Yamaha M406 Mod

[Sonicblue99]

So I recently got a Yamaha M406 and am planning on doing BXT403's mod Now, I thought I would put it out here before I do anything for some brainstorming. Particularly, I'm tempted to do the channels in pairs with different output transformers for each pair (cinemag & Lundahl or Sowter) and would love to hear any ideas (what the ideal turn ratio for the trannies would be.) Also, since I will be bypassing the mixer for my direct outs, what are some options for routing the signal to the VU? I feel I could go from either the echo out, the post fade out, or perhaps somewhere else. What would be the ideal place to tap from? I was thinking of routing the echo out to a multiswitch that then goes to the meters. Any help, tips, ideas would be great.

[mattbeme]

I have a M406 which I just finished a repair on so I am fairly familiar with its circuitry.

First, you have to decide if you want to use just the R PGM/Echo meter or if you want to be able to use both meters.

If you are going to pair the inputs, I would suggest using both meters, then you could use the Left meter for 3 pairs and Right meter for the other 3 pairs.

Let's say, ch. 1-3 on Left meter; ch. 4-6 on Right meter. Or, odd # channels on Left meter; even # channels on Right meter.

I can give guidance as to where the best points are to tap and send the signals.
You may have to keep the PGM L , R or ES (Echo Send) boards in place if the meters require some circuitry on them to be driven properly and depending on whether or not you want just the Echo meter or both meters.

Matt B

[mattbeme]

BXT403 mod: For VU meter use = a single 1/4" trs input on front of unit:

To patch in any single channel output to check its level. Why TRS? To keep the 'semi-Balanced' transformer signal levels unchanged as much as possible.
According to this mod: he had added an output transformer to each channel direct out. This meant that each output was now being semi-balanced and there were now 2 output signal wires that had to be separately connected to the VU board. He had run the TRS jack to the L and R inputs directly on the Meter board.

Otherwise with a TS jack: the signal level would drop and the level on the meter would not be an accurate read of the actual output level.

-Single 1/4" trs input on front of unit =This seems to be the Best way = keeps the 'semi-balanced' signal levels = accurate VU read. Easiest / cheapest = no other wires being tapped off the signal. No switches with more wiring to add with their possible losses of signal levels.

It is just a minor hassle to have to patch in the output cable from the rear to the front; one channel at a time to check the level and then move the cable into the receiving device to receive the signal. You could put this TRS VU jack on the rear. Just make sure its as close to the meter board as possible with the wires as short as possible to avoid picking up EMF.

I would probably do this same mod if I were setting up my M406 as direct out preamps and needed the VU.


Matt B


BXT403 mod:

Yamaha M406 modified to mic preamp!

[Ike Zimbel]

Note that this would give you half the signal on each VU meter (Pin-2 on Left Meter, Pin-3 on Right). Another idea would be to put a six position rotary switch on the front panel and run the pre-transformer signal from the direct out's to the switch. Or, you could also add a single switch to each channel to use as a Solo function.
best,
Ike

[mattbeme]

Found this:


.....input impedance on your mic amp. A 10:1 ratio is good for load transfer and optimum signal strength....

[Sonicblue99]

Wow, okay, you guys have given me some great options and ideas. I'll be starting this mod next week (assuming my parts get here by then.) I've had some suggest to leave the pre's as unbalanced out. Since this will be mounted in my desk no more than 3 feet from the patchbay and my cables run won't be that long, I'm temped to go this route. Outside of long cable runs, is there really any reason to go balanced on the outputs, other than to add color?

Thanks for the help so far, and once I get this opened, I'll post pics and samples

D

[mattbeme]

I would send the pres out unbalanced.

To add better trannies like Jensen etc. would be quite expensive for negligible improvement. The output structure of the M406 is not 'true' balanced output anyways. With my own M406 I cannot hear a difference in sound quality, apart from signal level strength, between the PGM outputs, the Sub Outs or the Echo Send out. Perhaps others with more experienced ears could hear a difference.

Keep the cables short (as you said they would be)
Use Shielded cables with a Foil and Braid shield.
This will help keep noise free signals with unbalanced outputs.


Matt B.

[Sonicblue99]

Well, I finished making the mods. I cut the jumper on each card labeled (POST.FADEQ), soldered new leads in the hole that the jumper used to be in closest to the edge, and got the ground from the input section of each XLR terminal.

After that I decided to test to see where the phantom power came in. I'm almost 100% sure that I found it. Looking at the top of channel card, it is the 2nd pin from the rear (or to the right.) With phantom power off I got 0v, when I turned it on I was getting 39v off that pin. The good thing is that it's trace goes around the outside rear edge of each card. This means all we have to do is cut the trace and insert a switch in that cut (or you could simply cut the pin and attach there though I think the rear edge would be prettier and keep wires shorter.) This will allow us to have the switch for channel.

Now I left the main boards in hoping that I can somehow still use the main balanced out. I was hoping that I could effectively have the 6 direct outs + the summed main outputs, though I have not tested this to see if it will work. If anyone has tried this let me know, if I'm just chasing my tail. I figure worst case I'll just yank those cards out. Hopefully this weekend I will be able to test this bad boy. Once finished I'll post pics and a better how to.

[mattbeme]

Sonic, I apologise for not having replied to your request for info. on the phantom power mod; I've not been well.

It sounds like you have figured things out well and the info. you gave will help me when I add phantom power switches to my unit.

I think that if you were to keep the PGM cards in and use them for balanced outs simultaneously with the direct outs that it should work out fine.

Would this drop the output level on the direct outs? It may somewhat but this setup would not be harmful to any part of mixer.

Matt B

[Sonicblue99]

No worries. I'll try to post some final pics soon. I didn't notice much drop in the output. The great thing is, with the main boards still I still can use the meters as they were! Granted, it doesn't really matter but I just like seeing those old guys light and and the needle move...brings back memories of my dad's old gear. Anyways, hit me up if you need any help or pics.