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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 1370 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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AM is Not A Hobby - It's a "Way of Life"!
Timmy, Sometime in 2007 on a Mountain Far Away..
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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 35 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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AM is Not A Hobby - It's a "Way of Life"!
Timmy, Sometime in 2007 on a Mountain Far Away..
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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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AM is Not A Hobby - It's a "Way of Life"!
Timmy, Sometime in 2007 on a Mountain Far Away..
www.criticalradio.com
www.criticalbattery.com
www.criticaltowers.com
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Official Registered "Old Buzzard"
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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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