Nakamichi CM-300 Conversion to P48

[tchgtr]

  Some friends think I'm a frikkin' genius because I'm fearless about dismantling things, and tenacious about putting them back together in working order. After they saw my C12 clone build, they asked me to phantom power some old Nak CM-300s that had been lying around unused because finding and replacing batteries is so inconvenient.
  We decided to try just one first and see what we thought. I had previously resurrected some EV mics for a friend based on a circuit found online. A link can be found in the thread below, which was my starting point.
We start with a wonderful little circuit supplied by PRR in this thread:
http://www.groupdiy.com/index.php?topic=51383.msg653143#msg653143

    As always, I'm grateful for the support and help from this community, and PRR discounted a lot of the BS found online for these mics, based on his own experience with his own set of them, and his vast knowledge of electronics.
    This thread is my way of saying thanks and helping the next poor sucker who tries this, and finds this thread.

 

[tchgtr]

When I dismantled the mics, I decide to leave the switch soldered to the circuit board, as it's mounting screws hold together the casing of the mic, and provide support for the PCB and housing.
    I also decided to use the PCB for my own purposes, and removed all parts EXCEPT the transformer and switch. It wasn't too hard to find traces that would work for this simple circuit, tho I made sure to cut them between the solder joints and the switch, so there would be no surprise voltages later. In other words, the switches do nothing after the mod is finished.

PARTS LIST
1 4.7 uf cap
1 47uf cap
1 9.1 V Zener
1 2k2 resistor
2 inches of 3/4 inch dowel (smaller dia. is better)
2 nickel thumbtacks
3 inches of wire

Bought new capacitors, but used the 2.2K resistor that was already in the mic.
3/4 inch dowel is actually too thick, and I had to do a bit of sanding to reduce the diameter so it would fit the mic body.

    First, dismantle the mic by removing the two screws at the bottom of the tube near the xlr connector. You should be able to pull that end of the mic apart from the rest, but here is a ground tab that fits in a recess, and must be pushed with a pointed object of some sort to release the tube.
  There will still be three wires connected between the XLR housing and the PCB, so don't pull it apart to quickly.
  You must de-solder the battery spring from the circuit board before removing the two screws in the switch to take apart the housing and get to the PCB. As mentioned before, I desoldered everything but the transformer and switch from the PCB.
  Sorry about the blurry photo...

 

[tchgtr]

Using the traces on the PCB, I soldered the two caps between the  switch and transformer. You can see the zener diode soldered between the legs of the 47uf  cap.

 

[tchgtr]

I grabbed the P48 from the point where the two wires connect to the transformer, and used 1% resistors soldered to the underside of the PCB, where they can connect to the junction of the zener and the + side of the 47 uf cap. One resistor is hidden inside the shrink-tubing. From there the 2.2K resistor takes the juice to the capsule via the dowel.

 

[tchgtr]

  Here's a photo of the dowel with a hole partially drilled thru, so a wire can connect the two thumbtacks. The wires soldered quite nicely to the nickel tacks.

 

[tchgtr]

Photo of soldered thumbtack.

 

[tchgtr]

It all went back together neatly , but be sure that the (ground?) tab is in it's proper position, and solder the battery spring last, maybe after you've hooked it up to P48, and checked for proper voltage.

 

[tchgtr]

In this case, the mics had been neglected for years, and were in doubtful shape. I did a lot of cleaning to the -10 pads inside the bodies of the mics, under where the capsule holder screws on, as well as all the contact points between the capsule and pads, and where the head amp connects to the battery.
  One of the capsules was stuck in it's holder, but some light oil and careful unscrewing with pliers over a thick cloth got it off without any damage. Another capsule was rattling inside it's enclosure, and I unscrewed the ring holding it in, checked it out, and remounted the capsule. Seems to be fine, but work carefully, as the capsule might get damaged when out of it's housing.
  I ran a dry toothbrush over all the screw threads, so hopefully, it will be easier to screw and unscrew the capsule, body, head amp, etc..
  Here's a photo of the completed project, ready to be screwed back together. You can barely see the wire running thru the hole in the dowel.
  The owners would like to hear some sound samples, before giving the OK to convert the last two bodies, so I will try to put some files up when I get those done.
  First impressions are good by my own ancient ears, and the selection of capsule patterns is nice.
  Thanks again to everyone here. It's because of you guys that my friends think I'm so smart.

 

[reubenm]

nice werk. how does the mic run? looks like there was no phantom filter network-does it run without humming?and can i ask what is the console/interface your using? thanks,reuben.

 

[tchgtr]

Hi Reuben,
    Here is a recording made with the microphone and a cardioid capsule. It's going thru a stock Symetrix SX202 preamp, and there is no compression, other than dithering to 16 bits, and turning up the volume a bit.
  Mic is placed above the neck where it meets the body, about 4 inches away and pointed toward the sound hole.
  The pre is turned up to around 90% on the gain control.
  These mics use electret capsule elements, and sound pretty good. It's nice they can have a 2nd life. I have 3 CADs and two modded RS PZMs in my collection which I like a lot, so I would recommend these to anyone who finds a deal on a set, especially if all the capsule are intact and working.
Thanks for reading and commenting.

http://soundcloud.com/tchgtrst/cm-300

 

[reubenm]

what a gem... nice job sounds better than a neumann TLM to me.

 

[tchgtr]

    Long story short---to make the sound clip above, I randomly grabbed the cardioid capsule that was in the worst condition out of the 3 available with this set of mics.
    Here are some new ones:
http://soundcloud.com/tchgtrst
    They are labeled "Cardioid 2" and "omni". The other examples lower down with the odd titles are from an earlier blind test of RK-12 capsules from an older thread. Same guitar, tho.

  After noticing the "fuzziness" on the decay of the final chords in the first example, I checked the contact points between the capsules and the head amps. Sure enough, the one capsule I had picked at random had corroded plating on the contact point. The other capsules were much better, and I cleaned all contact points gently with the eraser of a pencil.
    You can still hear the noise floor come up on the final chords, but it's much better than in the first example.

    Would love it if anybody has any suggestions for repairing the worn plating on the damaged capsule.