Yamaha M916 Mixer Refurb Questions - schematic included

Hello everyone - I'm looking for some help with a Yamaha M916 mixer. I'm not an expert but am interested and would like to learn. I can solder, use a multimeter, find parts on a schematic / find those parts on the circuit board, and have a beginner's level understanding of what those parts do and how the signal flows.

This board works well and is in good shape, but I'd like to see if I can make it sound a bit more like my Speck Xtramix line mixer - that is, less noise, more bass, and clearer, more 'present' & 'defined' sound (if any of that makes any sense).

This board relies on unique op amps (Toshiba TA7322P) that don't have anything like a pin compatible replacement, so I'm probably not going to be rechipping any time soon.

So, my questions are:

1) The power supply is inside the mixer's case. There is a metal shield on top of it. Should I bother moving the power supply outside of the case into a seperate enclosure? I was thinking that might help reduce the noise.

2) Any suggestions for parts to replace in the power supply to improve its performance? I have a schematic posted here. From my reading I'm guessing the electrolytic caps should be replaced. Suggestions for cap values / other part replacements would be appreciated.

Thanks for any help / suggestions / ideas. There's a PDF of the complete service manual with schematics posted here if you're interested - note that it's nearly 20MB, though.

thanks again,

dave

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Hi Dave,
[quote author="lang_dave"]...I was thinking that might help reduce the noise...[/quote]What kind of noise are you hearing? High frequency hiss? White noise (sounds like a waterfall when cranked)? 60hz buzz? Can you isolate the noise by individually listening to different parts of the mixer like one channel at a time, line in, mic in, left main, right main, etc. If it?s on a mic channel, does the noise increase when you turn up the input gain?

Any suggestions for parts to replace in the power supply to improve its performance?...Suggestions for cap values / other part replacements would be appreciated.

Click to expand...

It may need a cap-job, depending on how old it is. I'd use Panasonic FCs, same values.

Maybe you can go up with the voltage to 27 or 28V without a hassle to the other components...however your opamps can take up to 30V supply voltage as the datasheet says...and the opamps are maybe the noisiest element of your desk (S/N ratio = 82dB A-weighted from datasheet)?
hmm... :?

Kind regards

Martin

PS: Panasonic FC :thumb:

If you want to measure your caps, do not rely on your MM only...
I posted some links to diy ESR-testers some time ago...
...

> more bass

Start there. A board this old, the electrolytic caps may have decayed. And there are over a dozen of them from in to out. At least, leave it powered-up for a week before evaluating, to re-form any caps that have deformed while idle. Then run frequency response tests, compare 1KHz and 20Hz. If it does not exceed orginal specs, you have rotted caps.

However, they may measure OK yet still be flabby. You might want to start re-cappping a channel and master strip, see what difference it makes.

Each stage typically has 3 coupling caps. For page 13, IC1 they are the 4.7uFd input cap, the 220uFd feedback cap, and the 33uFd output cap. These values are not bad, but because there are so many in cascade, I might double them all. Note that the input and feedback caps are polar, but the output caps are bipolar. Sadly, none of these can reasonably be replaced with film caps, though you might put 0.1uFd to 0.5uFd film across each electro.

> moving the power supply outside of the case

What noise, hiss or hum? I've had boards of that vintage that did need their power transformer moved outside. But that won't help hiss. And even the hum may be bad caps, not transformer field.

And on that thought: the power supply is fine, but the decoupling on each board is awful skimpy, as little as 10uFd for a high-current IC. Example: big PDF, page 13, IC1 has two 10uFd caps; I'd want to see 50 or 100uFd on each rail bypassed with 0.1uFd right at the chip.

> unique op amps (Toshiba TA7322P) that don't have anything like a pin compatible replacement

Data-sheet here. The TA7322P is actually a very good little amp, Yamaha's best phono preamp chip. If you ignore funny-pins 1, 4, and 8, you "could" cobble-in about any chip op-amp, but most will melt at +/-25V, and few are as good as these chips.

The LA1 opamp module is another little treasure. A Jensen 990 might be more musical, but this is a long way from 5558 and 5532 design.



The PRG output stage, if strapped for "+10dB", works at gain of 34 or 31dB. That may be a bit much for high dynamic range recorders.... output noise like 30uV, output signal like 1V nominal 5V max... well, that's still 107dB signal to noise with master full-down. Need to know more about where the noise is to figure what you should do. Maybe you just need to tune your knob settings.

25 years old and tons of connectors: get a soft rubber eraser and a pint of EverClear (151 proof drinking alcohol) and clean all the connectors, then get an ample supply of magic contact wipe like Caig Detox or whatever is faddish these days.

And remember: it is an older Yamaha. The competition in those days was pretty nasty (or MUCH more expensive). The Yamaha may always be "mellow" rather than "in-yer-face".

Thanks so much everyone for your help! PRR - your analysis / suggestions are the most detailed and useful info I've received about this board since someone found a copy of the schematic for me.

As soon as I get a chance I'll be acting on these suggestions and I'll report back with the results.

dave

Well, it's about time I dealt with this thing.

PRR's advice above was great, but some was a bit over my head. I've tried to learn about over the last few months, but still have some questions. Anyone care to expand upon his suggestions?

The two main points of advice were regarding the values for the coupling caps and decoupling caps. Here's what was said about the coupling caps.

"Each stage typically has 3 coupling caps. For page 13, IC1 they are the 4.7uFd input cap, the 220uFd feedback cap, and the 33uFd output cap. These values are not bad, but because there are so many in cascade, I might double them all...you might put 0.1uFd to 0.5uFd film across each electro."

I can find the three caps for IC1, but am not sure I've found them for IC2 & IC3 and was curious about the IC in the EQ section (it's actually a 4558 although marked on the schematic as a 6552). I put a smaller jpg of the schematic on the web. I marked the caps that I think are the coupling caps for each IC yellow. Did I get them all? The schematic is here:

http://davelang.com/m916/graphics/M916caps.jpg

There are some other caps I'm not sure about. I marked them red - should any of these have their values doubled?

PRR also wrote:

"the decoupling on each board is awful skimpy, as little as 10uFd for a high-current IC. Example: big PDF, page 13, IC1 has two 10uFd caps; I'd want to see 50 or 100uFd on each rail bypassed with 0.1uFd right at the chip..."

I've marked what I think are these decoupling caps in blue. Did I get them all? I plan to use 50uFd replacements and bypass them with 0.1uFd film caps but am not sure what pins on the chip to connect these bypass caps to. I think I know that there are two power rails on each chip - a positive (+) and negative (-) rail, but I'm not sure whether they are pins 2 (+) & 3 (-) or pins 5 & 9. And I don't know what pin(s) or connector the other leg of the film bypass cap should connect to.

Again, thanks for any help. I feel like I'm on the edge of the advice being "go learn some more then come back", but I'm trying to get started.

dave

[quote author="lang_dave"]
I marked the caps that I think are the coupling caps for each IC yellow. Did I get them all? [/quote]
Hi Dave:
Red marked caps and two 10/35 caps in the highpass filter are also the coupling caps.

I've marked what I think are these decoupling caps in blue. Did I get them all?

Click to expand...

All 10/35 caps (except those two in the HPF) and 47/100 cap in the phantom power are decoupling caps.

I plan to use 50uFd replacements and bypass them with 0.1uFd film caps but am not sure what pins on the chip to connect these bypass caps to. I think I know that there are two power rails on each chip - a positive (+) and negative (-) rail, but I'm not sure whether they are pins 2 (+) & 3 (-) or pins 5 & 9. And I don't know what pin(s) or connector the other leg of the film bypass cap should connect to.

Click to expand...

Pins 2 and 3 are the input pins; inverted (-) and noninverted (+).
The film caps may be soldered on the solder side of the PCB directly to the elco caps pins.

I feel like I'm on the edge of the advice being "go learn some more then come back", but I'm trying to get started.

Click to expand...

You may find more info about opamps in the "Op-Amps for Everyone", edited by Ron Mancini (Texas Instruments app. note SLOD006b).

Regards,
Milan

Hey Dave,

Even if you miss a cap here or there, you should hear a major improvment by changing those caps and upping some of the values. The bipolar and tant caps available today are superior and you should drop the THD nearly a decimal point just by changing them. Also you could try replacing the 4558s in the eq with 5532s. That should make the eq much smoother and quieter.


M@

Thanks Milan / Matt!

Couple more questions:

- I thought from PRR's message that the bypass caps for the decoupling caps should go near the opamp IC from the phrase "bypassed with .1uFd right at the chip". Can I just hang the bypass caps off the other side of the circuit board beneath the decoupling caps?

- what pins are for the power rails on the TA7322P chip?

dave

I've marked out another schematic and numbered the caps I'm planning on replacing and put it on the net here:

http://www.davelang.com/m916/graphics/m916inputcapmapv2.jpg

I marked what I believe to be coupling caps red.

I marked what I believe to be decoupling caps blue.

I marked what I think are the electrolytic caps in the EQ section green.

Coupling caps are going to be doubled in value and bypassed with a 0.1uFd film cap.

10uFd decoupling caps are going to be replaced with 47uFd bypassed with 0.1uFd film caps either at the cap or the chip, once I figure out where they should go.

I'm leaving the EQ caps alone.

I'm going to take Matt's advice and replace the 4558 (marked on schematic as 6552) with a 5532.

Comments? Any feedback on this plan? Did I identify the caps correctly?

Just in case anyone is intrigued by this project, you can buy your very own M916 here:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=011&item=320018168860&rd=1&sspagename=STRK%3AMEWA%3AIT&rd=1

They're dirt cheap.

It's not mine. Honest!

[quote author="lang_dave"]Just in case anyone is intrigued by this project, you can buy your very own M916 here:
They're dirt cheap.

It's not mine. Honest![/quote]

wow that looks like a really nice mixer, though a little big and heavy(same size as my broken Tascam M512). What does it mean though:

"This has 32 XLR inputs in an M1/M2 flip design."

Kiira

I don't know why, but I really like it. Maybe because it's cheap and clunky and doesn't have the hype (and, sadly, probably the sound) of other Yamaha mixers of that vintage like the PM1000 etc.

[quote author="kiira"]What does it mean though:

"This has 32 XLR inputs in an M1/M2 flip design."

Kiira[/quote]

It has two rows of inputs on the back. You can connect two different sources to each input channel and choose between them. There's a switch on each strip. The M916 comes stock with two rows of XLR inputs, mine has one row of XLR and one row of 1/4" TRS.

When ya gonna fix your M512?

[quote author="lang_dave"]I don't know why, but I really like it. Maybe because it's cheap and clunky and doesn't have the hype (and, sadly, probably the sound) of other Yamaha mixers of that vintage like the PM1000 etc.[/quote]

It has the same aura that my M512 has.

http://www.2tough.com/~kiira/rec/studio.jpg

It seems real solid and friendly. It seems like it really would be worth fixing up especially now that the pm1000s have all been dismantled and sold off as "mic pre known as "the Japanese Neve(not Langevin not api not great river not Hardy)".

It has two rows of inputs on the back. You can connect two different sources to each input channel and choose between them. There's a switch on each strip. The M916 comes stock with two rows of XLR inputs, mine has one row of XLR and one row of 1/4" TRS.

Click to expand...

oh ok I thought it might be something like that. The listing says it has xformers for both inputs and that doesn't make sense though... they could share a transformer if the don't have separate pres.

When ya gonna fix your M1512?

Click to expand...

I'm going to fix it reeeeeal good. I'm going to take out everything but the meter bridge and then put 12 of NYDave's single bottle line pres in it, a stereo summing amp of some kind and a direct out on each channel. Ha! :razz:

HHOS

Kiira

Kiira, how long have you had that lamp?

[quote author="NewYorkDave"]Kiira, how long have you had that lamp?[/quote]

er um.. since I was a kid. :roll:

I am working again on debugging the NYDave two bottle. I recorded the noisyness and i'll upload it soon. Maybe it will ring a bell with someone. I am wondering if I have a bad cold solder somewhere.

Kiira

I think my parents may still have my Charlie Brown bedsheets lurking in the back of their linen closet.

Keep us informed of your progress with the Two-Bottle!

Hey New York Dave! Care to have a look at this and see if I identified the caps right?

http://www.davelang.com/m916/graphics/m916inputcapmapv2.jpg

I marked what I believe to be coupling caps red.

I marked what I believe to be decoupling caps blue.

I marked what I think are the electrolytic caps in the EQ section green.