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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Fri 10, 2017 5:31 am 
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Chuck, you may need to work the alignment in reverse and set the a representative trimmer to a reasonable place in its adjustment range and then loosen the coupling and adjust the tuning capacitor to peak with the dial set at the correct frequency. I am sure you know that the bandspread must be at the "set" position. It also can be off in its indexing and that can affect the tuning as well. It is really a complicated equation and until you experiment like you are doing--checking the trimmer ranges at peak, you won't really know how the dials should line up with the positions of the tuning capacitors. It is only a few degrees that can make it all work or not work and these old radios may no longer align correctly in the factory position.

I think I played around with my last SX 42 for more than a few hours and probably ran through the RF alignment 5 or 10 (or more) times before I found a compromise I could live with. I shifted the main dial and bandspread dial a fraction this way and that until I got a feel for where it should end up.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 13, 2017 6:06 pm 
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While working on the above two suggestions, I discovered that I or someone before me had failed to make the connection from the top of the Sensitivity Control, R12, to its wiper.

Of course I dutifully corrected this error, but as I was doing so the question came to mind as to why this control had seemed to work OK even BEFORE this correction. :?:

The more I looked at the seemingly simple configuration, reproduced here:

Attachment:
Sensitivity Control.jpg
Sensitivity Control.jpg [ 8.03 KiB | Viewed 252 times ]


the more I wondered why the circuit would work any differently with or without the connection - shown broken by the X in the above picture of that portion of the schematic.

I suspect that I'm missing something obvious, as I seem to be getting better at doing as I "mature".

Please let me know your thoughts, even if it is, "Back away from the soldering iron...". :D

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 13, 2017 6:34 pm 
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Chuck,

I suspect the wiring as called for in the schematic is primarily to prevent the circuit from going fully open due to poor contact between the variable arm and the resistance element. Since there is DC flowing through this circuit having the full element connected via the shorting link ensures there is always a path even if there is a wiper issue.

But as you found out it generally works OK without the other arm connected since this control is functioning as a variable resistor rather than a voltage divider.

You realize your SX-42 project is going to be finished soon so what is next in the queue? :)

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 13, 2017 7:43 pm 
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Ah, that sounds like a good explanation, Rodger.

As for the queue, I'm leaning towards some test equipment repairs and restorations for a change of pace: 8656B front panel buttons intermittent, 2 Chanalysts, a Hickok 155 Traceometer and a TEK 422 just for starters.

When I'm ready to tackle another receiver, I'll likely start with an SX-101A (1 of two) that I already have out of its cabinet and about which I've read your many favorable comments. Behind that there's another dozen or so boatanchors waiting (HQ-180AC, (2) HQ-110s, SX-25, NC-46, NC-109, Drake TR-4 & MS-4, and on and on).

Obviously, at 73, there's not enough time left to get to these, let alone several dozen other pieces, so I'm actively spreading the word locally to find new homes for many pieces. Shipping boatanchors is not a trivial pursuit, financially or otherwise, as you well know.

Can you pull a trailer with that Corvette? :D

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 13, 2017 8:22 pm 
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WoodchuckTN wrote:

Obviously, at 73, there's not enough time left to get to these, let alone several dozen other pieces, so I'm actively spreading the word locally to find new homes for many pieces. Shipping boatanchors is not a trivial pursuit, financially or otherwise, as you well know.

Can you pull a trailer with that Corvette? :D


I can understand wanting to thin the herd some but don't put yourself in your grave before your time :)

At some point I will migrate back south and that will be a time to thin the herd here. The Corvette is allergic to trailers but I am getting ready to finally replace my 2006 GMC 2500HD diesel pickup with a 2018 replacement; I hope I don't regret the move with all of the added emissions stuff on the latest model but it is time to move onward and there are times when I still need to move heavy stuff.

I am tempted to work on something small next, an Eton E1 XM receiver which is a decent little Drake designed portable but it appears no schematic is available. Mine works OK except the passband tuning is inoperative (stuck at its maximum plus setting) and the S meter display is stuck at mid-scale even though the sensitivity and AGC are both fine so I suspect a reference voltage issue since both problems appeared at the same time. Unlike your SX-42 gain pot the passband pot in the Eton E1 is configured as an actual voltage divider which feeds one of the CPU inputs and I suspect the voltage is out of range. I just hate jumping into something like this with no schematic but it builds character.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 1:28 am 
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I'm 69 yrs old. I figure to do another couple of hundred restorations. Don't write yourself off too soon!

:lol:

Sandy


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 1:53 am 
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Sandy, Rodger, thanks for the advice. It's just that it's frustrating sometimes when the old brain just doesn't process like it used to.

I'm still working on conquering the alignment of the 455 IF xtal filter section. Another session with it this afternoon ended in frustration. Maybe 4:00 in the afternoon is too far down the back side of my synapse activity curve.

Maybe tomorrow.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 1:59 am 
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I would recommend that you do the HQ180 next. Wonderful receiver and pretty easy since they used ceramic discs ;-)

Been fun following this. I have a SX42 in my que, but did the HQ180 a few years ago and now radio daze has water transfers thanks to me lol.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 2:40 am 
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Chuck, the procedure for setting the crystal filter is confusing at best. I have befuddled my way through it many times. I consider myself still pretty sharp--but I am not sure I have more than a partial idea why the manual is asking me to do what it is asking. I don't have a generator that one can "rock"--because mine is set digitally to an exact frequency. That makes the process even harder to follow. Hopefully Rodger or someone more knowledgeable will explain this and I can go back and re-do both my 42's.

I probably have mine set about right--they have reduced output at the sharp, less reduced at the middle, and more at the broad, with the normal ranges having the highest output, but beyond that---I have no idea if the radio is correctly set. It may be time to get out the big old spectrum analyzer and "see" what I am doing with the adjustments. I am going to run into this again on the SX 62.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 3:04 am 
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K8, thanks for the recommendation on the 180. I may do that for a little respite.

Norm, the ability to "rock" is exactly why I'm more comfortable using my HP-606B, for many parts of the alignment, than I am with my HP-8656B. However, when the 606 was down temporarily recently for a bit, I did learn I could "rock" the 8656 using the up-down buttons, but it was clumsy - especially when I'd hit the wrong button and change something I didn't mean to change. :roll:

I'm very lucky to have both, but the 606B is my favorite for most of what I do.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 3:32 am 
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Here is what I think the circuit looks like for the filter.

Norm


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 4:06 am 
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Norm Johnson wrote:
Chuck, the procedure for setting the crystal filter is confusing at best. I have befuddled my way through it many times. I consider myself still pretty sharp--but I am not sure I have more than a partial idea why the manual is asking me to do what it is asking. I don't have a generator that one can "rock"--because mine is set digitally to an exact frequency. That makes the process even harder to follow. Hopefully Rodger or someone more knowledgeable will explain this and I can go back and re-do both my 42's.

Norm


Norm,

It is good to rock babies to sleep and if you are The Clash you can Rock the Casbah but I don't rock my signal generator.

Hallicrafters awkwardly worded service instructions are trying to make sure that you have the generator set properly to find the crystal peak and the "rocking" may make sense for a low quality service generator that is constantly drifting and has a very fast tuning rate but with a decent signal generator just set it at the crystal peak and leave it there and make the adjustments.

The single element crystal filter used by Hallicrafters is essentially the same as that used by many other manufacturers with the major difference being that Hallicrafters used user adjustable trimmer caps to set the broad, medium, and normal filter width while others mostly used fixed settings.

The person who originally penned these instructions for aligning the crystal filter must have also been a big fan of the moving vane meter used in the Heathkit AT1 and a number of other low cost transmitters because the rocking of those movements makes it equally difficult to truly know what is going on.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Wed 15, 2017 5:48 am 
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The alignment procedure in the SX 62 manual is worded differently. It actually makes the process a little clearer. I think they realized that the SX 42 manual was confusing. It is the same filter in both radios.

Norm


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 20, 2017 1:38 am 
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Rodger, thanks for the "rocking" clarification. That makes the process look a little simpler when you factor that in - or actually, out - of the process.

However, as I attempted to utilize this new-found information, I discovered the audio was once again garbled, more noticeably on SSB than AM or FM, but present on all. Monitoring the BFO on my DX-400 showed it was clean. As before, the HFO turned out to be the source of the garbling, as heard on the DX-400.

The garbling is to these old ears, not terrible but is noticeable.

Unfortunately, after swapping out the 7F8 two or three times to try to find one without the garbling, I'm out of tubes to swap. All of the 7F8's tested good with no leakage on a Sencore TC-109 which has one of the better leakage tests.

I'm suspicious that there's something besides the tube causing the issue. The 7F8 voltages are all within reason and don't point to a problem.

At one point I looked at the HFO signal with my TEK 453 and noticed distortion at the top of the sine wave peaks. (Sorry, at the moment I can't remember exactly where I was picking off that signal.) I'll be following up on that clue while I try to find some more 7F8's to try.

Norm, thanks for the SX-62 info. I had long ago -at Rodger's suggestion- gotten that info and have almost memorized it I've read it so many times :D

I also downloaded the SX-28 procedure by Bill Feldmann, N6PY, which added some clarity. However, magic lightbulb hasn't quite lit up yet.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 20, 2017 2:43 am 
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Chuck,

Does this oscillator instability occur on all bands?

The warble could be from the wiper contact on the oscillator section of the variable capacitor or a contact on the 7F8 socket; loctal sockets seem more prone to pin contact problems than any other type. Try some cleaner on the pins of the tube and insert it to see if that helps.

As I recall C38 (mixer cathode bypass) is one of the difficult to access wax paper caps so make sure this one was changed.

Typically the output from the HFO won't be a pure sine wave but that is OK.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 20, 2017 3:10 am 
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Rodger, I wouldn't exactly characterize it as instability. To my (admittedly inexperienced) ear, it sounds like mild hum superimposed on the audio. The scope trace also looked like such to me.

Thinking just as you suggest, when the tube swapping didn't help (since they're used they could all be bad) my first step was to thoroughly clean the 7F8 pins and socket with DeOxit. I then did the same for all the tubes in the RF and IF sections. The problem remains.

C38 was changed when the recapping was done, as I think I covered in an earlier post.

I appreciate any and all suggestions and will take a look at the wiper on the oscillator section of the tuning cap.

And, yes the garbling/hum is there on all bands.

Listening to the BFO on the monitoring receiver in CW mode I get a nice clear note. Listening to the HFO, I can clearly hear the garbling. Again, I'd describe it as garble/hum as opposed to the warble I'd expect from a wiper issue or other "mechanical" problem.

I don't mean to sound like I'm discounting your analysis. I'm only discounting what may be a poor description on my part.

Keep those cards and letters coming! :D

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 20, 2017 3:30 am 
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Does the problem go away if you turn down the RF gain? You may have an oscillation introduced from components or component leads providing feedback to a tube in the chain. I have had that problem before. You may try using the scope to work back through the chain. I sometimes pull a tube at a time, working back from the detector and see if there is a point that the warble or oscillation stops. the reason I ask about the Rf gain, is that if turning it down a bit makes the oscillation stop, that is another indicator that it is an unwanted oscillation that starts when the gain reaches a certain point.

Usually such an oscillation is pretty overpowering, so this may not be what you have, but I suggest it only as a possibility.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Nov Mon 20, 2017 5:04 am 
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Thanks for the question, Norm. Since I don't know the answer, that's something else I'll have to check.

And since you mention it, I may have that issue with an SX-101A I started to look at awhile back. But that'll be another thread perhaps if I can ever "stick a fork" in this one and call it done.

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