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 Post subject: Hallicrafters SX-62A troubles
PostPosted: Jan Fri 05, 2018 7:47 pm 
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Joined: Oct Wed 01, 2014 1:20 pm
Posts: 105
Location: Wood River, Ill.
Hi all,
I didn’t want to hijack woodchuck’s thread on his SX-42, so I come here, hat in hand, and must confess that I have now run into troubles of my own with my ‘62. Just to fill in on what has transpired,to date with my project: this receiver had some mouse damage. Specifically to the BC band coils. I successfully transplanted coils from a SX62 into this 62A. All paper/wax caps were replaced along with a hand full of OT resistors. The band switch and other wafer switches were cleaned by judiciously applying DeOxit to the contact surfaces. I ran thru a quick IF alignment on the 455 IF only, as I had a few of the 6SG7’s that were weak.

I ran into some overloading of local AM BC stations and distortion to the audio signal. While monitoring the AGC voltage which was floating around -3 VDC, I was able to stabilize the AGC up to around -7 to -7.5 volts by moving around and swapping out the 6SG7’s. It was sounding real good, distortion and overload went away, and I was feeling pretty good about things, allowing it to play off and on for several days. Then Murffey’s law kicked in. The distortion came back, along with the overloading. The AGC voltages are now running back around -3 VDC. I can’t get it any better than about -5 VDC playing around with the IF alignment. My order of 6SG7 came in, which didn’t help either.

I am thinking my problem lies in the AGC, but this may just be a symptom of a problem somewhere else, IF stages, perhaps? I am running out of ideas and need some guidance. Any help would be appreciated.

Thanks for reading.

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Chris
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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Fri 05, 2018 8:24 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
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Chris,

Start by measuring the resistance from pin 4 to ground of th first IF tube, it should be over 1 meg and this will give you an idea whether there is still a source of leakage to ground on the AGC bus. Some of the AGC bypass caps in receivers have a way of hiding.

Measure the grid voltage (with a strong signal from your signal generator applied) at the control grid of each of the AGC controlled tubes and look for a voltage that is less negative compared to the AGC bus voltage (or even positive) at one of the grids because the problem could be in one of the RF amp tubes or even your new tubes. Gas or grid emission can cause a positive voltage to be developed which will then throw off the negative bias of the entire AGC bus.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Sun 07, 2018 8:03 pm 
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Joined: Oct Wed 01, 2014 1:20 pm
Posts: 105
Location: Wood River, Ill.
Thank you Rodger for getting me restarted in my trouble shooting. Obviously I still have much to learn! I did the check at pin 4 of the first IF and found the resistance at that point was running no more than 0.88 meg. I spent the day running that down, all to no avail. I did go in and replaced the two 0.047 caps on the cathodes of the two RF tubes. They are very neatly buried above the band switch. (I had foolishly left them in the initial recapping as they looked as if they had been replaced at some point in time. Live and learn...)

They were leaky. It did not improve the resistance measurements on pin 4, though. When I powered up, the AGC voltage did improve much, getting up to around -7.5 VDC, but the distortion is still present. I did put an audio signal into the phono input, and verified that my problem is an RF or IF problem, and not AF. Reducing the RF gain does not remove the distortion, though.

Now on to your second suggestion; trouble shooting at the AGC controlled tubes with the signal generator. This is a learning exercise for me, anyway.

Thanks again.

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Chris
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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Sun 07, 2018 9:42 pm 
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Joined: Dec Sun 21, 2014 6:37 am
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Location: Portland, TN, USA
Thanks for the consideration, Chris, but the radios and the problems involved in this case make us one big happy(?) family - or not.

Anyway, you might get more and quicker help this way. I'll be following your progress closely, as we're stalking similar elusive dragons. We can be glad that Rodger, Norm, and others have our backs.

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Want Hallicrafters S-76 with good cosmetics; Can fix electronics issues.


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Sun 07, 2018 9:51 pm 
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Joined: Jul Tue 21, 2009 1:38 pm
Posts: 748
Location: SW WA state
rsingl wrote:
Gas or grid emission can cause a positive voltage to be developed which will then throw off the negative bias of the entire AGC bus.
Rodger WQ9E


This is good advice! I had a receiver that limited with anything more than about 10 uV: The AVC line wouldn't go negative enough, so the IF saturated, and this gave me horrendous distortion in recovered audio (AM receiver).
The problem was traced to a Nuvistor that was either gassy or otherwise shorted.
Replace tube, that problem was cured (one of many issues with that set).


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Mon 08, 2018 3:04 am 
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Joined: Oct Wed 01, 2014 1:20 pm
Posts: 105
Location: Wood River, Ill.
Thanks Chuck and Tom for the replies. Chuck, I did not want to mix in my issues on top of yours and muddy up the works, even though some of the issues may be similar. I thought the safest way would be to start this thread. I am really thankful for the help and advise from this group of experts. :D

I won’t state “problem solved” yet, but I have made progress since my last post. I found two issues, which, I must confess, were problems I created. One was a cold solder joint, and the other was the installation of a 68 ohm resistor where a 680 ohm should have been. Both dumb mistakes, but it happens! :oops:

The audio distortion appears to be gone and the tube pin voltages on the RF and IF tubes are now within reason. I won’t call this solved until it has been operating on the bench for a few days or more. At least I feel better about it this evening. :D

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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Mon 08, 2018 3:26 am 
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Joined: Jun Sun 19, 2011 2:31 pm
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Congratulations Chris and you learn a lot through finding mistakes whether they are new or those introduced in the past. More than one factory built rig has made it many years with mistakes that are later found during a full restoration.

Although the SX-42 and SX-62 aren't horrible to work on they are more complex than the majority of receivers of their era and with all of the running changes, both documented and undocumented, by the factory you will frequently find yourself doing research to figure out what differences were intended and which were not.

But when you finish you will have a nice performing receiver that covered a very wide frequency range for its day.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Mon 08, 2018 6:47 am 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1599
Location: Costa Mesa, California
I love my SX-62. I have many other radios, but the SX-62 is one that stays plugged in and when I am in the garage, I turn it on and listen to my local classical music station for that great sound. For FM it only needs a small antenna sitting on top of the radio to pull in the station loud and clear. There is almost no drift from turn-on. I don't know why it out-performs my other radios, but I love the look of that big dial.

Norm


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Mon 15, 2018 9:58 pm 
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Joined: Oct Wed 01, 2014 1:20 pm
Posts: 105
Location: Wood River, Ill.
Boy, do I feel like a dope, pt.2!!! :oops:

When I last reported on my 62 I stated that I thought that I might have solved the distortion issue. “Might” is the key word here! I didn’t have it solved completely, as it would return from time to time. This distortion only appeared when listening to music on the AM BC band. It was not present at all on BC FM at all. (It was probably present on other bands as well, but didn’t check it there. )

I have been working on this off and on since my last report. It turned out that the problem was sticking contacts on the noise limiter switch. I feel like a dope because I have experienced the sticking contacts before. This is something that I should have remembered, but failed to do so. :roll:

To make a long story short, it now sharply tunes and the slight distortion has gone away. Live and learn...again!

Feeling much better today! Very progressive use of a holiday!!! :D

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Chris
N9WHH


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Mon 15, 2018 10:20 pm 
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Well done Chris!

Noise limiters function more as distortion adders than noise reducers although under certain conditions the old clipper style limiters do work quite well. But they are always going to contribute some distortion and often a lot of distortion. A well implemented newer blanking type of noise reducer works extremely well on certain types of noise but they often degrade the dynamic range.

The RME-45 series offer a real "gotcha" with a noise limiter controlled by an unmarked "push pull" operation of the audio control. Even when the operator was aware that the noise limiter existed in his version the Mickey Mouse switch arrangement often failed to kill the noise limiter and I am sure many owners spent hours searching for the mysterious intermittent distortion caused by this switch.

And don't feel like a dope because we have all been there. I wasted 15 minutes trying to figure out the paper feed problem with a new Canon photo printer I installed this morning because the stupid quick start guide illustration showed the paper being fed into the rear feeder for the initial print head calibration and test print. The rear feeder has to be selected via software and the top feed is used by default. I should have spent more time looking at what the printer was doing because it would have been fairly easy to see the rollers for the top feed turning trying to load paper. Canon did a great job with the camera documentation but the printer looks like it was done by the summer interns.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Tue 30, 2018 7:41 pm 
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Joined: Oct Wed 01, 2014 1:20 pm
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Location: Wood River, Ill.
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First of all I am trying to add a photo to my post.
Secondly I thought I would provide an update. I think I have most, if not all electronic issues taken care of. I think I can safely slide the chassis back into the case. BUT, the case needs some work. The front panel looks pretty good, so it will remain untouched, other than cleaning and some auto wax/cleaner. I was planning to have the case powder coated, similar to what Norm has done on some of his projects. I have found a place to do the powder coating, I think, but they only offer sand planting using garnet media. I think this media is much too corse for this application for removing the original paint and what little rust is on the case. I was thinking more like soda blast, but am having difficulty finding someone local to do the soda blast. Has anyone tried chemical paint removal before powder coat. Good? Bad? Something else?
I can do the soda blast, but don’t have a good place to do it without ticking off the neighbors. The paint remover I can do when the weather gets better. Just looking for a little advice. Maybe I am all wet?

Thanks.

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Chris
N9WHH


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 Post subject: Re: Hallicrafters SX-62A troubles
PostPosted: Jan Tue 30, 2018 9:48 pm 
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Location: Costa Mesa, California
I am not sure what my powder coater uses for his abrasive, but I would suspect it is the skill of the person doing the blasting that would determine whether garnet is okay for your cabinet. Show them the cabinet and ask them if they are worried about getting a smooth finish.

Norm

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