Tascam M-___ Story...

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RFR said:
What if it is just a noisy mixer? What then?
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Uuuuummmmm...probably a combination of making some minor changes that might reduce the noise floor, and accepting it for what it is. It's a cool mixer. And working with it the last couple days is a treat. So if it's noisy, it's one-of-kind noisy.

But I might not be in bad shape. I tried some other things out tonight.

• Out of curiosity I measured the noise at the STEREO OUT R jack with all inputs muted, and the STEREO MASTER fader at maximum. I wanted to see just how much noise was coming off the Control Module alone. The DMM read 0.0mV. So that's better than -70dB noise floor.

• Out of further curiosity I set the source of the 8 installed modules to one of the two LINE inputs, and with the LINE trims set to maximum, channel faders at unity, and the STEREO MASTER fader set to unity, the DMM read the same 0.0mV. If I raise the 8 channel faders to maximum as well as the STEREO MASTER fader, the DMM displays 0.1mV. So 8 line inputs at max trim, channel faders at max, and master buss fader at max it's still about -70dB noise floor.

I just have somewhat noisy mic preamps maybe? A couple of the other modules have upgraded opamps in them. I might see if they are any quieter, though I don't expect the upgraded opamp will make much if any difference; I think those upgraded modules have a 2134 instead of the stock 072 for the mic amp.
 
Feeling somewhat accomplished...one big mystery as of late has been the powering of the BUSS OUT LEVEL boost circuitry. Each BUSS OUT jack (which includes the outputs of PGM groups 1-8 and AUX busses 1-4) are switchable -10dBV or +4dBu. There is a complicated (to me) network of 5 transistors, one side of a TL072 and a whole mess of resistors (and some other stuff) that make this happen. The transistor network is powered by a separate set of power rails than the main +/-15V audio power rails. The main PCB for each module has these rails labeled as +/-35V. I always assumed that labeling referred to unregulated power since in the M-500 mixers the same level boosting circuitry is powered by a +/-18V set of rails; I assumed in my naivety, based in the appearance of the guts, power was regulated locally in each module. So way back yonder when I was crafting a power supply for the M-__, I simply pulled the regulators from the +/-18V supply in the PS-520 supply I ended up using (leaving those rails at about +/-26V running wild) and have used that ever since. It recently dawned on me my assumption might be wrong about the nature of the power for the level boosting circuitry..."maybe it's supposed to be regulated...maybe those transistors aren't doing any regulating" I thought...a friend of mine who's been quite a help here and there throughout the years on this project told me I needed to draw up the schematic for the level boost circuit so we could analyze what kind of power the circuit needs...regulated is always better when powering audio...so if it calls for regulated power, does it have to be +/-35V as labeled, or could it be less? If less, then how much less? Answers to these questions will tell me if I can continue to use those rails in the PS-520 and just put some regulators back in place (by my calculations even +/-24V regulators will work in place of the stock +/-18V parts), or if I need to round up a separate supply.

Anyway, I traversed a great hurdle (for me) and finally got a rough draft of the schematic together...I need to clean it up and lay it out differently now that I've got it sketched out, but this is a big deal for me...I don't have any formal electronics training, so looking at a PCB and translating what I see physically to a schematic is a heavy challenge. I've been avoiding this one...but I've got the hardest part done...just getting it on paper.

IMG_7379.webp
 
I need help, analog community...does anybody have schematics for a Tascam M-50 mixer??

Okay, so here's why...I'm kind of freaking out a little...you know I've been working on drawing up the schematic for the PGM BUSS output amp and level boost circuitry on the M-__. This circuit is found on each of the 12 I/O modules, but there is also a pair of boards in the master Control Module that have a similar circuit on them to provide the selectable -10dBv/+4dBu/+8dBu output level boost for the STEREO OUT.

Remember, the M-__ is sort of a missing link between the early generation Teac/Tascam modular consoles like the Model 15 and M-16 consoles (with their plug-in modules and integrated per-module backplane), and the later generation M-50, and M-512/520 consoles with even some carryover into the M-300 series. The EQ section on the M-__ is also very close in functions and features to the EQ channels on the Tascam PE-40 rack mount parametric EQ unit. The M-__ is chock full of features found nowhere else in any Teac/Tascam mixer that made it to market, but the ancestral and descendant elements that do exist have been occasionally helpful in troubleshooting or analysis over the years since there is no documentation that came with the M-__. For instance, since the EQ sections between the M-__ and PE-40 are so similar, I can use much of the PE-40 schematic when referencing the M-__ EQ circuit. Well, this whole level boost thing has been important because it is powered by a dedicated higher voltage set of power rails. I have no documents for the M-__ other than what I've reverse engineered, and it came with no power supply. So it's been a bit of a guessing game what kind of power the level boost circuitry actually requires. The boards in the modules are all labeled +/-35V relative to the power for the level boost circuitry, but that seems really high, and furthermore it's been a mystery whether that power should be regulated or unregulated. I'm reasonably certain as of late the power should be regulated, but the amplitude is still a mystery. Once I can determine the proper amplitude, I can finish the work on the power supply (or source a separate dedicated power supply if it truly needs to be +/-35V...the most I can get out of the PS-520 power supply I'm using will be +/-24V). In circuitry of similar function (but completely different topology) in the M-512/520 mixers the power is +/-18V. So how excited do you think it is to realize the M-50 appears to have the same level boost circuitry as the M-__ STEREO OUT level boost circuitry? I think I've been aware the M-50 had the same level boost features as the M-__ for the STEREO OUT, but I assumed the circuitry was the same as the M-512/520 and different from the M-__...nope. Note the M-50 is the predecessor to the M-512...very similar to each other but with some notable differences...like the master outputs...M-50 features dedicated level-selectable balanced outs just like the M-__, whereas the M-512/520 have fixed unbalanced -10dBv outputs for the STEREO busses, with an independent patchable fixed -10dBv to +4dBu 8-channel balance amp section. Anyway, the point here is I was doing a regular eBay trolling this eve and something caught my eye...there's a fella whose got a bunch of listings for bits from a parted out M-50...the listings have been up for weeks or maybe months...seen them maybe dozens of times, but tonight I notice the tiny thumbnail image for one of the listings, a PCB assembly, looks like a spitting image of the balance amp boards in my M-__ Control Module. I open it up and zoom in and sure enough it looks very, very close...even right down to the component layout. Looks like my M-__ assemblies use higher quality polystyrene caps in the feedback loops of the opamp, but otherwise it's really, really close. It's hard to describe how exciting it is to identify an assembly in a production unit that is so close to my M-__...because there exist technical documents to the production unit.

This is what one of the boards looks like on the M-__:

IMG_7534.webp


And here is the M-50 version:

IMG_7532.webp


Isn't that cool??

More evidence of M-__ elements that actually made it to market!

So I need help tracking down schematics for the M-50. The operation manual is available online, but I can't find the service manual. Can anybody help me?
 
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Were you actually able to download the manual though? I've found that site before but couldn't figure out how to actually download the document.
 
Yes, If you look in third line of text below the preview, you will see this;

This file is downloadable free of charge: ...processing...

After a short while, it will change to this;

This file is downloadable free of charge: Get Manual


Then just click on, "Get Manual".

It took me a while to figure it out the first time I downloaded anything from there.

Also Sent you a PM with a link to the copy I've downloaded.
 
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Oh man!! Thank you so much!! :D :D :D

That is so cool...that's the circuit I already reverse engineered on my own on the M-__ almost exactly...and, wow...it is indeed powered by +/-35V rails in the M-50, and unregulated...! :eek:

So now I can send those documents to my friend whose been helping me, and decide if it has to be +/-35V unregulated or if it could be regulated and +/-24V. If it can be the latter, that would be great because I can get that out of the current power supply...and regulated power is always better when bundled and being distributed along other regulated power rails vs unregulated noisier power running next to regulated.

If it's gotta be the +/-35V power, we'll then I'll have to come up with a solution.

But this is huge just having this info!

Thanks! :)
 
You're such an inspiration Cory........just wanted to add that.

Every time I read one of your posts, I'm spurred on to better my knowledge of electronics....it's one of my biggest regrets in my education......the amount of useless stuff schools try to fill your head with....instead of 'filling buckets, they should light fires' (paraphrased from a kid that went to my school and had a teacher that did do just that...he's nice the Vice Chancellor of the University in my home town).

And well done Quiberon for pitching in with the help.....like I said, it's refreshing to see.
 
I'll second the inspiration aspect.
These threads not only enhance, or at least open the door to electronics, but keep the art and skills alive.
We live in such a throw away world. Repair shops are a thing of the past.

They (these threads) also help keep the interest in analog gear alive. I'm probably not the first to want to pick up a piece of gear to fiddle with and eventually use.
 
Thanks guys. That means a lot. Really does.

I got some answers from my friend about the level boost circuit. Before I go into that I need to correct some misinformation I posted about the Tascam M-50 outputs...I said earlier the STEREO OUT jacks on the M-50 were switchable output level, as compared to the M-512/520 with its flexible 8-channel -10dBv unbalanced to +4dBu balanced BALANCE AMP section. Well the M-50 is actually kind of like the M-512/520, but it is a 2-channel balance amp, and the output is switchable +4 or +8dBu. Furthermore all the unbalanced PGM outputs and the two stereo AUX buss outputs are switchable -10dBv or 0dBu. Just wanted to correct my information. The biggest difference between the M-__ and M-50 as compared to the M-512/520 is the M-__ and M-50 use a high headroom +/-35v powered opamp/transistor array circuit, whereas the M-512/520 use a more garden variety +/-18V powered opamp-only circuit. So the M-__ and M-50 are unique in that regard.

Here's what my friend had to say which is wonderfully informative:

ok, the deal is that the opamp [in the level boost circuit] needs +/- 15v, and the output stage will take as much as it can get, to the limits of the transistors.

this output stage has voltage gain, so it can take the +/-15v output of the opamp and get it to about +/- 35v.

It's all in the name of headroom, or peak output capability.

the power supply rail represents the maximum peak output of the circuit...there's no way for the output voltage to rise any more than that.

but that's peak voltage, and we are used to RMS readings, so to convert peak to RMS, then you multiply by .707. We'll use +/-34v with a volt taken for circuit losses. 34 * .707 = 24v RMS

convert that to dBu: 20 * log(24/0.775) = +29.8 dBu, which is the sort of level that we are used to seeing from old tube gear and earlier studio gear (before opamps).

If we reduce the supply rails to +/- 24v, then we have +26.8dBu, still respectable.

If your working signal level is +4, then you have 23dB of headroom. That's a bit of magic/marketing there, the reality is that if you have 20dB of peaks in that +4 signal, then the peak signal level is +24dBu, which is still 3dB from peak clipping.
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So now the decision is to go with good, better, or best in terms of the amplitude of the power rails, and the corresponding headroom.

Good = put in 24V regulators on the current unregulated +/-27.5V rails I'm running right now for more stable power, and have "respectable" headroom.

Better = leave things as they are with unregulated +/-27.5V rails and continue having better than respectable headroom.

Best = figure out a way to bump the power to +/-35V as Teac designed. This would require a completely different, or an additional secondary main transformer in the PS-520 power supply, but would provide headroom on all my buss outs and the main out akin to tube/pre-opamp gear.

Guess which one I'm leaning towards? I like things to be "right". I'm working on a solution.
 
Just curious (and I know it's back there somewhere....) . What's your headroom now?
 
bachelorb said:
Just curious (and I know it's back there somewhere....) . What's your headroom now?
Click to expand...

Running the current +/-27.5V unregulated rails, the calculated headroom on the STEREO, PGM and AUX BUSS outputs is about 23.6dBu above +4dBu when switched to the +4dBu nominal output level.

I found an error in my friend's calculations (no discredit meant to him in any way...). When he was calculating the headroom if the power rails were reduced to +/-24V, he forgot to subtract 1V for circuit losses. So at +/-24V the calculated headroom would be about 22.4dBu above +4dBu.

Again, at +/-35V the headroom would be about 25.8dBu above +4dBu, which is over 50% more headroom than if the power rails were reduced to +/-24V (signal level doubles every 6dB).

I have to say, one thing I've always noticed about the M-__ was that it just seems to be able to handle gobs of signal at the PGM summing busses and be completely clean. Like I remember when I first started running audio through it thinking "Man I shouldn't be able to throw that much signal to the PGM groups like this...it's like it can handle really hot signal levels." Now I understand why.
 
So this has been an interesting process trying to tackle how to accommodate the power needed for the +/-35V rails. There are voltage considerations, max current considerations, and physical space considerations.

Problem: the current multi-core main transformer in the PS-520 power supply I'm using does not have an output that will produce +/-35VDC power at the output of the power supply. About 25VAC is needed to produce the 35VDC. It is not feasible or reasonable to modify the transformer, so either a separate dedicated transformer needs to be added to the system, or a completely separate multi-core transformer needs to be sourced. Whatever transformer or transformers are utilized, they need to have the proper voltage outputs AND current spec. It is beyond me to be able to determine what the max current potential is for each of the outputs of the existing main transformer (there are no specifications available), and I don't know how to calculate what the maximum current draw might be with the different sections of the M-__. This reality has shut the door on using an alternate multi-core main transformer (I have spares from a Tascam 48 and a Tascam 58 that have all the right voltages except for what's needed for the +48V phantom power rail, and I have a main transformer from a Tascam M-308B that fills that void), because I have no way to know what the max current spec is of each core, nor do I know what the console requires. I had another possibility: there is a main transformer for sale on eBay from an M-50, which includes the correct output to power the +/-35V supply (because the M-50 has +/-35V power driving the same level boosting circuit as found in the M-__). The problem with this option turns out to be two-fold: the M-50 transformer doesn't physically fit inside the PS-520 chassis without significant modifications to the case, and I also don't have any way to determine if the M-50 transformer can provide for the current needs of the M-__ level boost circuitry; there are only 4 of these circuits on the M-50, but there are 16 on the M-__. The reasonable worst case scenario is a max current draw of about 750mA from the level boost circuitry in the M-__ (my friend did the calculation for me...we can figure it out with that circuitry because of its purpose as an output driver, and the relative simplicity of the circuit as opposed to trying to calculate for an entire console). But there's no way to find out if the M-50 transformer 25V output will provide up to 750mA of current. So that's out...so...

Solution: knowing the voltage and current spec needed, source a new transformer that physically fits in the space inside the chassis, mount it and wire it up.

I found one from Hammond Industries...and I think it will fit.
 
Maybe Mercury Magnetics in Los Angeles could source or build you one?
 
I think the Hammond one will work...have to confirm the depth of the spot where I want to put it in the chassis, but I think it will fit nicely...$34 at Mouser.
 
Yes! The transformer is 2 1/8" wide and there's a perfect spot right in front of the existing main transformer that is 2 1/4" deep. Perfect! :D
 
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