Topic: Nakamichi CM 300 housing question

[weedwacker]

I want to build some custom mic body housings for some Nakamichi CM 300s and need to find out the following if anyone here can help out.

1. Does anyone know what thread size is used for the attenuator housing(is it a standard thread like a pipe thread or a custom thread size)?
2. What is the overall current consumption of a standard unmodded attenuator and switch/xlr connector? I want to attempt to make a shortened mic housing using different batteries than a 9v neda 1611 transistor battery and am looking at possibly using CR1612/1616/1620 3v lithium ion coin batteries and am trying to determine the overall battery life using 3 3v based on the mAh capacity of the batteries. If the original manual is correct the CM-300 should draw less than 1 mA which means using 3 50 mAh CR1616 3v batteries should give me approximately 50 hours of battery life before I need to change them.

[SHINN]

The thread is a M20 X .75  It's not a pipe thread. I'm not sure about the cap, but could measure it at work if you need to know.

[Roger Gustavsson]

2. What is the overall current consumption of a standard unmodded attenuator and switch/xlr connector? I want to attempt to make a shortened mic housing using different batteries than a 9v neda 1611 transistor battery and am looking at possibly using CR1612/1616/1620 3v lithium ion coin batteries and am trying to determine the overall battery life using 3 3v based on the mAh capacity of the batteries. If the original manual is correct the CM-300 should draw less than 1 mA which means using 3 50 mAh CR1616 3v batteries should give me approximately 50 hours of battery life before I need to change them.

I think you can go for three CR1632 (125 mAh). I will come back with more information. In a hurry right now.


Roger

[weedwacker]

The thread is a M20 X .75  It's not a pipe thread. I'm not sure about the cap, but could measure it at work if you need to know.

Do they even make M20 X .75 taps? As far as I am aware and on a quick scan the smallest fine thread metric tap I've found is M20 X 1. And duh on my part I should know better that if it is made outside the U.S. it will be in metric.

[SHINN]

I saw one on ebay for 28.00 bucks.  I was thinking of making a custom housing for my naks at work, but our machines only go to 20mm.  To bad cause the thread could be made easy with a single point tool or a thread mill. The franken mod seems like the way to go for me...with a small battery in the line. The small 1.5v batteries can be stacked up to make 9v

[Roger Gustavsson]

I think the M20 x 0.75 thread has to be made on a lathe or milling machine. I would certainly like to have it done but I have no connections to any machine shop.

Here is a picture of my chopped CM-300 with an internal battery (3 stacked CR1632).



Here is a comparsion of an original and a chopped one with right-angled XLR-connector.




Roger

[SmokinJoe]

So Roger, I'm guessing you just disassembled it, cut off most of the barrel where the battery and transformer go, reusing the thread where the attenuator body screws on.  The only machining was drilling a hole for that screw at the back?  Cool idea.

[Roger Gustavsson]

Yes, you are right! I did the cut just leaving the Nakamichi writing intact, the barrel is 38 mm (1½ inch) long. If I could have a new thread made in the tube that is left over, it can take 3-5 CR1632 in the shorter battery compartment and still be used.

Roger

[SHINN]

Roger, I like your chopped cm 300's !  Do you have to use a 3 pin connector for this setup? Would it be possible, or advisable to use a rca connector (for a 4' cable)? What do you think about hard wiring them without a connector, would the battery go dead cause there's no switch?

I took apart my old 2 xlr into a single 1/8 mini jack cable (for my stock cm 300's). The 1 and 3 pins were joined at the mic end. Then both mic lines went into the single 1/8 stereo jack.  Is this the wrong way to wire the mics?

I want to do either a chop or a Franken-nak mod but I'm not really sure of the pros and cons The only schematic I have shows the cm300 active pre mod and cm300 active post mod. But not the wiring to the battery and deck. It looks like the signal on the post mod doesn't go through the battery maybe?

Thanks for your help

It's snowing like crazy here in ct!!

[Roger Gustavsson]

Hi SHINN,

You can use a 3 pin connector. Pin 1 and 3 can be connected inside the female XLR, acting as a power switch. Without a switch, the batteries will die quickly. As there is no capacitor in series with pin 2, the mic preamp must be AC-coupled! I think most are.



I am using a 4 pin connector, in case I want to make the output impedance balanced. Just a resistor between pin 4 and ground. I have not tried it yet as my preamp does not have differential inputs.

You can also hardwire the chopped ones. Just make a suitable insert with a clamp and lock it with the small screws that normally holds the XLR insert. I am not in favour of RCA connectors, I prefer locking connectors that stay in place.

Roger

[runonce]

Here's my TEAC ME-80s that I had chopped down at my local gunsmith.

I could never make the mod work(i forget why) - and then soon decided I didnt like the original franken-nak mod - and gave up.

Now - Im favoring Rogers approach and thinking of trying that...

Problem might be - did I chop them too short!?

[jbell]

The chopped Teacs don't work???  Would you be interested in selling them?

Here's my TEAC ME-80s that I had chopped down at my local gunsmith.

I could never make the mod work(i forget why) - and then soon decided I didnt like the original franken-nak mod - and gave up.

Now - Im favoring Rogers approach and thinking of trying that...

Problem might be - did I chop them too short!?

[runonce]

The chopped Teacs don't work???  Would you be interested in selling them?

Here's my TEAC ME-80s that I had chopped down at my local gunsmith.

I could never make the mod work(i forget why) - and then soon decided I didnt like the original franken-nak mod - and gave up.

Now - Im favoring Rogers approach and thinking of trying that...

Problem might be - did I chop them too short!?

I think for now - I'll take another stab at getting them to work...somehow.

[SHINN]

Thanks for the help Roger,

I decided I don't want to cut the bodies right now so if wanted the mics can be brought back to stock. Plus I have those shotguns I never used.

For know I want to use your schematic for the mods, but with the battery in the line. Then down the road I'm going to buy that m20 tap on ebay and try to make some bodies from scratch with the battery in the body. I think I can thread the tube if I use a bridgeport to line up the tap. If that works and anybody wants to use the tap when I'm done they are welcome to it.

Also, does anyone know what material the bodies are? They look like stainless. I was thinking aluminum would work but would need to be a little thicker.

Thanks again,
Shinn

[Roger Gustavsson]

Shinn,

Have you found the correct M20 x 0.75 mm tap, where? Digikey have a cable clamp with M20 x 0.75 thread, HR1389-ND.
The bodies seems to be made of some alloy of brass, but I am not sure. I would be most interested in some bodies like the ones you mention! I have some extra bodies but it is a waste to cut them...

M20X0.75, Hand Tap, X150200075     http://uk.osgeurope.com/products/catalogues/OSG-UK_%20SPCL_TAP_%202007.pdf




Roger