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 Post subject: SX-25 How do I identify coils
PostPosted: Sep Thu 23, 2021 4:40 pm 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
I have another thread here following my work on a Hallicrafters SX-25. I posted this because I haven't had much luck getting answers (with a few exceptions).

See below a schematic of this receiver. One of the biggest challenges I have faced is identifying components physically within the chassis. For example, at the moment I'm troubleshooting a no reception condition on band 2 and band 4. BC, band 3 and 3B are loud and clear. It has been suggested that I have band switch problems, either dirty contacts or contacts that are not connecting properly. I understand this. What I do not have confidence with is identifying which coils are which, like antenna coils, oscillator coils, etc. So, in the schematic shown, can someone familiar with this receiver help identify the coils in the red rectangles. Also, in the bottom most group of coils are the points within the green circles connected.

Many thanks, Frank


Attachments:
SX-25 schematic   questions for forum.jpg
SX-25 schematic questions for forum.jpg [ 671.51 KiB | Viewed 576 times ]
SX-25 schematic connection  questions for forum.jpg
SX-25 schematic connection questions for forum.jpg [ 77.4 KiB | Viewed 576 times ]
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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Fri 24, 2021 4:52 am 
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angelfj1 wrote:
One of the biggest challenges I have faced is identifying components physically within the chassis.
Is this what you are asking about? (labelled your schematic and included physical layout).
Attachment:
SX-25 COILS.png
SX-25 COILS.png [ 835.42 KiB | Viewed 539 times ]
Attachment:
SX-25.png
SX-25.png [ 46.18 KiB | Viewed 539 times ]
Curtis Eickerman

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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Fri 24, 2021 5:21 am 
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Hi Frank, No, the points in the green circles are not connections.

Regards, Larry


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Fri 24, 2021 10:52 am 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
Eickerman wrote:
angelfj1 wrote:
One of the biggest challenges I have faced is identifying components physically within the chassis.
Is this what you are asking about? (labelled your schematic and included physical layout).
Attachment:
SX-25 COILS.png
Attachment:
SX-25.png
Curtis Eickerman


Curtis, Yes! That's exactly what I was looking for.

Thank you, Frank :D


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Fri 24, 2021 10:55 am 
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Posts: 346
Location: SE PA 19335
Lar H wrote:
Hi Frank, No, the points in the green circles are not connections.

Regards, Larry


Larry, Thank you. Without the "dots" designating a definite connection, this has always been confusing to me.

Thanks for confirming this.

Frank


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Fri 24, 2021 11:53 pm 
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With help from the forum, I can now identify each RF section. I understand that these circuits consist primarily of air core transformers with variable capacitors on their secondary, thereby allowing the circuits to be tuned to a specific frequency. See below, I have marked up a pic of my receiver's RF section to identify the trimmer capacitors. The trimmer designations are from the Hallicrafters operating manual that Curtis posted.

Attachment:
rf section-Frank's SX-25-R1.jpg
rf section-Frank's SX-25-R1.jpg [ 338.55 KiB | Viewed 488 times ]


It would be desirable from a troubleshooting standpoint, to be able to identify each transformer and trimmer cap. I hoped to identify the transformers by their respective trimmer. This is not so straight forward for the following reason. For example, the antenna section trimmer caps, as shown in the above pic, are designated, Ca, Cl, Ch and Cd. However, the schematic identifies the same trimmers as C50A, C50B, C50C and C50D. So, given this confusion in nomenclature, how can I positively identify each transformer and trimmer capacitor. Your help and suggestions would be appreciated.

Best regards, Frank


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Sat 25, 2021 12:13 am 
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Location: Peekskill, NY
The coil with the thinnest wire and most turns will be the lowest band. The one with the fewest turns of thicker wire will be the top band. The trimmers and coils are all in the same order as you go across the chassis.

Dead bands: the topmost band in most sets like this are pretty quiet as the the gain stages are pushed above their real abilities. 2nd band however should
work well - check to see if the primary winding on the first RF hasn't gone open from a lightnng strike in the past. Just use an ohmmeter across the antenna terminals with the bandswitch on band 2. Check to see if the LO is running on that band as well, look at the plate with a scope.

Older hallicrafters sets can show wonky behavior on the coil sets. I think on some the ground connections are via the two coil mounting brackets and the screws can loosen up - and some are near impossible to get at. It's a tight spot to do any trouble-shooting in.

As mentioned try to visualize all the contacts on the bandswitch. I had a carbon trace in an sx28 switch that was putting B+ on the grid of the next tube.
Tube did get rather warm....


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Sat 25, 2021 4:04 am 
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angelfj1 wrote:
It would be desirable from a troubleshooting standpoint, to be able to identify each transformer and trimmer cap. I hoped to identify the transformers by their respective trimmer. This is not so straight forward for the following reason. For example, the antenna section trimmer caps, as shown in the above pic, are designated, Ca, Cl, Ch and Cd. However, the schematic identifies the same trimmers as C50A, C50B, C50C and C50D. So, given this confusion in nomenclature, how can I positively identify each transformer and trimmer capacitor. Your help and suggestions would be appreciated.
You might shoot dtvmcdonald a PM with regard to aligning an SX-25. He just finished one up a couple of years ago. https://www.antiqueradios.com/forums/vi ... 0&start=20

He might have the magic decoder as to which coils and trimmers are which.

Curtis Eickerman

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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Sat 25, 2021 10:23 am 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
Jim, Curtis, many thanks. I have sent a PM to dtvmcdonald and hoping to hear from him.

Frank


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Sat 25, 2021 2:17 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
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Location: Champaign IL 61822
Since the coils are not adjustable there is no chart ... you have to follow wires or use a
good grid dip meter, or use a signal generator connected to each coil in turn (through say a 2
or 5 pH capacitor and also to a scope with a different 2 pF cap, and finding the resonance thereby.

Aligning is a nightmare. If you've replaced a paper cap in that section with a different sized one, or
even one with a different lead diameter, its going to discombobulate the tuning of
the top two bands .... badly. So does doing something as seemingly innocuous as moving a
ground wire connection a half inch! And that means even if you change a wire for a lower band it can ruin
alignment for the top band.

So you have to check each attachment point carefuly, then align from lowest frequency band up.

The "gimmick" caps added sometime after the first sets are the most super-critical. I had to actually remove one of them
to get it to align well. The tube type used as convertor is an abysmal design. The gimmicks
were added to try to neutralize out conversion transconductance killing direct feed of the oscillator
into the second (signal) grid. This is discussed in tube literature. This is one of the reasons
the Europeans went to triode-hexode or triode-heptode designs, and stuck with them even though
the 6BE6 is in fact a better design up to say 30 MHz. Above 40MHZ any hexode or heptode turns
out to be seriously inferior to a proper triode or pentode convertor, with triodes being clear
winners above 54 MHz.


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Wed 29, 2021 4:15 pm 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
dtvmcdonald wrote:
Since the coils are not adjustable there is no chart ... you have to follow wires or use a
good grid dip meter, or use a signal generator connected to each coil in turn (through say a 2
or 5 pH capacitor and also to a scope with a different 2 pF cap, and finding the resonance thereby.

Aligning is a nightmare. If you've replaced a paper cap in that section with a different sized one, or
even one with a different lead diameter, its going to discombobulate the tuning of
the top two bands .... badly. So does doing something as seemingly innocuous as moving a
ground wire connection a half inch! And that means even if you change a wire for a lower band it can ruin
alignment for the top band.

So you have to check each attachment point carefuly, then align from lowest frequency band up.

The "gimmick" caps added sometime after the first sets are the most super-critical. I had to actually remove one of them
to get it to align well. The tube type used as convertor is an abysmal design. The gimmicks
were added to try to neutralize out conversion transconductance killing direct feed of the oscillator
into the second (signal) grid. This is discussed in tube literature. This is one of the reasons
the Europeans went to triode-hexode or triode-heptode designs, and stuck with them even though
the 6BE6 is in fact a better design up to say 30 MHz. Above 40MHZ any hexode or heptode turns
out to be seriously inferior to a proper triode or pentode convertor, with triodes being clear
winners above 54 MHz.



dtvmcdonald, thank you for this info and your opinions. With additional assistance from folks here, I will soon have a map of the air core transformers and trimmers in the RF section of this receiver. In the course of troubleshooting, to gain better access to the wafer switches, I have carefully removed the rivets holding the trimmers to the metal dividers. The rivets will be replaced with 4-40 SS screws. There was quite a bit of corrosion (not rust, white oxidation) at these attachment points. When re-attaching the trimmers, I'm thinking about soldering a jumper from the metal divider (ground) to the trimmer common bus. There are many instances in this receiver where rivets and screws are used for a ground connection. I don't trust a mechanical fastener whether screw/nut or rivet for a ground connection, especially in a 75 year old receiver.
Attachment:
20210927_134258-R1.jpg
20210927_134258-R1.jpg [ 255.35 KiB | Viewed 369 times ]



I also came across several of these little bugs.
Attachment:
20210927_142213-R1.jpg
20210927_142213-R1.jpg [ 325.47 KiB | Viewed 369 times ]



I'm told that they are ceramic caps. However, I have found nothing written about them and they are not shown on the schematic.



Any thoughts?



Best regards, Frank


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Wed 29, 2021 5:14 pm 
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angelfj1 wrote:
I'm told that they are ceramic caps. However, I have found nothing written about them and they are not shown on the schematic.
They are indeed tubular ceramic capacitors. Since it is obviously original to the set when it was aligned in the factory and it is in a tuned circuit, I would leave it alone. Any failures of these would be extremely rare unless someone breaks a lead off of them.

Curtis Eickerman

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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Wed 29, 2021 5:40 pm 
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I have at least one in my 25:

Attachment:
Totally_Tubular_Cap.jpg
Totally_Tubular_Cap.jpg [ 727.91 KiB | Viewed 355 times ]


So, I would guess that it is original whether the schematic documents it or not. By the 1950's Hallicrafters was fairly good but not perfect about documenting radio designs and changes. In the 1930's theas documentation was more of a guide than a holy scripture. I would leave them alone, I agree that they are unlikely to fail unless they have visible damage.

Curiously you can get SMD versions of tubular caps!:

https://www.apitech.com/products/emi-filters-components-magnetics/ceramic-capacitors/tubular-capacitors/


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Sep Thu 30, 2021 7:31 pm 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
I clipped some of these out of a parts receiver and thought I'd measure them. They all came in at approximately 110 pf.

Attachment:
20210930_133812.jpg
20210930_133812.jpg [ 133.63 KiB | Viewed 301 times ]


I also carefully duplicated one of the gimmick capacitors using the exact same wire from the parts receiver and got 6 pf.
Attachment:
20210930_133945.jpg
20210930_133945.jpg [ 111.19 KiB | Viewed 301 times ]


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Oct Wed 06, 2021 1:23 pm 
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Joined: Oct Wed 06, 2021 12:37 pm
Posts: 1
I'm told that they are ceramic caps. However, I have found nothing written about them and they are not shown on the schematic.
----------------------
They are indeed tubular ceramic caps and they should show on the schematic as:
c9=35pf
c13=5pf
c14=35pf
c48=105pf
c55=700pf (sometimes changed to 1000pf)

From what I hear, they usually don't go bad unless they're physically broken...just as well, they're a bear to get at.


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Oct Sat 09, 2021 7:01 pm 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
Spqr915 wrote:
I'm told that they are ceramic caps. However, I have found nothing written about them and they are not shown on the schematic.
----------------------
They are indeed tubular ceramic caps and they should show on the schematic as:
c9=35pf
c13=5pf
c14=35pf
c48=105pf
c55=700pf (sometimes changed to 1000pf)

From what I hear, they usually don't go bad unless they're physically broken...just as well, they're a bear to get at.


Thank you! This info is helpful in identifying these little caps. I have measured those that I was able to access and their values have remained very close to original spec.


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 Post subject: Re: SX-25 How do I identify coils
PostPosted: Oct Sat 09, 2021 7:24 pm 
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Joined: Oct Sun 12, 2014 1:02 am
Posts: 346
Location: SE PA 19335
Update!

I have finally identified all of the transformers/coils and trimmers.
Attachment:
RF 20211004_111440-R35.jpg
RF 20211004_111440-R35.jpg [ 605.8 KiB | Viewed 152 times ]


Looking at the above photo, please note that I used my Dremel with an abrasive stone and removed the rivets (2 each) which secure each trimmer capacitor bank to the metal dividers in the RF section. I decided to do this to gain access to the coils and wafer switch contacts.

I think I have good news. I say, "I think", because I'm not sure. However, I discovered a wiring error in the antenna coils. Actually, I may have been the one who made the error. :shock: Why do I say this? Well, earlier this week a package arrived from the Boston area. It contained an SX-25 parts radio, which I bought for basically the cost to ship it. It's in terrible condition externally, but the under-chassis components and wiring are all original. Yesterday was the first time I had to take a good look at it. So, I put both radios on the bench, side by side, with their undersides facing me. That makes it easy to compare wiring of each of the RF sections.

I discovered that my radio had the A2 antenna wire landing on the wrong coil terminal.

I corrected the connection to agree with the parts radio and then continued checking as many connections in the RF area that I could access.

Next, I returned each of the trimmer banks to their original position. I had previously ground off the (2) rivets holding each of the trimmer assemblies to the RF section dividers. I did this to improve access to the band switch sections. With the trimmers out of the way, I could observe the action of the band switch. Some of the stationary contact "fingers" had lost their springiness. Using some of my soldering probe tools and dental picks, I was able to improve the action of several contacts. I watched the moving contact rotate. Where the moving contact engaged each stationary contact, I looked for slight movement (deflection) of the stationary contact finger. If I saw no deflection, I gently probed with my tools, until I could see a little motion. I went through this exercise at each switch stage that I was able to access and see what I was doing. I can't overstate how very tedious this job is and it took a lot of time. Eventually, I replaced each of the rivets with SS, 4-40 screws, lock washers and nuts.

After checking for foreign objects that may have dropped into the chassis, I used compressed air to blow out any dust that had accumulated during this troubleshooting work.

Finally, I replaced all of the tubes and powered up the receiver using my variac + dim bulb supply. After approx. 30 seconds of warmup, and at approx. 105 volts, I was pleased to hear a little reception on each band. But I say reception, not actual information. What I mean is, previously, before I was aware of the wiring error, I didn't even have background noise. Now, I can hear that familiar hiss and some very faint activity on every band. (including WWV on band3, but it was very weak)

I should point out that all of this testing took place in my basement , which has full height ceilings. So, basically I'm underground and using a 6 foot hook-up wire antenna. Previously, when I was reporting some completely dead bands, I was on the first floor of this house. That was before I found and corrected the A2 connection error.

Now, I need to decide what to do next. I was thinking that I might try a basic alignment. I did have to disturb lots of wires in the RF section and had to unwind one of the gimmick caps, so I'm sure the tuning is way off.

More updates soon.

Best regards,

Frank


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