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National MB 40 SL




 
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Author Topic: National MB 40 SL  (Read 7085 times)
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W4RFM
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« on: December 29, 2008, 12:35:59 PM »

I have always wanted to build a transmitter with one of these and I finally got one. Now, how do you hook it up and use it? I received no instructions with it.  I am guessing the connections are made on the terminals that come out the back of the coil area.  Any help will be appreciated. I looked in the old ARRL handbooks and see there is an "in" and a bias connection and an output connection, but where is it, and once it's connected what do I do? Man I feel like a dumb a**!
Bob
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BOB / W4RFM  "I have looked far and wide, (I also checked near and narrow)"
w4bfs
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« Reply #1 on: December 29, 2008, 01:03:35 PM »

when in doubt, reverse engineer .... make schizmatic .... mo' fun   73   John
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Carl WA1KPD
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« Reply #2 on: December 29, 2008, 03:49:14 PM »

Bob,
Here is a schematic with an amp using one
http://www.wb4gwa.netfirms.com/final.htm

Carl
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« Reply #3 on: December 29, 2008, 04:12:14 PM »

Bob,

My brother Bill (W2WM) found data sheets and application notes for the National MB-150 and scanned them to .BMP files and emailed them to me.  I would be happy to forward them to your email if you would like.   I looked at the schematic on the MB-150 data sheet, and it looks identical to the MB-40 that Carl posted, except the MB-150 has settable taps on the link coil. 

We are not sure of the difference between the MB-40L and the MB-40SL you reference, however Bill also located a detailed advertisement  in QST Dec 1950 page 77 describing the MB-40L in detail.

Hope this helps, let us know if you would like the email.

73,
Rick  W8KHK
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Rick / W8KHK  ex WB2HKX, WB4GNR
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W7XXX
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« Reply #4 on: December 30, 2008, 09:48:43 AM »

ARRL Handbook 1954 page 196 has 6146 amplifier using MB 40 SL. Another handbook in the 50's had an 807 transmitter using the MB 40. It must have been 1956 as I am missing this handbook and none of the other 50's have it. SL just means swinging link as opposed to the MB 40 that had fixed link. I have had several of these and built neat low power xmtrs. I have seen several homebrew rigs using these in the grid circuit as shown in a post above.
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w8khk
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« Reply #5 on: December 30, 2008, 12:59:17 PM »

ARRL Handbook 1954 page 196 has 6146 amplifier using MB 40 SL. Another handbook in the 50's had an 807 transmitter using the MB 40. It must have been 1956 as I am missing this handbook and none of the other 50's have it. SL just means swinging link as opposed to the MB 40 that had fixed link. I have had several of these and built neat low power xmtrs. I have seen several homebrew rigs using these in the grid circuit as shown in a post above.


Sam, I located the article you referenced in the 1956 ARRL HB page 186-193.  It is a 75-300 watt rig, with vfo control.  The MB-40 is used in the plate circuit of a 5763, driving a 4-65A.  Thanks for clarifying the "SL" designation.


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"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan
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« Reply #6 on: December 30, 2008, 01:04:14 PM »

I am finding some of the prints you talk about, but I still have the question: where is the connection to the grid of the final, and where is the connection to the bias?
The way the black wires connect everything together, I am not sure just from the schematics.
Most times I cant see the forest for the trees.  This is one of those times. Thanks everyone.
Bob
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w8khk
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« Reply #7 on: December 30, 2008, 01:08:13 PM »

I dug out my MB-40s.  One is a standard MB-40, with a fixed link, the other is an MB-40DL with a swinging link, and somewhat larger coils. 

I will photograph them if necessary, but let's try with just text first.

If you look at the schematic of the WB4GWA amplifier referenced earlier in this thread, you will see the rotors of all variable capacitors are tied together, but not grounded.  The frame of the MB-40 has the rotors grounded to the frame, as usual for a variable cap.  The 'GWA schematic shows an RF choke from the center of the main (rear) inductor to the floating capacitor rotors.  To connect the tuner this way would require the tuner to be mounted on porcelain stand-off insulators, or similar hardware.  It is not necessary to float the capacitor to apply bias.   The RF choke need not connect back to the capacitor rotor, simply connect one end of the RF choke to the center tap on the main (rear) inductor, bypass the other end of the RF choke, and apply bias to the bypassed end of the choke.

I noticed MY MB-40 has a connection brought out for the center tap of rear inductor, but the MB-40D with swinging link does not have a center tap brought out to a connector, it just loops from one half of the inductor to the other half, with no connection.  If this is the case with your tuner, you may either connect the bias RF choke to the center tap by removing the insulation, or connect it to the opposite end of the coil from the RF output. 

Looking again at the 'GWA schematic, the left pair of variable capacitors is to the rear of the MB-40, and the right pair of variable capacitors is in the front of the MB-40.   The grid of the driven amplifier is fed from the stator of the rear-most variable capacitor section.   If you are using bridge neutralization then the NC capacitor should be connected to stator of the second capacitor section from the rear (not as shown on the 'GWA schematic).  This would be the bottom left variable in the schematic.

It should be obvious where the link input should be coupled.  Some MB-40s have only one link, others have another link in parallel, coupled to the other main inductor.

Hope this info is helpful.  If not clear, I will try again, or add photos.  By the way, I will be at the Lawrenceville GA fest on Januar 10, perhaps an eyeball QSO there?

73,
Rick

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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan
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« Reply #8 on: December 30, 2008, 01:44:00 PM »

Now I got it! Thanks Rick, you are very kind and very patient.  My wife said last night "after 30 years of radio station engineering, I'd think you would know all of that!"
Well, I do now. 
Thanks everyone.  Hopefully the next post will be when I fire it up.
Bob
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BOB / W4RFM  "I have looked far and wide, (I also checked near and narrow)"
w8khk
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« Reply #9 on: December 30, 2008, 01:49:47 PM »

Happy to help, Bob....   Looking forward to hearing that rig STRAP!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan
w8khk
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« Reply #10 on: December 30, 2008, 02:38:41 PM »

Just a bit more interesting info on the MB-40 and MB-150 series of tuners....


* mb140data.jpg (591.59 KB, 1334x2005 - viewed 796 times.)
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan
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