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Shure M67 Mods
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RodC
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PostPosted: Wed Mar 15, 2006 12:06 am    Post subject: Shure M67 Mods Reply with quote

I have been stealing the transformers out of these units and I often wonder why they dont get used as much as some of the other units of this vintage. The transformers are pretty good, heck they are mu metal shielded and are larger than some of the others of the era. The unit does get a bit too much of its gain from the transformers, which does color it more. Heck they recorded woodstock with these, not sure how because that leads me to the biggest problem, distortion. Put a 57 on one of these and get 10 ft from a guitar cab and it sounds like shit. So I set out with a few goals.

1 Make it a 4 channel preamp.
2 Tackel the distortion problem.
3 Maybe the hardest: use very little new parts and make the mod so anyone who can solder can do it.

I think I have something that sounds pretty good, Im going to setup some compairisons soon, but with some loud guitars and drums today it seems to have that descrete transformer sound we all like. Here is the mod, I will post a schematic tomorrow.

There may be a better way to tackel the gain/distortion problem, but this seemed like the easiest.

Shure M67 Mods
3/14/06
Rod_Childers@hotmail.com

Goals of this modification:
 Convert mixer to 4 channel mic preamp
 Eliminate low cut filter
 Tame high gain so preamps are useful for loud sources (Guitar amps, Drums) do this by relocating the gain control between the Mic transformer and the transistor stage.
 Make as few changes as possible, use minimal new parts. The only new parts required are ¼” jacks for the outputs.
 Optional, remove unused componets/switches.

Using channel 1 as an example:
1. Remove the Orange wire going from the center of R9 (Channel 1 Gain control) leading to S2 (Low Cut).

2. Remove the Blue wire going from S2 to R10 on the PCB. Note where the 4 33k resistors are on the PCB (R10, R17, R10, R24) This is where the individual preamps are connected to the mix bus. You will remove these 4 blue wires, one for each channel.

3. Leave all the black ground wires that connect to the top of the gain controls in place.

4. Repeat 1 and 2 for channel 2

5. Mount the output jacks:
Remove the headphone jack; use this hole for ouput 1.
Remove the 2 lugs for the line out.
Use the 3 spot for output of channel 2.
Use the 2 spot for the output of channel 3.
Remove the ground lug and ream out the hole or use a step drill bit, use this hole for the output of channel 4.

6. The wires on the bottoms of each Gain pot will be the outputs for each channel. Cut the wire from each pot, not the PCB. Run each wire to the proper output jack.
Brown = Channel 1
Red = Channel 2
Orange = Channel 3 (Extend this wire)
Yellow = Channel 4 (Extend this wire)
The Master is yellow also, but we won’t need this pot
Ground all the jacks at one point. I used the lug that all the input transformers connect to.

Still using channel 1 as an example:
7. Unsolder the white wire from the PCB that connects to the output of the Mic transformer for channel 1. Connect this wire to the bottom lug of R9 (Channel 1 Gain)

8. Solder a wire from the center of R9 to the 1 spot on the PCB. (Where the white wire from the mic transformer was removed in step 7)

9. Repeat 7 and 8 for channels 2-4

EDIT 9/6/06:

The last couple I did I left channel 1 in the mix bus so it has the gain of the summing amp. Channel 1 then has 2 controls, the original volume and the master becomes sorta like a master gain.

This way you end up with 1 channel with lots of gain and 3 others that work great on loud guitars and drums.


Last edited by RodC on Wed Sep 06, 2006 9:10 am; edited 1 time in total
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djgout
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PostPosted: Wed Mar 15, 2006 12:23 am    Post subject: Re: Shure M67 Mods Reply with quote

interesting. we were discussing modifying these things today to have direct outs. i absolutly love the pres on kick drums. i'll probably try out your mods.

-justin
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ubertar
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PostPosted: Wed Mar 15, 2006 1:08 am    Post subject: Re: Shure M67 Mods Reply with quote

This is awesome Rod. Do the same mods apply to the M68?
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RodC
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PostPosted: Wed Mar 15, 2006 9:15 am    Post subject: Re: Shure M67 Mods Reply with quote

ubertar wrote:
This is awesome Rod. Do the same mods apply to the M68?


I'm working on the M68 right now. It should work just as well. Its funny when you look at these mixers they all are a built different. Even the 677 extender for the M67 has a different circuit.

I'm betting you could do the same to the M68, it seems to get a bit less gain from the transformer, but I dont have anything for specs except the test voltages on the schematic.


Last edited by RodC on Wed Mar 15, 2006 9:53 am; edited 1 time in total
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RodC
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PostPosted: Wed Mar 15, 2006 9:16 am    Post subject: Re: Shure M67 Mods Reply with quote

djgout wrote:
interesting. we were discussing modifying these things today to have direct outs. i absolutly love the pres on kick drums. i'll probably try out your mods.

-justin


Dang, I would love to know how you got it to work? The ones I tried always broke up and sounded real gritty before I moved the gain control.
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RodC
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PostPosted: Wed Mar 15, 2006 9:45 am    Post subject: Re: Shure M67 Mods Reply with quote

I posted these instructions with a schematic after the mods. Just scroll to the bottom of my homepage.

http://www.beyondsanityproductions.com/


You can get the original manuals with schematics for the M67, M677 and M68 at:

http://www.shure.com/datasheets/guides-discontinued.html
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RodC
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PostPosted: Wed Mar 15, 2006 10:09 am    Post subject: Re: Shure M67 Mods Reply with quote

*** BTW *** if you dont want to relocate the gain control, this is the most radical part to this mod, you can disconnect the blue wires and use them for your direct outs. When I tried this I still had way too much distortion to use on loud sources.

This will be the 4 blue wires that lead from the low cut switches to the PCB that is mentioned in step 2:

Note where the 4 33k resistors are on the PCB (R10, R17, R10, R24) This is where the individual preamps are connected to the mix bus.
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hyde maintenance
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PostPosted: Wed Mar 15, 2006 10:47 pm    Post subject: Re: Shure M67 Mods Reply with quote

RodC,

I was wondering if you were familiar with the M267?
It looks like a mod may be trickier as the pots + switches are pcb mounted, but are the guts similar otherwise?
I happen to have one sitting around is why I ask.

Thanks.
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RodC
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PostPosted: Wed Mar 15, 2006 11:21 pm    Post subject: Re: Shure M67 Mods Reply with quote

hyde maintenance wrote:
RodC,

I was wondering if you were familiar with the M267?
It looks like a mod may be trickier as the pots + switches are pcb mounted, but are the guts similar otherwise?
I happen to have one sitting around is why I ask.

Thanks.


They are pretty similar in layout but the M267 is all Op amps. I think they could sound OK. I would start by seperating out one channel and see what it sounds like. If it suffers from distortion I would then relocate the pot like this mod, put it between the transformer and the first stage. This relocation would probably work better on this model because the gain pots are 100K and would affect the impedance of the tranny less. You could offset most of this affect on this unit by removing the shunt resistors on the trannys (R201, R214, and R213).

If you look at the schematic each channel enters the mix bus between a Cap and a resistor.

Channel 1 between C301 - R304
Channel 2 between C302 - R308
Channel 3 between C303 - R312
Channel 4 between C304 - R316


First seperate channel 1 by removing R304, one side of R304 connects to C301, use the hole you removed R304 from that connects to C301 as the out for channel 1. If you want you can just clip the resistor and use the lead that is sticking out to put a clip lead on to do some testing.


If this point is very distorted and you have to relocate the pot I'll try to give you some pointers.

Get yourself a copy of the manual/schematic here:

http://www.shure.com/pdf/discontinued/m267_english.pdf

Let us know how it turns out, I think a lot of these mixers sound much better than lots of stuff that we pay 10X more for these days. I know mine sounds better than my SM Pro PR8 LOL
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hyde maintenance
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PostPosted: Sat Mar 18, 2006 3:42 am    Post subject: Re: Shure M67 Mods Reply with quote

Wow.
Thanks Rod. You really step up. I have taken note of your directions and will be trying this out when I get a chance (I will let you know my results).
I don't think I'll be getting to it too soon though as I have had some amazing luck these past couple of weeks and have a bit of a equipment overload on my hands. So much stuff to try out, it'll take some time.
My head is still spinning, but I'll probably post about it soon.
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honkyjonk
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PostPosted: Sat Mar 18, 2006 1:20 pm    Post subject: Re: Shure M67 Mods Reply with quote

Were the M67's always made the same way? The reason I ask is because the prices for these things vary wildly. Let me guess, the 60's ones are better than the 80's ones for some reason. No really, are m67's the same or no?
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RodC
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PostPosted: Sun Mar 19, 2006 5:15 pm    Post subject: Re: Shure M67 Mods Reply with quote

honkyjonk wrote:
Were the M67's always made the same way? The reason I ask is because the prices for these things vary wildly. Let me guess, the 60's ones are better than the 80's ones for some reason. No really, are m67's the same or no?


I havent seen any too different. I have seen some variations in the components. The service info from shure does not mention any differences.

A lot of the difference in the price on Ebay is how they are advertised.
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Paul Fury 161
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PostPosted: Mon Mar 20, 2006 9:37 pm    Post subject: Re: Shure M67 Mods Reply with quote

Hi Rod, just posted replying to another thread over in gear talk, then found this one, doh!;-p I'm interested to know if these units run only with 110v as stock (i'm in the UK), quite tempted to shel out for a few if they are easy to mod for uk mains, or come switchable. Sorry for the idiot question, thanks in advance,
Cheers
Paul Fury
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RodC
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PostPosted: Mon Mar 20, 2006 9:39 pm    Post subject: Re: Shure M67 Mods Reply with quote

They come in a 240V version, but im sure it would be cheeper to get a 120 and change the tranny.

Look for a M67-2E the only difference is the transformer.
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Paul Fury 161
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PostPosted: Mon Mar 20, 2006 9:41 pm    Post subject: Re: Shure M67 Mods Reply with quote

Thanks - I presume the 2E is the one that comes with the 240v trany as stock?
If you think I could get a replacement tranny for cheap that'd work ok, i'd be tempted to have a go with a us 110v unit - heck, if it comes out ok, I might buy 4!
Thanks for the great thread and advice!
Cheers,
Paul
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