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 Post subject: Hallicrafters SX-101 help with Sensitivity--not wrk as expec
PostPosted: Sep Sun 26, 2021 12:49 am 
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Hello all,

I am in the process of restoring an SX-101 Mark 3. I've replaced almost all of the paper caps and old electrolytics and gone through the alignment and things went well.

However, the Sensitivity control does not seem to work correctly. It drops the signal very quickly--when I go from 10 to 9 or 8 the speaker goes silent.


I traced the signal back from the sensitivity pot to the three tubes responsible. I found one resistor that should have been 180 ohms actually measure at 242 ohms. I replaced that thinking it would help but still the same. The other two resistors (100 ohm) are fine. These resistors are all decoupled with 0.02 mf disc caps. I did not check the ceramic disc caps because in my limited experience with tube equipment ceramics hardly ever fail. I checked the three tubes with a Sencor tube tester and they all test ok. I also tested the full range of the 10k sensitivity pot and it is fine.

Maybe this isn't a problem, but it just seems way too sensitive. Do others of you with SX-101 have the same experience with the sensitivity pot or is it just me?

BTW, if I do have to replace those 0.02 mf bypass caps, do you think I can replace them with mylay rather than ceramic? I have the proper sized mylar caps--I'd have to order the ceramics.

Thanks,
- Bill


Last edited by wmdino on Sep Sun 26, 2021 1:10 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 Sun 26, 2021 1:08 am 
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On my SX 101, if I am listening to a really strong station on 40 meters with the S meter popping at 50 over, and I turn down the Sensitivity, by 5, I can’t hear it at all. At seven I can hear it okay and then it jumps up by the time I am at 10. So yours may be normal. Don’t listen to noise. These are pretty quite receivers. Get a strong station and try that.

Norm


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 1:12 am 
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Thanks for the info.

I am injecting an RF signal into the antenna terminals with an HP 8640B signal generator. I'm putting in about 10 uV, not sure exactly how much of that is getting to the receiver due to impedence issues. Even when I run the signal up to 300 uV it doesn't seem to make a difference with the sensitivity.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 1:22 am 
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You should hear a signal at .25 microvolts. Certainly at 2 microvolts you should hear the signal. Your signal generator puts out the signal at 50 ohms impedance so it should match up fine. You may need to go through it again. Are you touching up the match with the antenna capacitor? I usually run through the full alignment at least twice on a receiver that has been sitting a long time or one that I don’t know the history. I find the alignment the best troubleshooting procedure. It identifies what isn’t working quite right.

Norm

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 3:37 am 
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Thanks Norm, I'm definitely not hearing a signal at 0.25 uV, perhaps a bit more than 10 uv best case and maybe 30 uV worst case. I will try to match the sig-gen output closer tomorrow and perhaps I should send the unbalanced output of the siggen directly to a PL259 rather than alligator clips on the antenna terminals.

It appears I have something else going on that isn't right. All the bands are more or less the same. I was very careful with the IF frequency alignment and that seemed to go OK. I did use a high-impedence, modern, DMM instead of the VTVM recommended when doing the alignment. Everything seemed to peak correctly with the IF frequencies. I was a little unsure when injecting the signals from the sig-gen into the test points for the two IF frequency checks. I used the lowest level possible. (The 8640 doesn't go down to 50.5 kHz so I used a later model HP synthesized sig gen for that part of the alignment only.)

For each band, there are upper and lower frequency alignments. All the upper frequency alignments require de-soldering the trimmer caps which I did not do (too worried I might damage something), so I only adjusted the lower frequency for each band. Things did not seem that far off in any case.

Any suggestions as to what the problem might be for reduced sensitivity across all bands? It would be nice to get it to 0.25 uV.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 10:15 am 
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In the alignment under “IF Sensitivity Check” there is a procedure to see if the radio is working correctly at that point. That should tell you if the problem is before or after V2.

As far as the antenna connection, make sure the jumper is in place at A2 and GND. A1 is connected directly to SO5, so no problem using the screw terminal. The audio goes through a contact on the headphone plug. I always clean that with some switch cleaner and a piece of paper I slide between the contacts a few times.

There are a couple capacitors in the “doghouse“ sub-chassis. I wouldn’t worry about those unless the tube socket pin voltages are off. That should be the next place you check if the alignment just can’t get you there. I did not unsolder any adjustments when I did mine.

Norm

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 12:07 pm 
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Hi Bill,

A few thoughts.

You say you have another generator, is the receiver's "sens" the same using the other generator? If so, equipment problems are "out". I don't think how you are connecting the generator to the receiver's front end makes ANY significant difference in the receiver's performance. They mention using a resistor in series with the generator output, but when that was written most hams didn't have anything more than some sort of "Kit" oscillator, let alone an 8640.

Did you do the sens check called out in step 4 of the alignment procedure? What were the results?

I don't see anywhere them mentioning the removal of padder caps in the alignment process. You always adjust the "C's" on the high end, and the "L's" on the low end of each band, several times until there is no further interaction between the two adjustments.
========
Another thought for low sens is the possibility of low injection level into the first mixer. I wouldn't necessarily suspect the tubes since you say the sens is basically the same on all the bands. Because the performance is "the same" on all bands I'd be looking for cap/resistor problems, but I see you've looked at the resistors. As for the bypass caps, lift one lead of whats in there and tack what you have in place and see if the condition improves. I've used MYLAR CAPS in HF receivers as RF bypasses and they worked fine.

You might also solder a jumper from the arm of the "sens" pot to ground and see if that makes any difference

=====
Norm,

Nice looking 101 you have there.

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 2:03 pm 
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Thanks Norm, I'll review the "IF sensitivity Check" to see if the problem is before or after V2

Thanks Mike,

I'll do what you suggest.

You mention: "I don't see anywhere them mentioning the removal of padder caps in the alignment process. You always adjust the "C's" on the high end, and the "L's" on the low end of each band, several times until there is no further interaction between the two adjustments." The "C's" have their adjustment screws soldered to the cans so I can't check them without clearing the solder--that makes me nervous. Also, the adjustment screws don't have slots, it seems I'd have to use a pair of needle nose pliers to rotate it.

Do you think there is any problem using a modern DMM to measure the voltages rather than a VTVM?

I had a VTVM many years ago and I recall the probe may have some type of special tip, whereas the DMM just has test leads. The DMM has very high impedence (I think, I should check).

Good to know about the mylars.

I'm a retired EE and started out with tubes as a teenager but things quickly switched to solid-state devices and my knowledge of vacuum-state devices has waned considerably. Also, never did too much in the RF area in 50 years so this is definitely stretching my abilities.

I appreciate all the help everyone is giving.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 4:25 pm 
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Norm, Mike,

I'm trying to confirm the IF sensitivity check as you both suggested. The alignment states a 1/2 W receiver output (I assume at the speaker terminals) should occur with 20 uV at pin 1 of V2 with a 1650 kHz signal injected (with a .05 uF cap). When I do this, I need about 25 uV to achieve 1/2 watt. ( I have a 520 ohm resistor across the 500 ohm speaker terminals).

I am assuming that when the manual says "approximately 20 uV" that 25 uV is probably OK?

So does this tell me my problem is before V2 (the first mixer)?

There are a lot of tuning slugs in the front end (before and after V1, the RF Amp) and I am wondering if I followed the procedures correctly, or if there is a problem in the RF Amp area. I am pretty sure that I got the IF frequencies alignments correct.

Thanks all,
- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 5:10 pm 
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Bill, I think you are in range and can start by checking the signal paths before V2. I would ignore the individual bands and look at common connections. For example, for a given band, jump the switch wafer to eliminate a contact problem. Because V1 is connected to the sensitivity network, be sure the cathode has the correct resistance to ground (about 180 ohms). Another test would be to skip the antenna coils and inject a signal at the dial frequency directly into the grid of V1 before C5.

Norm

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 5:19 pm 
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Also, be sure the AVC is off and the AVC switch is working correctly. Be sure the Receive Standby switch is working. Some of these switches are never used and can have problems over time. They need to be direct shorts to ground.

Norm

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 5:50 pm 
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Thanks Norm,

I had previously found that 180 cathode resistor on V1 was bad (it measured 242 ohms) and replaced it. I will check the switches and do the signal injection tests you suggest as well.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 7:12 pm 
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Norm,

I did as you suggested and the Receive/Standby switch seems to work ok, with about 2 ohms to ground when in the receive position. Just to be sure, I put a temporary jumper in on the sensitivity pot directly to ground.

I next injected a signal directly into the RF amp before C5. I tuned the dial to the frequency set on the sig gen and the best I could get before the signal disappears is about 1.5 uV sensitivity. (I'm injecting through a 0.05 mF cap; the reactance should be pretty low at that frequency and not a factor).

I did try the full tuning procedure for the 10-11 meter band, including removing the solder on the osc cap and there was little in the way of adjustment that made a difference in the sensitivity.

If I recall what I did correctly, it did not seem like adjusting L12 (the mixer slug for the high end of the 10m band) made a lot of difference and it almost seems like the slug is falling out when I get the highest volume output, BUT the background noise and the injected signal are both increasing as I back out the slug. Not what I expected.

I'm thinking there is maybe a problem still in the RF amp section since the sensitivity tests for the mixer seemed OK (injecting 25 uV produced the correct output at the speaker terms).

Thoughts? I am assuming here that I should be getting better than 1.5 uV sensitivity with injecting the signal directly before C5.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 10:05 pm 
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I think that the 1.5 microvolts is good, seeing that you are more than that at the antenna. I think your problem is before V1. May be a switch wafer or lack of continuity at a crucial connection—like the ground end of the antenna coils, or the ground for the antenna trimmer capacitor.

Norm

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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Sun 26, 2021 10:54 pm 
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Bill,

The IF sensitivity check shows how well the 6BY6 is operating as an IF amplifier and not as a mixer. With low sensitivity, if you have the tubes available try substitutes for both the 12BY7 HFO and the 6BY6 miser tubes. A tube tester is NOT a good test for whether these tubes will actually perform properly as an oscillator and mixer respectively in the HF range. Reduced oscillator output or poor mixer conversion gain will both kill performance and the 12BY7 is known for losing output at RF frequencies with age, this is exacerbated in the later SX-101 series models (includes yours) which have a separate filament transformer that keeps this tube filament on whenever the receiver is plugged in (that was a drift reduction strategy).

The SX-101 series are very good vintage receivers and you will really enjoy your set once it is working properly. I used a SX-101 as a novice receiver and I own several versions of this set.

Note that for AM, this set always receives only one sideband at a time. This allows the SX-101 to provide the same recovered audio bandwidth with 5 Khz. bandwidth selected as a traditional set would with a 10 Khz. IF and allows the SX-101 to be highly immune to interference that is primarily on one side of the desired station. If you aren't used to using this type of setup, until you get the hang of it try tuning AM stations with the BFO on and the pitch control set to zero and tune for zero beat (no beat note between the AM carrier and BFO). With a properly aligned SX-101, this places the carrier slightly down the IF response curve which is where it belongs. Once properly tuned, switch to AM mode and choose the sideband with the least interference. You can instantly switch sidebands on AM without retuning and that makes this series of Hallicrafters receivers my favorite vintage receivers for AM operation.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 1:38 pm 
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Thanks Norm and Roger, I'll do what you both suggested.

Will I have to go through the alignment procedure again when I change the tubes?

Roger, I recall seeing some concerns about the 12BY7 filament issue. I was thinking of rewiring the primary of that filament transformer to go through the switch just as the main transformer does, so when it's off, it's off.

- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 1:54 pm 
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Bill,

I would rewire that so it is switched if I were you, you could also plug the 101 into a power strip and shut it all off when not in use. That is what I do with vintage gear because even after restoration you are dealing with older power switches and other parts that could fail unexpectedly while the set is off but plugged in. Most of my vintage setup is in a finished basement and all of the outlets for the vintage gear are fed by a separate panel controlled by a shutoff switch so I can kill everything with one switch when it sin't in use.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 4:30 pm 
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Thanks Roger, the power strip idea sounds like an easier solution. I always worry when working on old equipment that I'll do something to damage an irreplaceable part!

I'll try changing the 12BY7 and 6BY6 later this morning--I think I have some NOS replacements around somewhere.
- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 6:18 pm 
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All,

I changed the 6BY6 mixer tube and also the 6CB6 (A) RF Amp tube and my sensitivity has almost doubled to under 1 uV now when injecting a signal directly into the antenna input. I had to order the 12BY7 tube as I can't find one in my parts bin. When it comes in I'll do a full alignment again.

I want to thank everyone for their help with this problem which seems to be solved. I haven't achieved the 0.25 uV sensitivity I saw in an earlier post but perhaps when I do the full realignment and the 12BY7 tube things will improve even more.

But.... :!:

I may have another problem. On the lower bands (40/80/160 meters) there is a lot of loud snap/crackle/pop static on the speaker. The bands above 40 meters are all relatively quiet.

I haven't looked at the schematics yet but I am wondering if there is something common to the 40/80/160 bands that could cause this type of static. The RF sensitivity is still seems good on these bands (although hard to really make out a signal with all the snap/crackle/pop noise). Maybe I should make this a new topic since it seems to be a different problem? I did do a good cleaning (de-oxit) of all the wafer switches when I started this project. Most all of the paper caps have been replaced with Mylar as have the power supply electroylytics.

Thanks again,
- Bill


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 Post subject: Re: Hallicrafters SX-101 help with Sensitivity--not wrk as e
PostPosted: Sep Mon 27, 2021 8:23 pm 
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Bill,

Does the snap/crackle/pop disappear when the antenna is disconnected? If so, it is coming from outside of the receiver and it is likely something in your house or nearby.

Glad to hear the sensitivity picked up!

Rodger WQ9E


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