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Hallicrafters SX-101A Mods for better AM




 
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Author Topic: Hallicrafters SX-101A Mods for better AM  (Read 4771 times)
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WA1HZK
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« on: March 05, 2019, 01:27:22 PM »

I just finished capping and repairing my SX-101A. Nice rcvr. but seems to have been designed with mostly SSB in mind. When tuning AM you can select one sideband or the other to listen to. I figured I should ask if anyone has made any improvements in this rig to open up the bandwidth a bit for better AM? Maybe there's a trick I should add before I horse this beast back into it's cabinet and move onto the next one.
Keith
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MikeKE0ZUinkcmo
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« Reply #1 on: March 06, 2019, 07:05:46 AM »

I agree the "pick a sideband" approach is certainly not a Puritan's idea of REAL AM, however, after using my 101 for some time I grew to like the "one side or the other" approach, in that, if the radio is properly aligned, you use the BFO to zero beat the signal on one sideband, switching to the other sideband can really help with certain interference issues.  Further, 6Kc really is plenty of bandwidth if the receiver is properly tuned.  

The 101 does have a distorted audio issue due to the noise limiter implementation.   Using a DPDT in place of the original SPST unit will improve/fix the issue.    This is not my idea, I found it years ago somewhere but can't remember who's idea it was, so I can't give credit where credit is due.



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WA1HZK
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« Reply #2 on: March 06, 2019, 09:57:34 AM »

Thanks
I agree with what you said and I did notice that distortion issue. I'll get out the clip leads and see what works.
What I did notice was the quality of the components in the RF section and the fact the osc. tube remains lit and there is a small electric heater unit that runs even if the power switch is turned off.
Keith
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« Reply #3 on: March 07, 2019, 12:12:32 AM »

Quote
... the osc. tube remains lit and there is a small electric heater...
Guess they just wanted a quick fix, rather than work on improving the oscillator design.
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« Reply #4 on: March 14, 2019, 10:57:25 AM »

I must admit, after listening to 75 in my office for the last couple of weeks, I really like this receiver. It's dead nuts stable for SSB but my use is AM and the 5KC bandwidth into a decent speaker is giving me good sound. I turn down the volume on 3885 and flip on the ANL switch. Then it sounds like a day at the beach with even with S9  background noise. When I hear someone talking I reach over and flip off the ANL and crank the audio up. The AVC seems to work just fine and signal strengths appear normal. So, now to finish the HT37 mods. I had that done to the point of wanting to connect a microphone. I plugged the mic into the 528 processor and...... nothing. Someone had removed the preamp chip. There will be a slight delay in the story until the chip arrives from China.
Keith
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« Reply #5 on: March 15, 2019, 06:35:34 PM »

I had some luck with an SX-101 Mark-I years ago with stagger tuning the low frequency IF cans. I was going to play with the values of the coupling caps between the cans too, but never got around to it. In the audio department a much larger AF output transformer and inverse feedback helped a lot.
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MikeKE0ZUinkcmo
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« Reply #6 on: March 16, 2019, 07:17:14 PM »

Quote
...I really like this receiver...
I agree,it is an excellent receiver. I bought a straight 101 because it had 160.   Never converted to the "A" product detector, again because I use it primarily for AM and what little sideband work I do, the receiver is adequate.
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« Reply #7 on: March 17, 2019, 08:48:47 AM »

I have an SX-101 MKIII, the last in the line before the "A" series with the product detector and VHF dial scale instead of 160 Meters. The audio was slightly distorted in AM mode. I disabled the noise limiter (otherwise known as distortion multiplier), which made a slight improvement. Then I happened upon K6STI's article in Electric Radio #171, "Improving Modulation Acceptance".  He discusses the components in the detector load circuitry of typical receivers.  Long story short, he recommends decreasing the value of the resistor at the bottom of the string from cathode to ground by a significant factor. In my receiver, I identified R53 (270K) as the problem child, so I tacked another 270K resistor in parallel with it, to decrease the resistance value to half, or 135K. The difference was dramatic! Now AM reception on my receiver is clean, a big payoff for a simple mod that's easily reversible if desired. I'm not familiar with the detector circuitry on the "A" version of the SX-101, but for the older receivers, this is worth a try.
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« Reply #8 on: April 04, 2019, 04:10:19 PM »

OK
There's an issue that's come up and before I spend hours looking at the schematic I figured I would ask if anyone else has come across this issue.
No Upper Sideband at all in CW, SSB or AM? Anyone got any suggestions?
Keith WA1HZK

PS, got the HT37 transmitter all done and that came out nice.
Now it's got a Low Z input for the 528 and the audio chain was all cleaned up.
Got to get these two together.
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« Reply #9 on: April 04, 2019, 05:44:53 PM »

Hmmmm
I guess it's T5, bottom slug.
Beast is going to need to come outta the cabinet. Brass screws.
This weekend.....
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« Reply #10 on: April 04, 2019, 10:43:20 PM »

Yep one or the other, or, the crystal itself.   Just take the extra effort to make the two sideband responses equal.
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« Reply #11 on: April 06, 2019, 01:12:31 AM »

Here is one simple fix for reducing distortion in the 6SC7/6K6-GT combo:

Run a 3.3 Meg resistor from Pin 3 of the 6K6 to Pin 5 of the 6SC7, or

use the schematic below for more extensive updates.


Phil - AC0OB  

* SX-101A Linearized Audio PreAmp.pdf (105.81 KB - downloaded 198 times.)
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WA1HZK
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« Reply #12 on: April 06, 2019, 02:54:15 PM »

OK
I got lucky. The missing upper sideband was just an alignment issue. I did all the other audio mods & added the feedback while I was in there. It sounds pretty good now. Station is now just needing a set of cables made up and I'll fire it up later today or tomorrow.
Thanks
Keith
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« Reply #13 on: April 08, 2019, 01:05:19 PM »

Interfaced with HT37 so will be testing with W1IA around 3:00.
I need a close station that knows what I sound like so I can adjust the 528 processor.
PS, demonstrated the new sound of the RX with the "Linear" mods. That's a big thumbs up!
Next Project, my R-390....... Arrrrrrr!
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« Reply #14 on: April 08, 2019, 04:31:55 PM »

Glad it worked out for you.

My SX-108 has a similar audio circuit and it sounds great! The 6SC7 and the 6K6 make a great audio combo.

BTW, if you ever have an intermittent signal loss where the signal dips after warmup and then comes back up, replace C20 and C42. Those EL-Menco mica caps are nothing but trouble.

Phil - AC0OB
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W3KW
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« Reply #15 on: September 23, 2021, 08:46:18 PM »

GE gents-

I need a bit of direction. My SX-101A has awful distortion in the audio and I'd like to use at least some of the mods listed above to clean it up. I'm a layman, and the full linearized preamp mod seems a bit complicated. I will knock out the noise limiter mod to lift C81 when the NL is off, and add the 3.3 meg resistor between pin 3 of the 6K6 and Pin 5 of the 6SC7.

The linearized Audio Preamp schematic above looks daunting. Does anyone have any photos of the changes that they can share? Maybe it's easier than it looks?

I'm about to open her up and do surgery. New caps, repair an inoperative remote standby on the accessory plug, and complete at least the easiest of the audio mods above. If I get some good intel and direction, I may attempt to go a bit further.

My degree in EE is from YouTube, QRZ forum, AmFone forum, building Heathkits with my Dad as a young man, and by hands-on restoration of HX-11's, DX-40's, DX-60's, DX-100's, and others.

73 and thanks. Wes W3KW
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« Reply #16 on: September 24, 2021, 01:32:12 PM »

My SX-101A has awful distortion in the audio and I'd like to use at least some of the mods listed above to clean it up. I'm a layman, and the full linearized preamp mod seems a bit complicated. I will knock out the noise limiter mod to lift C81 when the NL is off, and add the 3.3 meg resistor between pin 3 of the 6K6 and Pin 5 of the 6SC7.

Wes,

   I would not assume that the stock audio design is causing all the distortion that you are hearing. Heck, the IF alignment could be all jacked up as well.

I would review the basics first. here are some that come to mind:

Are the tubes good?
Are the resistors in the audio area in tolerance?
Are the capacitors leaking? The audio coupling Cap to the 6K6 grid would be most suspect.
Are the power supply electrolytics in good shape? Any original electrolytic should be defined as bad.

What about following a voltage chart on the audio stages?

FWIW, I just went thru a Hammerlund HQ-110 restoration. Once the IF cans were swept and aligned, the audio was pretty fair from 200hz to 3200hz. Below 200hz, it was awful, and it turned out to be the AVC was getting modulated by the lower audio frequencies, and that modulation was grid modulating the two AVC gain controlled stages. Before I realized that, I added a beefier audio output transformer, and a NFB loop. Lesson learned: I jumped the gun with modifications, and did not take the time to figure out what was going on. Also, the power supply as designed had hum in the speaker all the time. That was a simple fix...BUT in stock form, it was fine, 200hz to 3200hz.

Jim
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W3KW
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« Reply #17 on: September 25, 2021, 07:39:08 AM »

Jim-

Thanks for the advice. I will first do a substitution test on the tubes. Second, though the audio is dead silent of AC hum, I will be replacing all the caps. Third, I will check all the resistor values. Something I always do anyway. I usually test the rig periodically as I complete work to make sure I havenít caused new issues (read made a mistake). I think the external mute will be an easy fix. The accessory plug was difficult to insert so likely a mechanical issue there. Plus, thatís a simple circuit component (likely). The voltage chart for the audio section is a great idea. I assumed since Iím not the only one that dislikes the audio that all the SX-101ís were in the same boat by design. Bad assumption.

Iíll update the thread when I have results to post or questions. One issue bothering me is dial drag. The top of the needle drags behind the bottom. Is that a difficult repair? I have to say, Iím using this receiver because Howard Mills has my 75A-4 in for a tune-up and Iíve really come to like this rig. Itís super heavy, and gets very hot, but feels like a high quality design and build.

Anyway, thanks. Also, internal pics always welcome if anyone has any.

73 Wes W3KW

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« Reply #18 on: September 28, 2021, 07:33:37 PM »

Watch out for that master oscillator filament onóall-the-time feature.  Not a good thing for tube longevity.  A couple of years ago I acquired an SX-101 MK III that exhibited a very unusual frequency instability - it would slew up and down about 100 Hz at about a 1 Hz slew rate.  Oddly, the magnitude of the instability was greatest on 160 meters and wasn't even noticeable at all on ten meters.  Swapped in another 12BY7 master oscillator tube and problem was eliminated. 
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