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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 10:14 pm 
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Roger,

I disconnected the antenna and then grounded the antenna input so I think it's pointing to a receiver problem. What I didn't do is turn off a lot of the equipment I have in my shop that powers up when I turn on the lights. I'll do that tonight and report back.

It seems odd that it is consistent on the 40//80/160 meter bands and is completely gone on the upper frequency bands. I briefly looked at the schematics and I don't see anything (yet) that might point to something specifically common to these three bands.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 11:56 pm 
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PROBLEM SOLVED! (But still need advice :) )


When I turned off the lights and all the test equipment in my shop the snap/crackle/pop mostly went away.

In fact, just the act of moving my hands too close to the RF section around the band selector alters the noise.

Perhaps I missed something from alignment 101, but should I be doing the band alignment while the unit is still in the case with some shielding?

Also, the original SX-101 only has a two pronged plug, should I replace it with a three prong plug so there's a power line ground?

Just the act of connecting the ground lead of my Fluke 45 bench DMM to the low (ground) side of the speaker terminals made it worse. I have a lot of florescent lights and other test equipment in my shop that turns on when the lights turn on. Should I do the alignment with only the minimal complement of equipment and incandescant lights?


Last edited by wmdino on Sep Tue 28, 2021 3:49 am, edited 1 time in total.

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Tue 28, 2021 3:01 am 
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I install a three wire cord in almost every piece of gear that go across my bench. It is a good time to make sure that the radio is properly fused and wired correctly (load/black/hot side of line goes to fuse and then the AC power switch, neutral should NEVER be broken by a switch or fuse).

A good RF ground is also necessary to help reduce interference in the set, the line bypass caps are part of the noise reduction network to help keep noise from other devices coming in via the power line but unless the chassis is at RF ground potential, they do little to no good and the same is true for the shielding properties of the case. Your power line ground often is not a good RF ground, especially at higher frequencies where a long length of wire to the ground point has high inductive reactance.

In general, if possible the receiver should be in its case for the front end alignment because the case does provide stray capacitance and an alignment done with the case off won't be perfect once the case is on. Many radios provide access holes for alignment, some (i.e. many National radios) want you to put a metal sheet near the adjustment points to simulate the case during alignment. This is mostly an issue for front end alignment, shielded IF transformers won't be impacted by the presence or absence of the case.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Tue 28, 2021 3:52 am 
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I took a look at the underside of the case and it in fact does have multiple holes drilled.

I'll add the three wire cord and do the other checks you suggested.

I think I am on the home stretch with this and hope to wrap it up when the 12BY7 arrives.

Thanks for all of your help!

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Tue 28, 2021 3:56 am 
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Rodger's description of the sideband switching is one of the reasons I really like the 101.

Glad to hear the tube subs made an improvement in your sensitivity, but keep in mind that on the lower bands the atmospheric noise is usually greater than a couple Micro Volts in city environments.

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Tue 28, 2021 2:32 pm 
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Thanks for the graph Mike.

I am in a rural area, but the the peak is rather interesting!

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Tue 28, 2021 3:03 pm 
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Bill,

I am in a rural area also (farm country, nearest neighbors about a mile away). But it still takes some work to keep things RF clean. I have had to filter some devices in the house, before I dumped Direct TV I had to do serious work with their receivers. I put in underground wiring from the power distribution pole to my house and the various outbuildings and I use a mains surge suppressor that also provides RF filtering at the main distribution point which comes out of the automatic transfer switch for my 40KW standby generator. Each of the structures has its own breaker panel which is equipped with the same surge suppressors/RF bypass units.

My noise level is typical around S-4 on 160, S-3 on 80, dropping to around S1 on 40 and up. Even in a quiet area, .25uv sensitivity is of marginal use on 20 meters and only becomes truly useful on 15 and 10. My FTDX-101MP specifies at least .16uv across the HF range increasing to .12 for 6 meters but realistically that is going to be noise limited almost all of the time even at the higher end of the range.

Several years ago I ran into an issue with an intermittent rough powerline like noise peaking on 3850 Khz. that would show up only sporadically for short periods of time. I tried various solutions because the source was located some distance away and propagated on the HV lines. I used a loop for 75 meter receive operation which was very effective at knocking out that noise.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Wed 29, 2021 3:37 am 
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I'm learning a lot about spurious noise in these frequency bands that I was never aware of. My back ground as a retired EE had little to nothing to do with RF. This has been a great learning experience and I appreciate all the help.

Today, while installing a three-prong cord and wiring the 12BY7 filament transformer to the switched side of the power line I found the power line neutral attachment on the terminal lug strip was a cold solder joint. It was so "cold" that I could actually rotate the wire around the lug. I would guess that probably didn't help my noise problems. I'll know more tomorrow when I finish it all up.

I rewired the power cord so the hot lead comes into the fuse, then from the fuse to the switch, then back to the hot side of both transformer primaries. Both neutral lines tie together on a new terminal lug I installed right next to where the power line comes in. The new terminal strip also includes the power line ground. Before the changeover, I really wouldn't consider the design to have followed any modern standard.

Working on these old units requires a keen eye I guess.

I would imagine with more and more electronics (with little to no shielding) showing up in a modern house that it becomes increasingly difficult to filter out all the noise on all bands. As everyone has pointed out, the lower frequencies seem much more susceptible. I should have a new 12BY7 tube in before the end of the week and I'll wrap the alignment all up (with the case on!) and report back. Thanks again for all your help.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Thu 30, 2021 9:50 pm 
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Update! I replaced the 12BY7 and the everything has improved greatly! Thank you again for all your help.

I would like to go through the alignment procedure again now that everything seems to be working.

I did the 50.5 kHz alignment, no problem.

While reading the alignment for the 1650 kHz IF, the instructions say to "tune through the passband and observe the shape of the frequency response. If it is symmetrical, the adjustment is complete; if not, reset the generator frequency to the center of the passband and repeak T1 and T2.

Two questions: First, do I peak the top and bottom slugs of T1 and T2? (It's not clear to me because part of that alignment paragraph discusses what to do if the osc. drops out.)

Second, the center of the passband is 1.6496 kHz. Is that close enough, shouldn't it be centered around 1.650 kHz?

Third (sorry, I said two), isn't it always going to be symmetrical around some center frequency, in my case 1.6496 MHz?

The whole "center of the passband" and its relationship to 1.650 MHz has me a bit unsure.

Thanks,
- Bill

Thanks,
- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Fri 01, 2021 1:06 am 
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Bill,

I am glad the 12BY7 replacement helped.

I am going to throw you a bit of a curve ball since you are going back in to do a careful alignment. Before you go any further, check to make sure that the 1600 and 1700 khz. crystal controlled oscillators are on their stated frequencies. The design behind this selectable sideband receiver has the final IF aligned at the absolute frequency difference between the 1600 and 1700 khz oscillators PLUS .5 Khz (this puts the "carrier" frequency slightly down the slope of the IF response for the desired recovered frequency response), they changed to PLUS .75 Khz. in later sets but the difference in performance is negligible.

HOWEVER, for proper performance the actual final IF frequency must be at 1/2 the absolute difference between these two oscillators plus the .5 (or .75) khz. offset for proper selectable sideband operation. If one or both of the crystals have drifted, your choices are to get an expensive custom replacement crystal or calculate a new target final IF frequency. Say for example the 1600 Khz. oscillator is exactly on frequency but the 1700 is now on 1701 Khz. The absolute difference is now 101 Khz, divided by 2 your new target alignment frequency for the final IF is (101/2) plus the desired .5 Khz offset or 51 Khz. instead of 50.5 Khz. NOTE that your BFO frequency with the pitch set to zero should also be equal to the correct absolute difference/2 between those two oscillators (50.5 instead of 50 Khz. using the example of the 1700 Khz. oscillator having drifted 1 Khz. high).

If you are lucky, both oscillators have stayed where they should but check them. The adjustment for crystal activity (T9) will slightly pull the frequency. I adjust the slugs of T9 for a peak and then go slightly past the peak on the side of peak where the output drops slowly and this provides for sure and stable operation. If you leave it set at peak, the oscillator likely won't start reliably. The second part of the T9 adjustment is to compare the overall gain in the upper versus lower sideband positions and adjust the two to match by slightly adjusting the oscillator for whichever sideband produces greater gain to produce less output and less gain so that receiver gain is equal for both sidebands.

As to the 1650 khz IF, you do align both the top and bottom slugs of both IF transformers but these two "garden variety" 1650 Khz. IF transformers provide a very broad passband so if the center frequency is off a bit when you align it or the symmetry isn't perfect around 1650 it won't have a noticeable impact. The alignment of the final IF (and setting the BFO to the correct frequency) is highly critical but the 1650 Khz. alignment is less particular.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Fri 01, 2021 4:38 am 
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Thanks Roger,

I'll go back and check the two crystal frequencies and do as you suggest. If I understand correctly, adjusting T9 can pull these frequencies in a bit if they are off? (Or should I just do the calculation you provided and peak it for that frequency--if the 1600 and/or 1700 is off?)

My take from what you said is that the very slight difference I have in the 1650 IF (1649.6) is probably good enough, is that correct?

I may have more questions on the 1700 kHz T9 adjustment but maybe things will fall into place.

At 7 MHz receiver frequency I am getting about .3 uV sensitivity at the antenna without doing anything so far, that seems pretty good and way better than anything else I've managed to obtain. I haven't checked the other bands yet since I was going back to tweak the IFs.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Fri 01, 2021 2:35 pm 
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You are welcome Bill,

I generally do a rough alignment and then check the two conversion crystal frequencies, after that I go back and do a final alignment based upon that check.

I haven't kept an accurate count of the SX-96,100,101,115,117 receivers I have worked on but a rough guess is around 1/4 of those that crossed my bench needed to have the 50 Khz. IF aligned at a slightly different frequency due to crystal drift with age.

The 1650 Khz. IF is so broad that as long as you are close it isn't going to matter. A single IF stage at 455 Khz. is going to have an 8 to 10 Khz. bandpass using typical IF transformers and the 1650 Khz. IF has a much broader bandpass.

It sounds like your receiver is working like it should.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Fri 01, 2021 3:54 pm 
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Roger,

I measured the two crystal frequencies at the cathode of V5 and got 1700.025 and 1600.026 respectively for the upper and lower sidebands. Using your formula, the actual IF should be 50.4995 rather than 50.5. However, that seems so close as to probably not make any difference so I think I can stay with what I have for the 50.5 alignment.

In for a penny, in for a pound....

I did not change several of the old paper caps, some of them (6) are tightly wrapped into the selectivity switch and there's one of those pink ones on the BFO. I am nervous about messing with the selectivity switch too much for fear of damaging the wafer switch and ultimately junking everything I've accomplished so far. The one on the BFO I guess I could change but worried about having to do a major BFO realignment. I'll take a look at the schematics and see if failure of these caps could cause any type of catastrophic failure elsewhere--unless perhaps you know from your experience?

Thanks,
- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Fri 01, 2021 4:17 pm 
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Bill,

Your crystals are still on frequency for all practical purposes so there is no need to modify the alignment frequency.

Before you do final alignment on the front end, make sure that you clean and re-lube the thrust bearing in the end of the tuning capacitor. The old lube is sure to have dried out and turned to crust by now which increases drag and frequency instability.

Tune the set to the low frequency end so that the capacitor if fully meshed, loosen the lock nut, and remove the thrust bearing holder. The bearing may come out with the holder or it may stay in the end of the capacitor shaft-make sure not to lose it. I use WD-40 to clean the remains of the old grease but any decent solvent will work. I use Mobil 1 chassis grease (same stuff I use for my tractors and the front end of my pickup) for grease and it will hold the ball bearing in place.

Sometimes multi-section variables aren't perfectly made and the most critical section is the oscillator section. Looking carefully from above with good light and a magnifier if needed, tighten the thrust bearing just enough to perfectly center the rotor plates between the stator plates of the oscillator section (this is the front most section). Then hold the thrust bearing adjustment with a screwdriver while you lock it in place with the nut. Re-check clearance because sometimes locking the bearing holder will slightly change the clearance.

For caps in delicate locations, I cut the leads and use those to attach the new cap rather than chance damaging terminals/wafers from heat or stress trying to remove the old leads which were often made mechanically secure before solder was applied.

The caps associated with the selectivity switch aren't going to cause collateral damage but they will reduce the performance of the IF if they start acting as an additional resistor in parallel with the capacitors.

Be sure to replace C92 in the BFO section and C97 through C99 in the audio section if you haven't done so.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Fri 01, 2021 4:35 pm 
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Thanks Roger, I will go through the bearing lubrication as you suggested.

I will replace C92 in the BFO section, I have not done that yet. I may role the dice on the selectivity caps or maybe do some selective (no pun intended) replacement there.

I have already replaced C97,98, and 99 so that's all set.

Since I'm into this, I also found a mod on this forum for improving the noise limiter by adding in a double-pole switch to replace S6 (around V7 at test point "B")) with a cap to ground off one of the switch contacts.


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Sun 03, 2021 12:05 am 
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Roger and Norm,

I have the SX-101 up and running. I realigned it while in the case (only needed some tweaking on the higher frequency bands).

I set up a crude outdoor antenna and I am getting a lot of traffic on the 7 and 14 MHz bands. The upper two bands and the 160M band are relatively quiet. This is all new to me and I'm not entirely sure what to expect.

I am going to start working on my ham license next.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Wed 13, 2021 9:55 pm 
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I had endless issues with my SX-101 involving the 12BY7, which stays lit all the time along with the damp chaser stick. I wasn't using it regularly and after some months, firing it up would produce signals with AC ripple and all sorts of other issues, all immediately cured by installing a new 12BY7. My impression is that leaving the 12BY7 filament lit without applying b+ and screens for long periods causes a rather obscure issue called cathode interface in the 12BY7. Some 12BY7's would even test shorted after being fine when installed 6 months earlier.

When I get to it my solution is going to have to be to just divorce the filament transformer for the 12BY7 from constant power and put it downstream of the on-off switch. Of course I then impair the drift specifications but that is cheaper than buying 12BY7's all the time. There was once an article in Ham Radio Magazine about solid stating tubes in HF transceivers and I think a 12BY7 was one of them. I may try that provided it doesn't throw the oscillator too far out of alignment.

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Oct Thu 14, 2021 3:35 am 
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Hi Geoff,

I did just that.... I put the 12BY7 filament transformer on the control side of the on/off switch. My damp chaser was not working anyway so I just removed it when I did the change to the transformer.

I am intrigued by the transistor conversion but part of this was to restoration project and I wanted to stay "in period." However, if you do find the conversion info could you please share it?

Thanks,
- Bill


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