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 Post subject: SX-25 restoration
PostPosted: Feb Tue 06, 2018 5:44 am 
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Joined: Feb Tue 06, 2018 2:59 am
Posts: 5
Hello all,
I've been reading the threads for awhile. I am currently restoring a Hallicrafters SX-25, I believe it to be a 1945 vintage. It appears to be mostly intact, although there are some problem areas. I'm putting some pics up of one of them. Coming off from pin 4 of 6SQ7 is an orange wire that meets a black wire coming from the pitch control pot. the two wires aren't connected electrically, but their bitter ends are twisted together. (see photos)
Is this the "Twisted Pair capacitor" or something else? there is the same thing between to variable condensors- green wire
Any help would be appreciated.


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Tue 06, 2018 1:37 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5540
Those were commonly called "gimmick capacitors" and are used when the capacitance value required is extremely low. They are commonly found in a number of sets. Try not to disturb them because although the capacitance value usually isn't critical you don't want to move this wire pair where it couples with something else where it shouldn't.

Have fun with the restoration. The SX-25 is a nice receiver one step down from the Super Skyrider series and anything named "Super Defiant" has to be pretty good :)

Rodger WQ9E


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Tue 06, 2018 4:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3060
Location: Seattle WA US
The serial number of your SX-25 would be from the same sequence as SX-28 serial numbers. You can estimate manufacture date by checking the serial number registry at http://radioblvd.com/SX28Notes.html
-Chuck


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Wed 07, 2018 2:58 am 
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Joined: Feb Tue 06, 2018 2:59 am
Posts: 5
Thanks for the replies guys!
rsingl - I'm very glad to know that those twisted wires are intentional, and not something else I need to chase down.
I have all the caps and resistors coming, can't wait to hear what this sounds like.

K7MCG - Thanks for the info!The serial # is HA 12479

MONTH/YR. SERIAL NO. RANGE

Aug 1940 to Dec 1940 . . . . . . . . . . H-115,000 to H-124,000

Jan 1941 to Dec 1941 . . . . . . . . . . H-124,000 to H-145,500

Jan 1942 to Dec 1942 . . . . . . . . . . . H-145,500 to H-166,500

Jan 1943 to Dec 1943 . . . . . H-166,500 to H-183,000/HA-1000

Jan 1944 to Dec 1944 . . . . . . . . . . . . HA-1000 to HA-8,500

Jan 1945 to Dec 1945 . . . . . . . . . . . HA-8,500 to HA 35,500

Jan 1946 to June 1946 . . . . . . . . . . . HA-35,500 to HA-55,500

Looks likes its a 1945 indeed.


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Sun 11, 2018 12:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1125
Location: Cedar Rapids, Iowa, USA
There are two areas in these I have had trouble with. First is adjusting the I.F. to the crystal frequency and setting the B.F.O.
Second, and more serious, is the push-pull audio output to that oddball transformer. Check the capacitors and resistors in the output stage, and that the transformer primary has continuity from the center tap to both ends. A common failure is that the output must be selected for 500 or 5000 ohms, and matched accordingly. People evidently would connect a 4 or 8 ohm speaker, turn it loud enough to hear, and burn up the transformer.
Hint: if one side of the primary is open, do not trash the transformer. Replacements are very hard to find. Carefully open it, lift up the end bell, and inspect the wires. They are very fine, and could be open just where they attach to each other and the center tap lead. If so, with care, they can be re-connected and the transformer can be put back together and back in service.
The same remarks apply just as well to the SX-28 because the circuit is almost identical despite using different tubes. In fact, I have put an SX-28 transformer into an SX-25.


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Sun 11, 2018 9:30 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 743
Location: Peekskill, NY
My biggest trouble getting the frequency set to the crystal frequency was temperature.

Finally I left the set on for hours to do this. Also the orientation of the set, and having it be
in or out of the case, made a noticeable difference. My suggestion is to let it warm up
fully, and to have the set in the case with the cover closed for quite some time before
doing this step. Seemed to me that even putting it on its side would be enough to
make a noticeable shift in the IF frequency.


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Mon 12, 2018 3:54 am 
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Joined: Feb Tue 06, 2018 2:59 am
Posts: 5
Thanks for the advice! I have finished the cabinet, with the exception of putting on the decals from radio daze. I used hammered black matte from rustoleum with self etching primer. I realize its not a match for the original, I just liked it. I have resistance from pin 2 of the transformer to pin 8 of both 6f6 tubes. Going from my schematic that looks like center tap to both sides of the primary, unless I missed something, the schematic is a little sparse.
I have a box of capacitors and resisters coming tomorrow, the real fun begins then ;) Jim, Thanks for the tip, when I get to the tuning stage I'm going to have some questions, I've never done that!
I uploaded some picks to my Flickr account in the meantime https://www.flickr.com/photos/134392624 ... 9470763119
Thanks!


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Mon 12, 2018 4:33 am 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 743
Location: Peekskill, NY
Another suggestion, learned by me the hard way: Don't uncase the radio when it's time to
do the alignment. Don't remove the bottom plate with the holes in it either.

My first step: align the set out of the case. Re-case it, alignment is gone.
Next step, put back in case, with the bottom off. Repeat the above effect.

By the time I aligned it in the case, with the bottom on, I got pretty good at doing it fast.


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Mon 12, 2018 7:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1125
Location: Cedar Rapids, Iowa, USA
It is a wise Hallicrafters owner who knows where all his cover plates and hardware are.


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 Post subject: Re: SX-25 restoration
PostPosted: Feb Wed 14, 2018 1:06 pm 
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Joined: Feb Tue 06, 2018 2:59 am
Posts: 5
I have a couple of questions about this micamold cap (it says micamold on the cap) in this photo.With the arrow, it goes black,brown,black then brown. It would appear thats 0 , 1 , with a 1X multiplier, 10% tolerance, 600 volts. So this is 01 what? Pico? my meter reads it at 1 nano farad. it meters to infinity on my fluke. Is this an actual mica cap, good to use or is it a paper in disguise?


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