Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
SX-28A- Sensitivity - AF Question




 
The AM Forum
April 30, 2017, 02:38:28 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: SX-28A- Sensitivity - AF Question  (Read 631 times)
0 Members and 1 Guest are viewing this topic.
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1417



WWW
« on: March 20, 2017, 04:26:23 PM »

So I have spent hours on my SX-28 acquired 6 weeks ago and ALL caps and out of spec resistors have been replaced. It works very well on the first 3 bands, up to about 12 MHZ but I as I move past there not only do signals get weaker but the audio goes down. In fact there seems to be little- no background noise.

For example, on the BC band, stations are loud, and I can them across the band. If I go up to 20 meters, the speaker is pretty much dead. If I turn on the BFO  I can locate several signals but even cranking AF gain up they are not very loud. But there is little, or no background noise. Almost like there is no IF gain at the higher frequencies.

I'm used to the older rcvrs being less sensitive on the higher bands, but still having background noise.
Was the internal design of this particular receiver so good that there was little background noise, or is something else going on?

Any thoughts, or is this the nature of this beast and current band conditions?
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
Jim, W5JO
Member

Offline Offline

Posts: 2390


« Reply #1 on: March 20, 2017, 04:31:59 PM »

What does your sig gen say?  Currently the upper bands are terrible.  No sunspots and none forecast.
Logged
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1417



WWW
« Reply #2 on: March 20, 2017, 04:36:38 PM »

Hi Jim,
Seemed to work fine with the gen during all alignment. I don't have an output meter, but I know on all bands I had to back off the amount of RF level from the gen substantially as I went through the steps. In fact it was pretty hot compared to some I have worked on.
I know the bands have been funky, what I find unusual is the lack of floor noise.
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
Jim, W5JO
Member

Offline Offline

Posts: 2390


« Reply #3 on: March 20, 2017, 05:17:40 PM »

Does your gen have calibrated output?  Can you modulate it with 400 Cy or 1KC?  If so, you can attach a AC voltmeter to the speaker terminals and get an idea of how it performs Carl.  I have a Simpson 260 that I use for that purpose along with a WV 98 that I can use.

I forget the specs. on that receiver but it does lose sensitivity as the frequency increases.  I have an NC 183D that is in good condition and I tuned up from 20 to 30 meg after I posted and the noise floor was very low except for the line noise that I have in places across that frequency spread.  Also if you have an antenna mismatch that can affect the receiver up high. 

I assume you tied one side to the balanced input to ground at the antenna terminals, and if I am right, balanced impedance is around 2-300 ohms.  Your gen is probably 50 ohms and that does make a difference at that frequency.

I assume you enjoyed working in the front end compartments.  Isn't that just fun that everyone should experience?
Logged
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1417



WWW
« Reply #4 on: March 20, 2017, 08:33:13 PM »

Hi Jim,

The output is not calibrated. I use the "RF Output Pot Set" method of calibration Smiley.  It does have a modulated output. Its a simple Leader Generator that usually is good enuff for what I do. I use the Flex as a means to set the freq accurately.

I also used my Simpson 260, and, as called for in the procedure, a 400 ohm resistor across the ant input as a dummy load.

I'm listening to 75 right now and the AM sigs are pounding in. So I think I will leave well enough alone. I'm going to pair it up with the HT-9 and use it on 160 so I'm probably worrying about something that will not be an issue.

That front end was a long tedious process. I did one section a day. What drove me crazy was I am a firm believer in just turning the rig on after doing a few parts in a section of a radio to make sure I did no harm. That was not possible in that project.

The worst part was I had to open an IF can because an internal resistor was way off. First off, nowhere does Hallicrafters tell you where the parts are located, they don't even show a  line around the IF can to show what is in there. In the schematic it shows up near the AVC tube. Someone on another board knew where it was. I thnk it was R-62. 

Because this is a military version all wires are tightly bundled and the entire thing coated in varnish. So all color coating is lost in that process, making tracing by sight impossible. In addition, a long terminal strip goes directly over one of the IF can nuts. Because of the varnish I had to get a good solid purchase on the nut, so all I could do was break the strip. Because the leads all turned brown varnish and went into bundles I had to snip the leads at the can and then reverse the process. UGH.

I'm looking for several small knobs and one RF coil slug, so if you come across a junker, please let me know.

Thank for your help
73
Carl 

Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
Jim, W5JO
Member

Offline Offline

Posts: 2390


« Reply #5 on: March 20, 2017, 10:47:32 PM »

I went through one of those back some 20 years ago and share your grief.  If the signals are very good on all but the highest band, I would say everything is hunky dory.  That says the IF section and the low band are good.  You will know more when the upper frequencies work better.  Right now is not a good time for them.

 Mine was not the military version, I quit working on military gear that had the fungicide back when I was a Conditional, circa 1959 or so and bought some surplus with that stuff.  It is almost impossible to get off.

Even though, those old cloth cover wires lose their color over the years.  I had an SP 600 that had not been touched and suffered from the lack of color on the wires to the RF deck.  One thing about that SX 28 is a sense of accomplishment when you finish and it works.  Congrats on a fine receiver and now you have something in common with Harry Truman.  

Hope to hear you on the HT 9 come next winter on 160.  If I see a parts radio I will let you know.

Logged
KB1VWC
Contributing
Member
*
Offline Offline

Posts: 7


« Reply #6 on: March 21, 2017, 06:24:59 AM »

  Carl,

  Check the 6SA7 mixer and Oscillator tubes. If you have a high impedance probe, check the LO injection level. They may test ok in a tube tester, but some will have better oscillator output as well as mixer performance. In mine I substituted a 6SB7Y in for the 6SA7 mixer. It's a direct drop in replacement. The performance on the 10 meter band was dramatically different. The MDS went from about -125dbm to -135-137 or so. You will have to re-align the mixer stage afterwards, but it is well worth the substitution. It actually performs quite well on 10 meters, considering it's age.

  Steve
KB1VWC

   
Logged
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1417



WWW
« Reply #7 on: March 23, 2017, 04:57:07 PM »

I checked the resistance on the osc and mixer and found that 3 pins (tied together) were off by a significant factor. Unfortunately to get to the resistors I had to get under the osc trimming caps and coils. Luckily I did not have to pull everything apart this time but it was a job. A 6.8K resistor had drifted to 55K. Interesting the 10 ohm one was right on. With my big paws it was a chore but I got it replaced.

Interestingly, I found a wire in that compartment that was pinned between a bracket and the top of the chassis. See attached picture.  It must have been that way since assembly I was going to loosen the bracket and move the wire, but the screw heads are hidden under a sub chassis. So I would have had to pull the whole osc section apart.
Considered replacing the wire but have been burnt enough times by just making one more little improvement and given that tiny working space I decided that it is not subject to stress or movement and has been that way for 75 years, so I would leave well enough alone. Surprised this got through QC.

I will try the 6SB7Y when I get one, probably at Nearfest.

Net effect, some improvement on the two higher bands, but not great. Not really an issue as I don't roam up there much. However 40 meters sounded much better so you sure nailed an issue.
I'm done, I've got about 5 weeks into this project and its working very well for my purposes. I'm going to pair it with the HT-9 on 160

Thanks for the help


* 20170321_172515.jpg (69.09 KB, 302x403 - viewed 53 times.)
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
KB5MD
Contributing
Member
*
Offline Offline

Posts: 527


« Reply #8 on: March 23, 2017, 06:57:19 PM »

I recapped ONE sx-28....make that ONE sx-28...never again.  There are definitely easier receivers to work on.
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 6580



WWW
« Reply #9 on: March 24, 2017, 08:51:05 PM »

For me, it's one beautiful and great-sounding receiver and I'd recap it again if I had to. Mine has always been poor on the highest frequencies. Maybe I should look into the good advice here.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
K4RT
Contributing
Member
*
Offline Offline

Posts: 417



WWW
« Reply #10 on: March 24, 2017, 09:09:48 PM »

Carl,

Nice meeting you at Frostfest in Richmond.  I take it that this is the SX-28 that followed you home from Richmond.

73,
Brad
Logged

Brad K4RT
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1417



WWW
« Reply #11 on: March 24, 2017, 09:15:11 PM »

Hi Brad,
That is the one. Ithe did work, and the guy had done a lot of work, but it sure took a lot more then I expected.
Now you need to come up to Nearfest.
By then the smoke smell should be out of the rig Smiley


Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
KB1VWC
Contributing
Member
*
Offline Offline

Posts: 7


« Reply #12 on: March 24, 2017, 10:53:10 PM »

  Carl,

  Check the Rf Gain pot as well. I have found on more than one receiver that the RF gain pots never actually get to 0 ohms. Even though they are at the stop, they may measure out at 60 to 100 ohms. Try shorting out the wiper arm to gnd and see if the sensitivity goes up. If so you need to open up the pot and " blueprint" the rf gain pot. Make it really make 0 ohms at full stop.

    Good luck. They really are a great receiver, especially once fully restored.

  Steve

  KB1VWC
Logged
K4RT
Contributing
Member
*
Offline Offline

Posts: 417



WWW
« Reply #13 on: March 24, 2017, 11:18:44 PM »


By then the smoke smell should be out of the rig Smiley


Carl I still get a chuckle out of that!

Planning to get active on 160M AM soon, so maybe I'll run into you on air.  NearFest is on my must-do list but it won't be this year.
Logged

Brad K4RT
Carl WA1KPD
Contributing
Member
*
Offline Offline

Posts: 1417



WWW
« Reply #14 on: March 25, 2017, 10:52:01 AM »

  Carl,

  Check the Rf Gain pot as well.
  Steve

  KB1VWC
Steve,
That is an excellent idea. In all the receivers I have done I have thought to check that, but it makes perfect sense.

This is the second one I have owned, sold the other one about 10 years ago and have regretted it. So when this came up at a price that I could justify, I grabbed it.
Logged

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
w1zb
Member

Offline Offline

Posts: 18


« Reply #15 on: March 27, 2017, 02:47:41 PM »

My SX-28's performance on 10M is a little down from the lower bands but still very usable. Several times in the past I worked into VK land on 10M AM with just my Johnson Ranger, the SX-28 and a beam. I found that a preamp wasn't required on 10M as it amplified both the signal and the noise, so no improvement in reception.
73 Jerry W1ZB
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.055 seconds with 19 queries.