Conversion from C12 to ELA M251 build photos

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shot

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Friends,

This is my step-by-step pictorial on building microphone pcb for ELA M251 from Chunger's C12 kit.
I went to build it and figured I can test my new camera and document every step. I hope that is fine with creators of this kit.

All the credit goes to Matador & Chunger for development of the C12 kit
and Category 5 for guidelines in adopting ELA M251 circuit to C12 board
I just built it and took some photos :)

This pictorial covers only the microphone pcb!
Please follow the build guide for C12 on how to assemble the capsule and transformer
https://groupdiy.com/index.php?topic=51377.0

and Universal Passive Tube PSU build guide on how to assemble PSU for this mic
https://groupdiy.com/index.php?topic=58054.0
Please note that ELA M251 doesn't require bias from the power supply so you can omit that wire

Refer to the conversion schematic
http://musicalsparks.com/images/251/m251_ht11a_schematic_only.pdf

If you prefer not to hunt for single components by yourself, be sure to visit Chunger's shop store.studio939.com and get
- C12 kit AND ELA M251 conversion kit
- OR just one ELA M251 kit (listed in C12 dropdown menu)
(you don't need both)
Conversion kit contains extra components on top of C12 kit needed for your ELA M251. You'll be left with few components from C12 but I'm sure you'll use them someday :)

------------------------------------------------------------------------------

First step is to clean the boards with isopropylene before any soldering.
It is highly recommended to clean after every soldering step (but watch out not to go near styroflex caps with isopropylene!)
9i5l3l.jpg




Cut the trace between RC turret and the one below it
6jewk8.jpg




Check with DMM that there is NO CONNECTION after cutting the trace!
261nl20.jpg




Clean again (there will be a lot of debris from cutting the board so be obsessive about it)
n5ist4.jpg




Mount only selected turret pins for now.
You will mount other turrets later
x5usnk.jpg




Flip the board using peace of cardboard box to make sure turret pins don't fall down before soldering
osqsqr.jpg




...and solder those pins
zvofus.jpg




Clean again
155jxad.jpg




Start by soldering resistors
Follow the schematic for ELA M251!
R15 & R16 are 30M and R14 is 8M.
Don't go by silkscreen values since they are for C12
2h7qknk.jpg

Please note that I forgot to mount two turret pins (red arrows)
and notice the blue arrow pointing to the trace cut (you already did that few steps before, right?)



I corrected my mistake and soldered two more turret pins
3145bhd.jpg




Now solder two more capacitors. They are non-polarized so orientation doesn't matter.
Notice that styroflex cap is to be mounted on the outside. Make sure it's not touching turret pin since we will be soldering to this pin and the heat can damage it.
hwznrp.jpg




Flip the board and mount five more pins
2h6en9j.jpg

You will have two unused pin holes - that's alright!




Use a resistor leg leftover and shape it like this...
20hr2nk.jpg




...and solder it in this position
fda93a.jpg




Now you can solder the styroflex cap
2ykgtpc.jpg




Use tweezers to solder this small smd resistor on the tube pcb
(it's super simple - don't worry if you haven't done smd yet - use regular soldering iron and keep your hand steady)
14o2dlj.jpg




Solder the tube socket in it's place
6fb2pd.jpg




and solder the tube pcb to the main pcb
Be sure to solder all the pads! They are not only here to hold the tube pcb but they also carry different signals.
21ditfa.jpg




Solder one more wire between pins
20qnvae.jpg




Prepare one small wire
2m4rvkg.jpg




Solder that wire from main pcb to the TP1 on tube pcb
68ya9y.jpg




Prepare next wire. Make sure you use insulated wire!
2e67yab.jpg




Solder it in this position. Try to bend it to be as low as possible since it'll be crowded around here
257fyon.jpg


You are using your brush and isopropylene after each step, right?


Mount a next capacitor
2rhbvde.jpg




Take a closer look to this pic!
Make sure you bend this resistor's leg near the tube pcb. It should be as close to the boards, but not too close to touch the pads!
It was very hard to focus the camera on this one - the leg of the resistor should connect to TK1
293vlua.jpg




Next you solder the capacitor to the grid (mid turret pin)
2uonxfm.jpg




I've lost focus on camera for this one!
Electrolytic cap is to be mounted from out turret to mid turret.
CHECK POLARITY for this capacitor! Plus to TK1 (or to the resistor leg that connects to TK1), minus to outside!
2a5ir9e.jpg




Mount the transformer and tube, mount the capsule and solder the capsule wires!
Note that backplate wires go to the same turret pin!
(RC = rear capsule)
2z6wi0o.jpg




THAT'S IT!

Now you have ELA M251


:)

Luka

p.s. did I mention that it sounds fantastic? !!!!
 

shot

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Joined
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Messages
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Location
Zagreb, Croatia
bruno2000 said:
What about C2, 22uF 6V?
Thanks!
Best,
Bruno2000

Maybe you missed it... it's in parallel with 1K8 resistor.
That one is on the blurry photo in my build guide... the brown one...


2a5ir9e.jpg

 

bruno2000

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Location
Atlanta GA
shot said:
Maybe you missed it... it's in parallel with 1K8 resistor.
That one is on the blurry photo in my build guide... the brown one...


2a5ir9e.jpg

But isn't ths mid term "PLATE"?
cap goes to CATHODE?
TNX!
 

shot

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Joined
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Messages
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Location
Zagreb, Croatia
Bruno2000 THANK YOU! YOU HAVE SPOTTED A MISTAKE IN DESCRIPTION!

Actually I have soldered it correctly but when I was writing descriptions I was referring to photos. And since this one was blurry I assumed wrong! I'll change the original post not to make any confusion.

So... positive leg of the resistor should go to TK1 position (or to the leg of the 1K8 resistor)

I'm sorry I made this mistake!
:eek:

Luka
 

bruno2000

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Messages
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One other question that I have that I don't recall seeing addressed is polarity.  The transformer that I have does not differentiate the top and bottom of the windings, so there seems to be no way to tell (other than with a scope).  I just plan on wiring it all up, then comparing with another commercial mic, and swapping the connections of the primary if needed.  Other suggestions?
Thanks!
Best,
Bruno2000
 

shot

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Joined
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Messages
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Location
Zagreb, Croatia
In my case, I went with Cinemag CM-13114 transformer.
Cinemag has all datasheets on their web so I referred to their data.
It's no brainer when they list you wire colors that correspond to connections.
I didn't document that in this thread. I've focused on pcb assembly. But yeah, maybe I should have done few photos of transformer and it's wiring.

If you're using other transformer it can be PITA finding out pinout by yourself...

:)

Luka
 

bruno2000

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Finished my 2 builds!  However, I burned up one of my 1nF styroflex cap so I subbed in a 1nF C0G cap, and that mic is about 6dB louder than the one with the styroflex!  Anyone have a suggestion as to the reason why?

Just to make sure, I swapped out the good styroflex for a C0G, and the mic outputs are now the same!

Thanks to Shot for the photos!  Mine were the first revision boards, so there were additional mods, but they're both working!!!!!!
Thanks~!
Best,
Bruno2000
 

shot

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Joined
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Messages
639
Location
Zagreb, Croatia
Oh! I didn't notice that tinypic service used in this thread is gone!
I'll try to find those pic and reupload them
:)

Luka
 
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