, IIRC, 10-15 dB down.  Stagger tuning the IF (top and bottom cores of each xformer a half turn either way off max at narrowest BW) doesn't affect the narrow BWs too much and gives a somewhat wider BW.  I'd like a bit more, but it's OK as it is.  Again, this is an easy and reversible mod if the user thinks the price (broader skirts at lower BWs) is too high a price to pay.  One thing I didn't try, and I perhaps will with the 101A that will soon be on the bench is adding some swamping resistance across the IF cans in the 5KC position to widen out the passband in ONLY that switch position.

5)  Using the 6BY6 product detector as an audio preamp is a poor design choice, as the component values in that circuit are not conducive to low-distortion, wide BW audio.  Simply bypassing that stage allows much better audio range with only a few dB difference in audio between SSB/CW and AM positions.  Again, this is an EASILY reversible mod (moving one wire and cutting a jumper on the switch).  

Rob, I welcome any further comments you have, and I'm willing to try suggestions because, as I noted, I'll have another one of these beasts on the bench soon.

73 John

Anything cooking on this project yet?
Keith

Keith, I ran into a bit of trouble on this... First, I had to do quite a bit of work getting my latest SX-101A working... then I did the mod (as I could remember it, I couldn't find my written notes) on the latest SX-101A and the SSB audio is dead, so I think I connected to the wrong terminals on the mode swtich... I gotta analyze the switch AGAIN

Sorry - I'll get this out as soon as I can, Keith.

73 John

No Sweat.
Thanks
Keith

John,
I have a SX-101 and have a 1650KC Mouse House BroadCast station in my back door.  I use a pre-amp and get rid of it 80-10 but can't get it out of my receiver on 160 mtrs??? any ideas??
73's Terry N4RQ

All I can say is make sure the case is grounded to the chassis well, make sure all the chassis screws are tight, etc.  You could build up a 1650 KHz trap or LPF... as you know the SX-101A doesn't have 160 so I haven't experienced problems with the receiver in its cabinet (on the bench, however, there's low level 1650 KHz BCB interference due to the radio being out of its cabinet).

Have seen this with SX-100s as well.

73 John

Hey John, do you have those SX-101 mods ready yet?

Have these mods and pictures been posted anywhere yet? I took my SX101A apart 6 months ago and am still looking for the audio fixes.
Keith

Not yet, Keith, I apologize as I got sidetracked with other projects and it's been too damn hot lately to work on this in the garage.  I gotta trace out that stupid mode switch again as I lost the notes I made.

73 John

No sweat. I turned on the AC the last few days and cooled the rig so I could talk to brent so I started getting the bug a month early.
Keith