Tascam M-___ Story...

[dementedchord]

minor correction re faders..., the fader sits inbetween the output of the channel and ground... and is always there so the output sees an essentialy constant resistence... the center tap moves from one end to the other such that a signal proportional to it's position is then passed to the next stage... think of it as a variable ladder resistor... when at zero you've essentially grounded the input to the next stage...

 

[sweetbeats]

Wacky...wackywackywacky...

This fader thing is a cool stumper.

BTW, thanks for the clarification, demented. In my staring at the fader board here and there these past few days your explaination ties it all together with how signal is flowing through the fader...the element on the board that dead ends on one end ties to the output at the other...that element is a constant resistance and low at that. The other element is the resistive one and goes between the input and ground. Like you said (I hope I'm understanding correctly), when the fader is all the way down the easiest path for the signal is for it to pass straight to ground. As the fader is raised the resistive path to ground increases in resistance gradually diverting more and more signal to the output. did I get that right? The wiper is actually two wipers tied together (that's two for each channel on a stereo fader...4 total)...one end contacts the output element, the other touches the resistive element which goes between the input and ground...so the wiper bridges the two elements together, and when the fader is down the output is physically furthest from the input. Its down at the ground end of the trace, but it is still easier for signal to go straight to ground rather than cross the bridge through the wipers to the output. And then when you raise the fader it is physically bringing the output closer to the input making that bridge the easier path for the signal.

So anyway, I was trying to figure out what is going on with my master fader. I actually pulled the fader completely apart so I could look closely at the wipers. They are made up of really fine stiff wires! The basic faders that were put in many of Tascam's mixers (not the tall-bodied chrome ones, but the short ones) as well as all the pots I've worked with have wipers with 3 to 5 individual contacts. The wipers in these faders have like 20? 25? 30 for each? Didn't count them but I did find a couple bent ones and I thought I had the problem licked...figured the wayward wiper wire was hanging over into the array of short traces (the mystery traces) on each side of the resistive trace. Cleaned everything up really nice...problem still there.

Then I got the brainiac idea to test the characteristics of one of the channel faders. Unplugged one, clipped on the DMM between the input and output, and the output level bounds up and down just like the master fader......! But the audio output doesn't present that way.

So I took a closer look at the mystery traces...If you look close you can see that each main resistive trace (the ones that go between the input and ground) is connected to an array of those small elements. Look...I circled 4 of the 5 little bridges that go between the resistive element and the array of elements in bright blue (there are 5 total, one at each end and 3 in the middle...you can't see the first one because it is behind the wiper chassis):

So I discovered that my drops in signal happen right when the wipers pass by those little bridges, which makes some sense if those bridges allow signal to go other places besides either ground or output...like looping or something. FWIW, just for clarification, all the black is conductive surface i.e. element. All the silver is a conductive trace, but non-conductive on the surface.

So anyway, I kind of figured this whole problem was related to me flipping the fader 180-degrees for wiring convenience. I don't yet understand why the phenomenon presents itself on both a channel fader and my master fader when out of the mixer (i.e. output bounding), but when in the mixer the master fader performs as it presents when out (audio level bounds when raising the fader), but the channel fader behaves like it should (i.e. even taper). It most assuredly has something to do with the easiest path to ground. I'd have to understand the nature of the main resistive element to have a real grip on this thing, but the bottom line is that I think I'm gonna need to rewire the rewiring job. My assumptions were not correct before, and hopefully they will be right this time (that all will be right when I put it back the way it was.)

It still leaves one big question: What on Earth are the arrays of short elements for??!!?

 

[dementedchord]

forget about this partcular fader... looks stereo to me... and frankl;y i haven't been reading all the posts on this... what i'm suggesting concerns understanding the circuit in a general sense... the output of the prefade section needs to see a load... which is provided by the total resistence of the fader and doesnt vary so that drive stage isnt taxed and is stable... the input of the next stageis tied to the wiper which can be at any portion of the total fader... so it sees a signal directly proprotional to it's position on the fader... in essence becoming a variable voltage devider... dont make the mistake of confusing things by trying to make it fit a specific part that your not sure of...

 

[sweetbeats]

Yes, stereo...its the master fader.

That all makes sense (thank you) except for "dont make the mistake of confusing things by trying to make it fit a specific part that your not sure of"...you lost me, but that's not new, and I figure not worth your time to straighten me out...

Anyway, thanks.

I think my biggest confusion at this point with this component are the element arrays...totally non-critical. Just curiosity, but has anybody ever seen anything like that?

 

[sweetbeats]

So How Does It Sound?

So I was doing some testing to ascertain if the LINE inputs on the M-___ will handle +4 nominal sources.

This is an important little question because I know the M-___ will feed the inputs on my Ampex AG-440-8 which are +4 or +8 nominal thanks to the switchable output level on the BUSS OUTs on the M-___ (-10 or +4). The question has been can the M-___ handle the tape returns at +4. I've had a hunch it could because of the way some other things were setup, but other mixers from this era in Teac's history were all -10dBV as far as line-level inputs.

I did some testing using a 1kHz tone (you can see the details in this post...) The testing came out indicating that the inputs likely were suitable for handling +4 sources, but of course 1kHz sine wave is not real-world unless that's what yer into listening too... So RRuskin thought I should try it with program material. So I did just that using this song. It is a good one for this kind of testing IMO because its familiar to me, it is dynamic in range and the style is clean.

Other test conditions:

• Monitoring was via Sennheiser HD280PRO headphones using the M-___'s headphone amp.
• Playback of the test song was via retail CD played from a laptop through a Tascam US-122 USB audio interface using the line outputs.

So my thought was to push the level on the program material throughout the chain to see how the system responded under that sort of demand, and here's what I did:

1. Connected the US-122 L and R line outs to the LINE1 in on channel 9 and LINE2 in on channel 10
2. I turned the respective TRIM knobs up until the OVERLOAD LED's lit occasionally...this correlated to the meters pegging and PEAK LED's lighting regularly when the input was monitored by the meter and the channel faders were set to unity. That corresponded to the TRIM knobs being set to about 2:00 for both channels.
3. I routed both channels to PGM group 7 and turned the BUSS MASTER knob up until the meter (when monitoring the BUSS) pegged and PEAK LED lit frequently which correlated to the BUSS MASTER knob being set to about 1:30.
4. I then made sure that the BUSS OUT level for #7 was set to +4dBu.
5. I then connected the #7 BUSS OUT jack to the LINE2 input on channel 11.
6. I turned the channel 11 LINE2 TRIM knob up until the channel OVERLOAD LED lit frequently (which by the way corresponded to the meter being absolutely buried like constantly pegged and PEAK LED lit almost constantly if I set the meter to monitor the input and raised the channel fader to unity). The TRIM knob was set to about 2:00 to get those conditions.
7. At this point I wanted to PFL channel 11 so I could hear how it sounded, but since the master fader is out of the mixer right now I can't use the STEREO or SOLO busses, so instead I connected the PRE OUT jack of channel 11 and brought that in direct to the master section using one of the ECHO RETURNs, and then routed that to the MONItor buss and the sourced that to the headphones.

So does that make sense? Basically pushing the TRIMs where the program material came into the mixer, pushing the sum of those channels, pushing the TRIM on another channel sourcing that +4 BUSS OUT and then doing a pre-fade listen...so how did it sound?

I was shocked. It sounded really good. Totally clean and pleasing to the ear. My notes I wrote at the time of the test state "Sounds beautiful. Full. Clean. Wow."

It was easy on my ears but I sincerely have never heard that song sound that good in the headphones. The bass and kick drum (kick coming in at 1:05) were quick and full but not overbearing. This was all in mono so I can't comment on the quality of the stereo field, but suffice it to say I think the LINE inputs handle +4, yes?

 

[A Reel Person]

Sweet!

If it walks like a duck & quacks like a duck, then it's a duck!

Jerry Garcia must be smiling down upon you!

 

[cjacek]

That is great, Cory and can you imagine how so much better it'll sound when you actually start mic'ing in-studio instruments, rather than using a CD played from a laptop through an interface?!

Someone once mentioned here that a tape deck is not a measuring tool but rather a musical piece of gear. The lesson here is if it sounds good, on complex program material, such as a musical instrument, with overtones etc..., piano, guitar, drums etc.... who the hell cares if it's not dead stable or super accurate on simple test tones or that bias is off by a fraction. How good does it sound indeed.

 

[sweetbeats]

Yeah..

Well, what's hitting me right now is this (after I laughed about Dave's Jerry Garcia bit and then read your post, Daniel): I've been wondering all along "how good is this mixer going to sound?" "Is this going to fit the bill or is it going to be a noisy pile of over-complex hunk of gear, useful for little more than a conversation piece?" I thought about those things mainly because the thing is dripping with opamps and standard-quality pots...lots of them, and some funky wiring...just lots of roadway for signal to have to go through (though of course there are a stoopid number of ways to patch signal more directly from the control surface or on the jack panel). But somehow the thing has gobs of headroom (compared to what I'm used to in my world) and sounds clean as a whistle and full as a freighter when everything on the control surface tells me I should be expecting to hear thin 'n' crunchy in the cans...and I'm left wondering why I'm even thinking about upgrading opamps...I still think there are a couple upgrades in the master section but I think I'm going to keep the rest of the upgrades to just the first two channels. Those will be my "super" channels and the ones I will use for critical sources, and also the PGM groups I will use for mixing down to stereo. I still plan on recapping everything over time just as a sort of insurance policy and extended lease on life for this thing, but I'm the kind of guy, surprise-surprise, that needs to be smacked in the face (figuratively speaking) before I reach the "oh now I get it" point. I get it...I think.

So this epiphany with the recent test means 1 of 2 things:

1. My other gear and ears and engineering skills really suck and so anything sounds better, or...
2. The M-___ is a pretty good sounding board.

Chances are the truth is somewhere in the middle but I have to say, all things considered, it is still pretty incredible that this kludgey mixer that has been relatively abused functions and sounds like it does.

 

[cjacek]

...and I'm left wondering why I'm even thinking about upgrading opamps...

That's actually a good question 'cause I think most of the time "upgrade" means, at least to me, changing the sonic character or capacity of a piece of gear, which can be, many a time, bad.

You know how many a time people totally recap a piece of gear, for instance, choosing the latest and greatest components and what inevitably happens is that they change that instrument's personality, making it sterile.

 

[sweetbeats]

Master Fader Finished

Finally got the master fader issue wrapped up. It was a struggle on several fronts because of the wiring and trouble sourcing push-nuts needed to hold the wiper chassis rails in place.

On the wiring, not only is the M-___ fader pin config different than, say and M-300 or M-500 mixer, the master fader is different than the channel strips. So I thought I could refer to the channel faders as a guide for determining what goes where in the fader plug...not so. All good now and works beautifully. All the cleaning helped. Smooth fade, no crunchy noises.

Here is a pic of the finished rewiring job and everything cleaned up...I decided to paint the sections that were lacking plating thanks to the rodents of days gone by:

Now regarding the wiper chassis rails...there are two rails, and each one is held in the body of the fader by a push-nut on one end of the rail. These get destroyed in the disassembly process...figured my local hardware store would have replacements...nope. Got impatient and improvised using #2 internal lock washers. I was able to get the washers into a conical shape using my spring punch, tap them onto the end of the rail using a small nut as a drift and a plastic mallet, and then using the same nut and the spring punch (with the point removed) drive the washer on the rest of the way. Worked really well.

Here is the fader assembled. The way these are built with the quality of the wipers and the finish on the element board, I imagine they will have a long life.

Now on to the problem with the 400Hz oscillator tone...

 

[sweetbeats]

Oscillator Switch

Pulled the oscillator switch out tonight.

Its clean inside, no obvious issues and a neat design. I will do a detail clean on it and test each leg for continuity.

Here's the switch:

And here is what it looks like opened up. You can see the neat rails with the sliding contacts on them in the background and the carriage inside the switch body in the foreground:

And with the carriage removed you can see the spiral of pins on the knob drum that actuated the carriage:

Here is a more closeup picture of the rails:

Next step is to clean it up a little more, and then start testing he cntinuity between the rails and the resective pin.

Also, I was originally thinking that I'd need to pull one leg of each resistor in each array to test, but with the switch removed I don't think I need to since the proximal end of each resistor (with respect to the switch) is on a dedicated trace to a leg of the switch, so that's nice...I can just take multimeter and check away.

 

[RICK FITZPATRICK]

I was able to get the washers into a conical shape using my spring punch, tap them onto the end of the rail using a small nut as a drift and a plastic mallet, and then using the same nut and the spring punch (with the point removed) drive the washer on the rest of the way. Worked really well.

Nice improvisation. I like it. the Mother of Invention strategy. If you ever need those pushon nuts, check a Napa automotive store. They have lots of these kind of products.

I must admit sweetbeats, your determination is outstanding. And I thought I was brave just taking the bottom pans/arm rest assembly off my M3700 Ha!

ooooohh, I completely forgot. I noticed you bought an Ampex 440 the other day. It just so happens, I have a 440 "parts deck" sitting out in the shop. I'll post a picture of it as soon as I can. Maybe theres something there you need. Maybe we can work something out if you do.(hint hint..I need some info on fixing the speed control on my Tascam 122b cassette deck) Let me know if your interested.
fitZ

 

[sweetbeats]

Rick, I'll PM ya, and thanks for the nice words...and oh yeah...I forgot to check auto parts. You're right. I'll check into that.

Tested some of the oscillator switch and I don't think its the problem. I tested the OFF, 64Hz, 400Hz and 1kHz functions and continuity exists where it should and doesn't where it shouldn't. I'll test the rest of the legs later. There's a little bit of oxidation on the rails so I'll polish that up and inspect the thing really close but outside of that I believe the problem with the 400Hz tone is somewhere else in the path so I'll test each resistor next and do it from from points one removed from the solder joints of the resistors themselves and in that way I'll also test those said solder joints.

 

[sweetbeats]

Totally Frustrated

Got the oscillator switch gently cleaned up and reassembled...fully tested. I'm confident the switch is not the problem. Checked each of the 5 resistors in each of the three sets (15 total) of resistors directly associated with the oscillator. The testing included a check of the pads to which the resistors are soldered. All is well, except for each resistor being about 13% off in value. The 400Hz tone still drops off. Can anybody help, or maybe recommend a resource? Reminder: I believe the oscillator circuit is very much like the one on the M-500 mixers.

It seems the DIM control in the Control Room section is not working...So many times I've wished I had this control at my fingertips, and I was excited that the mixer included it, but it doesn't work. Stared at the PCB for probably an hour tonight trying to figure the thing out...this is when I wish I could understand/follow this stuff better. There are transistors in the circuit and that just throws me. I need help. I don't know if anybody can and I realize its asking a lot, but I need help.

Yes, I could just let these things go, but they would actually be useful to me and it always raises the question "If those things aren't working right, what else maybe ain't right?"

Also can't get the output level switches to work on the stereo, control room and studio outs...putting that aside for now.

Can anybody help?

 

[RICK FITZPATRICK]

Totally Frustrated

Ah yes, the HR typical syndrome. I feel for ya. I'd bet most HR enthusiasts have either been through or are going through a "frustration" period.Me included..although in my case, sometimes I think its become permenent. Reminds me of that song..."momma said there'd be days like this...". When I was younger and had "days like this"..my mom used to say..."well, don't fret too much, tomorrows another day and every day brings change...even if you don't want it."

And sometimes you just have to let go of things. Even for a little while. If I push too hard on "irreconcilable" problems, I get real depressed. Thats why I've learned to put them aside and concentrate on those things that I CAN solve. Even if I don't want to. Success at something is definetly better than frustration. Case in point...my 122b cassette. Been sitting in the shop for over a year. I almost tossed it a few weeks ago...untill a glimmer of hope came in the form of a forum post. One titdbit of info that got me past a frustration period. It could turn out to be false hope...but that remains to be seen.

Have you thought of this Cory. This was a prototype..right? Could it be that maybe the electronic circuit design wasn't as well thought out as it could have been, or didn't work in the first place? Isn't that what prototypes are for? Perhaps you might have to settle for those features that actually work and be done with it? Although, I know you're a determined kinda guy, and maybe someone will come though with an answer. Maybe set it aside for a day or two and work on something else?
Anyway, I wish you the best of luck on this thing. You deserve it.
fitZ

 

[sweetbeats]

I'm heeding every word.

That is all wonderful advice. And I AM at a juncture where I could just set those nigling issues aside and so I'll have to think about that.

Maybe those functions DON'T work...maybe they never did. I thought about that and actually (maybe to the surprise of some) I could let it go if I could prove that that is the case. But I can't even do that at this time. The circuitry gets too complex for me partly out of lack of knowledge but also for lack of practical experience following the path(s). I get distracted and then confused trying to figure it out. GREAT mental exercise but more pain than gain at this point. My staring at the DIM circuit last night was actually more trying to answer the question 'was it ever functional' vs. 'what's broken' (there are 6 legs on the MUTE and DIM switches, a pair on each are common so I'm assuming that's where the audio goes out...the MUTE switch connects that audio path to ground when depressed but the corresponding legs on the DIM switch connect to nothing...no pads...I'd expect those legs to take the signal through some resistors...)

Anyway, and the oscillator thing has GOT to be a malfunction of some sort...the whole oscillator circuitry on the PCB and the switch itself...it looks very non-kludgey...well thought out as opposed to some other bits in the master module so I suspect it worked at one time.

More than anything your post helps me relax and be patient. I'm just anxious to put the dress panel and knob caps back on that master module! And I wanted to wait until I was DONE with it before doing so (don't want to keep having to remove and replace those things), so there's another driver.

I'm probably going to keep hammering away at this until its either fixed or I can determine if it never worked...and then I imagine someday I'll know enough to mod it so it works.

 

[A Reel Person]

I'm just mega-impressed with all the tinkering and progress,...

but you might want to put it in perspective and decide if you're a bench tech first or a musician, artist or producer? Eh?

No doubt, that's an awesome board. I'd love to see it all back up and running. You've made great progress & are close. You'll get it in time!

 

[sweetbeats]

I'm not a bench tech...experience and knowledge is too limited.

I am a musician, but I really enjoy the capture process so...

That makes me a kind of engineer I guess and/or producer on a very small scale, BUT...

Equipment is required to capture.

I believe I am an artist...I think there are artists that focus on restoring old works of art. So...there is an artistic outlet in fixing up the M-___ and associated bits, and since fully functional gear will help the capture process when I express myself through music...well...there you go. I'm enjoying it (the fixit-up process). Not burning out of it, mainly just frustrated that I don't know more to be able to figure this stuff out without burdening/waiting for others.

So with that said, I'll have you all know I desoldered the Control Room monitor switchrack (the set of 7 switches that determines the source of the Control Room outs, and also has the MONO, DIM and MUTE switches) because I wanted to know for certain how those switches operate/behave. It didn't take too long to remove it from the board and test all the switches and that helps me to start understanding better how stuff flows, but I'm certainly not there yet.

I'm also going to start drawing up a wiring diagram for the master module, which will lead to me doing PCB layouts and ultimately schematics, though the last item is where I'm going to need to find help.

Anyway, here is an exerpt of the circuit around the DIM switch. I'm still trying to understand how it works...and I'm not going be surprised if I'm going to need to extend the schematic to answer that question. It looks pretty clear to me that the output of the Control Room master level pot goes to the MUTE switch which passes signal to the DIM switch, and then there are some resistors in there but I can't get how those get utilized. When the DIM switch is depressed it would force signal to take a different path.

Anybody?

Is this just simply the wrong forum for this stuff? Honestly...

 

[Shoulderpain]

So incredible what you've done so far

It's inspiring to see this kind of discipline.

Outside of smoke tests and sound tests, have you been dreaming up what your first recording session will be?

 

[sweetbeats]

Colin!

How are ya??

Don't forget, everybody, shoulderpain is the one who brought this mixer to the forum...found it at a yard sale last year and then graciously passed it on to me.

I do think about what it will be like using it in operation. Those thoughts were whetted recently when I did a final systems test on a Tascam 48 I just sold and used the M-___ to route incoming stereo program material to all 8 tracks and manage the returns from the deck. It was awesome seeing 12 of the 14 VU meters dancing around.

This phase is proving to be valuable too with all the testing as it is getting me fairly aclimated to the control surface which was a little daunting at first not knowing what does what exactly, but those issues are melting away 1 by 1.

I think it will be a pleasure to use as the controls, in spite of their age, are proving to be in pretty good operational condition (nothing worse than having to fidget with controls to try and get a reliable cue mix to the talent), and the mixer feels like it has more headroom than what I'm used to and that's exciting.