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 Post subject: National SW-3 Thrill Box
PostPosted: Sep Sat 11, 2021 8:53 pm 
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Location: Wood River, Ill.
I recently acquired this sample of a National SW-3. How can one avoid something they call a Thrill Box?
Attachment:
outside.jpg
outside.jpg [ 1004.56 KiB | Viewed 765 times ]

I am just not quite sure of what I have. I have been looking at old posts and doing some other searches via Google, and have downloaded just about everything I can find for this model. It has at least one #58 that is marked, I assume another #58, though it isn't marked, and a #27 installed. The power cord has a 4-pin plug installed. This is leading me to believe that it is for use with an AC supply for 2.5 V filaments.
Attachment:
INSIDE.jpg
INSIDE.jpg [ 499.16 KiB | Viewed 765 times ]

Looking on the underside there is a very large wire wound resistor across the back side. The power cord, while having a 4-pin plug installed, appears to have 5 leads inside it.
Attachment:
UNDERSIDE.jpg
UNDERSIDE.jpg [ 1 MiB | Viewed 765 times ]

It came with two very nicely built home brew coils. There are no trimmers installed inside of them, so again am making the assumption that these are general coverage coils, with frequency range to be discovered.
Attachment:
COIL.jpg
COIL.jpg [ 469.02 KiB | Viewed 765 times ]


Inside there is a rotary switch, with a long shaft in the left rear corner of the chassis. There are also two 1/4" phone jacks on the back side of the box. One is connected to the audio output terminals and the other to the antenna terminals. I am not real sure what the long shaft switch is supposed to be for, but it is connected to the power cord, ground, and one end of the long wire wound resistor.

All the components appear to be original, or at least correct for the time period. The resistors are the National built ceramic forms with the molded lead caps with wire leads. The capacitors also appear to be period correct. I can't see where anything has been replaced or other work done, except perhaps around the wire wound resistor. The phone jacks may have been added, but the work was very good, and all looks to be factory wired to my untrained eye.

The overall condition is like new. No dust on the inside, and a little on the outside. The paint is very good with a few scratches here and there. To be honest, I am tempted to leave it alone. If I can find or build a suitable power supply, I would like to try it out. I do have an ARBEE III and wonder if anyone has used one with these. I think I can set the filliment voltage to 2.5 V and get a B+ of 135 volts out of it.

I think it would be interesting to get a "Thrill" out of this one and see if it operates as advertised.

Any insights would be appreciated. It appears that there is a pretty good following of these little gems here.

Thanks for looking.

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Chris
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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sat 11, 2021 9:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: S. Dartmouth MA 02748-1225 USA
Why would you want to avoid the "Thrill Box"?

The images suggest that this radio has been refurbished to the ninth degree or sat undisturbed for many years, still not enough info to confirm that.. It sorta looks too good.

These should be tube numbers engraved into the sockets and they "should be" riveted in. If the tube numbers suggest 2.5 volts or 6 volts that will determine which "Dog House" power supply is correct. The National power supply has slightly over-voltage filament supply as the cord has voltage drop, National accounted for that.

The radio should use one audio coupler, looks like a potted audio transformer. This has an internal resistor, capacitor and a choke. It has a very high impedance for the detector load, one of the essentials for The SW-3's fame. There are numerous other national innovations in the SW-3 that made it the most longest manufactured radio in history, sort of the Volkswagen of radios...

There are several web sites with extensive data on the SW-3, including coil charts.

https://www.prismnet.com/~nielw/sw3coils.htm

https://www.antiqueradios.com/forums/vi ... p?t=132217

https://www.qsl.net/wa9wfa/ha00008.htm

http://www.antiqueradios.org/gazette/SWevol.htm

The [67] in the base of the form suggests the lower frequency portion of the broadcast band.

Only certain coils in the Ham bands are band spread and will have a trimmer and a grid cap inside the form for tracking. The "67" also suggests that a 58 & a 6D6 are correct, but it depends on the OEM socket label...

Chas

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Last edited by Chas on Sep Sun 12, 2021 1:34 am, edited 1 time in total.

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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sat 11, 2021 11:22 pm 
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Joined: Nov Sun 22, 2015 4:51 pm
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Chas is handling the possible upgrades; they're merely challenges. :-) It looks like the original coupler -- which included a high-value audio choke, coupling capacitor and grid resistor for the set's audio stage, I think -- is still in place. When the potted-in coupling cap started to leak, those replacing the coupler or otherwise working around the leakage sometimes got creative.

That said, so far as I know, the SW-3 was not referred to by National as a "Thrill Box"; that was the contemporaneous SW-5, as the attached picture of a QST SW-3 ad -- possibly the first in QST, from page 2 of the October 1931 issue -- is careful to differentiate right off.

Attachment:
national_SW-3_ad_QST_October_1931_p2.jpg
national_SW-3_ad_QST_October_1931_p2.jpg [ 456.72 KiB | Viewed 730 times ]


An article describing the SW-3's electrical and mechanical design had appeared in the previous issue as James Millen, W1AXL, "A Combination A.C. and D.C Amateur-Band Receiver," QST, September 1931, pages 9 through 16.


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 1:07 am 
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Location: Tucson, Arizona U.S.A.
The SW-3 used plug-in coils with a proprietary pin arrangement. Your set has a standard 6 pin coil form. Therefore your unit has been modified.

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Who's that swimming in the punch bowl?
It's Walter Wart, the freaky frog!


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 1:13 am 
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Location: S. Dartmouth MA 02748-1225 USA
If the coil pins are all the same size and oriented with a group of three to one side and the other, it is a National coil form.
Quote:
The SW-3 used plug-in coils with a proprietary pin arrangement. Your set has a standard 6 pin coil form. Therefore your unit has been modified.
Why would one modify a set then automatically exclude all other coil combinations?

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Last edited by Chas on Sep Sun 12, 2021 3:58 am, edited 2 times in total.

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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 1:22 am 
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It doesn't have to be called the "Thrill Box" to be Thrilling! :D It was and is a regenerative set, pulling in exotic far away signals...Thrills galore!!! There you are with old high Z cans on your head and a long random wire out beyond the house, tuning around late at night and ignoring everything, such as your wife holding an oil lamp in one hand and a clock in the other. Election returns, football scores, glow in the dark crosses for sale, the Mercury Theater of The Air...who wants to sleep?

Now, notice how the cabinet is painted on the insides. Yep, you're supposed to lift the lid and look in there and be amazed, and show off the parts and craftsmanship to your envious friends. Also, it helped prevent rust.


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 1:28 am 
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I have an SW-3 w/doghouse, SW-3 DC and a SW-5, I find no difference in operation of the SW-5 w/doghouse and speaker, it shares or can share the same coils.

I have duplicate coils sets but will only swap for what I do not have and that is BC and long wave coils in my series radios.

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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 1:29 am 
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Joined: May Fri 01, 2015 2:33 am
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Location: Roslyn,Pa 19001
I'll try to address a few of your questions. I think your set started out in life as a 6vdc model and got rebuilt/restored to an ac model. AC sets had 4 wire cords DC sets had 5 wire cords, extra wire is for the screen voltage. Would have had 2x36 tubes and a 37 tube to be powered from a 6v battery and B battery pak or vibrator supply. Didn't have the big voltage divider resistor at the back of the chassis.

Frequency wise there is a number written on the bottom of the coil this is the key to finding the fq. https://bama.edebris.com/download/natio ... 3coils.pdf Looks like it sez #67 that's the lower half of the BCB.

Don't think it's the original wiring. Looks too large in diameter. The caps and resistors (- the divider) under the chassis look original. The metal can in the center has two bypass cap one is for the cathode of the output tube the other is the screen of the det. That one can give a lot of grief as it can cause the regeneration control misbehave if leaky.

The switch w/the long shaft was the off/on switch for the dc model. In the on position it grounds the B- and the gain control network to the chassis. This preserves the B batteries. The phone jacks are added. Originally the older sets had thumb screws on the top of the chassis. Later on it had a receptacle like the HRO that would except two phone pin connectors.

As for the tubes I wouldn't fool w/the 2.5v ones cuz you'll need abt 5 or 6 amps to power them. Grab a couple of 6D6's and a 76 or 37 and your good to go. That's only 900ma. Don't forget to swing the little bracket over and switch out the dial lamp out to a 6v one. You might want to check the dial scale to see if it's the proper one. Should be 0-199 going counter clockwise.

I think I touched on most of your questions.

Terry


Last edited by Radioroslyn on Sep Sun 12, 2021 2:46 am, edited 1 time in total.

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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 2:20 am 
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[/quote]Why would one modify a set then automatically exclude all other coil combinations?[/quote]
Someone probably got the set without coils, couldn't get any, and did what he had to do to get it working. Isn't that what the people on ARF frequently do? It's the difference between a usable piece of equipment and a partially complete (coils missing) shelf queen.

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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 2:42 am 
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I was told by an old timer that when the SW-5 was being designed 1929-30 time frame that there were no 6 pin tubes yet. So the 6 pin National coil forms are not so much proprietary by design but by necessity.

The SW-3 was a cost reduction model of the SW-5 not having the power audio stage. The oldest 6 pin tube I've run across dates abt 1932. That was a 2A5. Were 6 pin tube as we know them in uses before 1930?

I can't tell from the pic of the coil in question if it's 3x3 or 2x4.

Terry


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 5:18 am 
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Nice "Thrill Box". Yes, National used the "Thrill Box" moniker for several of the predecessors to the SW-3 initially because of the "Thrill" of DX shortwave reception but they also it on occasion to describe the SW-3 likely because it stuck. Here's the Chicago Radio ad where they used the name in Radio News 12-1931. But here's also an ad from National itself in 7-33 Radio Craft.


Attachments:
Chicago Radio ad for SW-3 and SW-5 in Radio News December 1931.JPG
Chicago Radio ad for SW-3 and SW-5 in Radio News December 1931.JPG [ 100.15 KiB | Viewed 667 times ]
SW3 ad by National in July 1933 ShortWave Craft at its lowest 'list' price, Note 'tooling and engineering costs now written off'.JPG
SW3 ad by National in July 1933 ShortWave Craft at its lowest 'list' price, Note 'tooling and engineering costs now written off'.JPG [ 108.82 KiB | Viewed 667 times ]
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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 5:31 am 
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So "thrill box" wnas advertising?

But "rush box" was applied to superrenerative receivers. That doesn't seem to be marketing.


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 6:27 am 
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Terry said
Quote:
I was told by an old timer that when the SW-5 was being designed 1929-30 time frame that there were no 6 pin tubes yet. So the 6 pin National coil forms are not so much proprietary by design but by necessity.


The old timer was correct. The SW-3 coils use a unique pin spacing, three pins on one side and three on the other. The gap between the sets of three is greater than the gap between the pins in each of the three. The coils you show were very well made but if they fit, the sockets were changed (as well as the tube sockets).

The one 58 tube socket is 90 degrees rotated from the other. Mine has both in the same orientation. As has been said, the set is very likely a conversion from the battery model, but whoever did the work knew what they were doing. The original also used pin jacks for output, not a phone jack. Those pins are hot with B+.

On mine the only capacitor that needed replacement was the filter for the regen control. In operation it only has about 20 volts or so on it. I did not replace that cap section, just disconnected it and hid a new one as best I could.

-- Rich


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 5:03 pm 
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Thank You all for the responses. It is appreciated. It may take me a while to digest all the information given. Since my last post, and after reading a few of the first responses, I have had the opportunity to look things over again. What I have found is that the two coils are in the 3x3 spacing format. The long stemmed switch is in the B- circuit, which connects the chassis and the wire wound resistor to the power terminal #3. Power plug pins #1 & #4 are connected directly to the tube filaments. (No switch) The tuning dial is marked "0" to "190". The audio output and antenna terminals are still there and connected, so the 1/4" phone jacks are connected parallel to the appropriate terminals.

All of the tube and coil sockets are held in place with screws and nuts, no rivets. The only tube socket with any kind of marking is the 227 socket in the audio stage.

Attachment:
InkedINSIDE_LI.jpg
InkedINSIDE_LI.jpg [ 504.58 KiB | Viewed 633 times ]

One other curiosity is the two insulators that are attached to the shields, circled above. I have not seen them in photos of other models, so am guessing they were added, but don't know why. The tube grid caps will fit onto them, but don't know why someone would want to do this, unless they were to be used when the grid cap was attached to one of the coils. Perhaps used as a junction point?

Terry: Thanks for answering my questions, as this makes a lot of sense. I will try the tubes you suggest, (I may have some of them). I was thinking that the 2.5 volt tubes in there might be more than the ARBEE III could handle.

I think what I am going to do is to just try it the way it is, with the 6 volt tubes, as Terry suggests, and see how it works. Like everyone has said, the work done looks to be very good, and I don't want to change anything, unless necessary. Will see how much Thrill I can get out of it.

Thanks again, all of you! :D

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Chris
N9WHH


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 5:31 pm 
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Location: Roslyn,Pa 19001
The insulators are for the grid leads when you use the bandspread coils. Grid leads from the coils connect the the tubes and the ones from the chassis slip onto the insulators. Originals were riveted in those look like they might be replacements, the whole thing not just the screw. I'm a little puzzled by the 227 engraved on the audio socket. Is it riveted in? With the switch it should say 237. The 227 would have been used in the SW-3 AC model.

A mid '30s modification if you were using 6v tubes was to swap the the output tube for a 38. It's a power pentode that will make 2.5w max 250v 10k load and it has a 5 pin base. A few wire changes, an opt and your speaker ready.

Terry


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 7:12 pm 
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It's not that difficult to build a power pack for a 2.5 volt model. You can do as Millen did, use a transformer that has a bit higher voltage with the umbilical cord length dropping the volts to what was needed. Had to do that for one of my FB-7 sets. Used the center tap on a 6.3 VCT transformer and added a bit of zip cord size wire to the supply to get the volts down to where the pilot light saw the right voltage. It doesn't take much extra wire to get the volts just right.

Millen wrote a column ad in QST after the FB-7 came out telling hams not to use off-the-shelf 2.5 volt filament transformers for a homebrew supply as that was inadequate due to the umbilical voltage drop, causing the oscillator to drop out.
The SW-3 coil forms are no longer made. If I needed more coils, one option would be to make a pair of adapters using banana plugs to 6 pin tube sockets, a non-invasive fix I used on a different regen. Then banana pins could be free-floating as the original coil sockets would hold them in place.

What Terry said: Use an OPT and an outboard speaker and you will be set with that coil for part of the BC band. Here's a pic of mine. Have fun!

-- Rich


Attachments:
SW-3 with power pack and homebrew speaker, top lid open .JPG
SW-3 with power pack and homebrew speaker, top lid open .JPG [ 672.78 KiB | Viewed 612 times ]
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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Sun 12, 2021 8:05 pm 
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Location: Roslyn,Pa 19001
Or if you like woodworking...
https://www.flickr.com/photos/42774321@N08/3939255767/
https://www.flickr.com/photos/42774321@N08/3940050414/

Terry


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Mon 13, 2021 12:08 am 
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Again, many thanks for the replies. It appears that this little receiver has many possibilities. It's like reading a murder mystery, with it's many trips and turns, a real thriller. (Pun intended.) I added a few photos this afternoon.

Terry, I have to make a correction: the audio output tube socket is marked 237 not 227. I misread the socket, due to bad lighting. :oops:

Attachment:
58 SOCKET.jpg
58 SOCKET.jpg [ 448.04 KiB | Viewed 579 times ]

Here I took a shot of the RF amplifier socket, where a type 58 tube is now installed. There are components above the chassis, with several mounted right on the socket. The tube shield has been trimmed to fit around them. This was my first clue that the radio had been modified. The trimming of the shield does not look like something that the factory would do.
Attachment:
SHIELD.jpg
SHIELD.jpg [ 407.82 KiB | Viewed 579 times ]


The detector coil has a Fahnestock clip on the top side leading to a tap on the outside (red) coil which is wound over the green coil.
Attachment:
DET-COIL.jpg
DET-COIL.jpg [ 540.12 KiB | Viewed 579 times ]


The detector coil socket and unknown, (although assumed to be another type 58) detector tube.
Attachment:
DSCN0471.JPG
DSCN0471.JPG [ 563.25 KiB | Viewed 579 times ]


Lastly is the main tuning capacitor, which is one piece two section, as are supposed to be on the later series SW-3 models, as I understand it.
Attachment:
DSCN0470.JPG
DSCN0470.JPG [ 578.52 KiB | Viewed 579 times ]

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Chris
N9WHH


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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Mon 13, 2021 2:01 am 
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Location: Roslyn,Pa 19001
Your 1st pic is of the det stage and doesn't use a tube shield or the base to one. As you might notice the regeneration control is right in front of it. The resistor and mica cap is the grid leak.

It's shame someone buggered up a good shield cuz they didn't know what they were doing. They are kinda hard to come by.

As for the BC coils I have a pair BUT one of them is a original (orphan) and the other is repro. The det and rf coils are different I think. The original one as a mica cap inside. I wound the other (rf coil) on a new form as per spec and it worked fine. It had a good peak across the whole range. The original det coil was fine. Do bear in mind the tuning cap is 90mmfd so it's range at lower frequencies is somewhat truncated. The good new is that instead of a normal 180 deg of rotation it has 270 deg. Even without the bs coils station are further apart.

Have no idea what the fahnnstock clip is for. I would ignore for now . Don't think it's a deal breaker.

Your next pic with a shielded tube and 1 mica close to the chassis is the rf amp stage.If you look at the front panel there's a trimmer cap in line with the tube. This is the antenna trimmer used to peak the ant. Between it and the regeneration control is a thin thumbwheel knob. This is the rf gain control.

The early sets did use those two separate tuning caps ganged with a coupler. On the tuning cap it's important that the rotor wipers are clean and make good contact. Just like any osc circuit.
Most of those had wafer style coil and tube sockets. If I had to guess I would say pre 1933 or so. Here's a pic of an early chassis: https://www.qsl.net/wa9wfa/Early%20SW-3 ... 20view.jpg Your set has the mid production coils sockets (oval) late model had square sockets.

Terry


Last edited by Radioroslyn on Sep Mon 13, 2021 2:31 am, edited 1 time in total.

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 Post subject: Re: National SW-3 Thrill Box
PostPosted: Sep Mon 13, 2021 2:30 am 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1795
Location: Roslyn,Pa 19001
Here's some pic's of the #17 coils. Same as the 67 coils just for the earlier models, has more turns in the slot winding.

Terry


Attachments:
#17 coils frt.jpg
#17 coils frt.jpg [ 159.09 KiB | Viewed 566 times ]
#17 coils top.jpg
#17 coils top.jpg [ 138.38 KiB | Viewed 566 times ]
#17 coils bottom.jpg
#17 coils bottom.jpg [ 208.97 KiB | Viewed 566 times ]
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