Tascam M-___ Story...

[sweetbeats]

So, random post. But maybe not so random.

The M-__ has mostly been just sitting under a custom LeCover for since my last post, but recently I’ve been prepping to capture a series of videos detailing the features and functions of the M-__. This is something I’ve been wanting to do for a long, long time…a video operations manual of sorts. I’ve shared that previously in this long forum topic. But anyway, because I’d like to finally capture those videos I’ve been taking the time to *finally* tease out details of the M-__’s functions and signal flow…I know it’s been, like, 18 years, but life has been crazy, so forgive me. I’ve never fully gone through nailing down ALL of the details of the functions and signal flow aspects of the M-__.

What I mean by that is detail stuff I can’t remember because I don’t use it enough, like the five different SOLO buss sources on each input/output module…where does each specifically tap the signal chain? The MONI TO STEREO switch that sums the monitor buss source on each module to the main buss…is it pre or post channel fader? The CHANNEL MUTE kill switch…what all does it actually mute? There are a total of 19 output channels from each input/output module…I knew off the top of my head the CHANNEL MUTE switch did NOT mute the DIRECT OUT jack. What else? Anything? Now I know. I have a couple other things I want to verify yet, but I’m nearly done going through the whole console with a fine-tooth functionality comb.

In the process, however, I became sidelined with finally clearing up some annoying issues…the signal flow to the STEREO buss has been problematic…both to the audio output as well as to the meters…intermittent…really annoying. And there has also been an annoying problem with the headphone output related to higher than normal noise level and DC popping in the headphones when any of the monitor buss global REMOTE source switches are pressed on the Control Module, which is what Tascam named the module that contains the master fader, oscillator, talkback functions and studio and control room functions as well as the master monitor and “remote” control functions. BUT I’m happy to say I’ve tackled those issues…some reflowing of solder joints as well as reinforcing the wire management took care of the intermittent STEREO buss signal flow and meter monitoring. I’m also finding it’s been enough years now it’s needing some maintenance on the edge connectors that interface the modules to the mother board, so checking the contacts in the sockets on the motherboard as well as a little gentle polishing of the contacts on the module edge connectors and then treating with DeoxIT D100…exercising, etc. It wasn’t bad, but it was time to do something and I want to take that variable out of the mix if I am troubleshooting a problem. Anyway, STEREO buss problem fixed. The headphone issue…I used to have a quote at the bottom of all my posts: “Never underestimate your capacity to be a complete ignoramous”…something like that. Well, I’ve done it again. Many years ago I cleaned up a VERY kludgey abomination of a circuit block on the “Phono Amp PCB” assembly in the Control Module. That’s the right-most PCB assembly of the four main boards in the module. I can’t remember what the function of that circuit block is at the moment, but I detailed it in this thread years ago…stacked opamps, one in the board and the other upside-on top of the first, just hanging in free space held only in place by the point-to-point wiring and components soldered to its pins…no judgement…it is a prototype device after all, and sometimes these kinds of things are even incorporated into production models by any manufacturer. But it was messy and I wanted better access to both opamps, so I modified things, moved one opamp into a vacant DIP8 site on the board, incorporated some jumper wires, etc. In that process I noticed the signal ground pins of the headphone level pot were referenced to signal ground via the shield of one of the output wires. So let me explain the level pot and the source switches for the headphone amp are on the right-most of the four PCBs, but the amp itself is on the left-most of the four PCBs. So there are wires from the two output pins of the level pot on the right-most PCB that go the amplifier input solder joints on the left-most board. The shields of those wires are also terminated on the PCB with headphone amp circuit block. But they also terminated the shield of one of those wires on the right-most board on a hand-added solder pad in order to connect the signal grounds of the level pot to signal ground on the PCB with the amp. Maybe that’s fine. But I wanted both channels’ shields to float at the pot so they could both properly shunt any induced noise interference to ground on the board with the amp…float at the source, shunt at the load. So I did that. And then found the closest place on the PCB with the level pot to strap the signal ground to the appropriate ground path…the headphone amp is powered by a +6VDC power supply, which also powers the meter lamps and LED indicators, so the ground reference for that supply is on almost every PCB in the console. So it made sense to me to just jumper the headphone level pot signal grounds to the closest spot where that ground was available on the same PCB as the level pot. Nope. What I didn’t understand at the time or think through at the time is the potential for DC offset by doing what I did. The original reference to ground through the shield of the one signal conductor strapped the signal ground of the level pot close to the amplifier circuit block ground path, as well as very close to where the PCB with the amp references system ground at the motherboard…and more importantly the +6V rail reaches the LM386 opamps in the headphone amp directly…no dropping resistor. However the +6V rail goes through a series dropping resistor on the PCB with the level pot. So that enabled some amount of DC offset in the power rail for the headphone amp between the pot and the amp (enabling the *pop* when I would latch any of the monitor buss global REMOTE source switches that are on the same PCB as the level pot, and the switches also power an LED indicator when latched), and beyond that actually increased the risk of noise interference in the headphone amp circuit…and in fact I think raised the noise floor of the headphone amp. I think the reason I didn’t notice after I did those mods years ago is because I wasn’t really focusing on the headphone amp after the mods (more focused on the modified amplifier circuit), and then probably didn’t do anything operationally with the console after that…I think I moved shortly thereafter and then the console was in storage for awhile. By the time I noticed the new problems I bet I didn’t associate them with the mods I’d done because substantial time had passed. Anyway, ground issue fixed…I ran a separate connectorized ground wire from the level pot signal ground pins to the left-most PCB on an available solder pad close to the headphone amp circuit block…no more pop, noise floor better, and proper attenuation. Fixed!

And here is the Control Module ready for final reassembly and reinstallation in the console frame, so I can tease out my last couple question regarding functionality…

Next step is to write out all the details of the control functions on pages I’ve printed with sections of each module…number each and document narrative like in an operations manual…and then capture a video series covering all of this and get it up on YouTube.

 

[sweetbeats]

Well dangit…not fixed…two new issues…one of which was likely there all the time, and the other probably the result of my reflowing of solder joints…

Problem #1: the right channel of the STEREO buss allows bleed from all sources assigned to it even when the master fader is all the way down…from the I/O modules, from the oscillator, etc. It happens regardless of how the main buss is monitored…at the buss’ output jacks, or via the from control room or studio monitoring banks…in the headphones, everywhere. I’m thinking this will be pretty easy to sort out.

Problem #2: some weird phase cancellation between the CR monitor and STUDIO monitor banks. If the same source is selected in both CR and STUDIO source select switchracks, and CR is also sourced in the STUDIO bank, the signal cancels if the selected buss’ level control (like the STEREO master fader if monitoring the STEREO buss, or the monitor buss level pot if monitoring the MONI buss) is swept to a certain level. This has likely been there all along and is the result of all the hackery that’s been present in the Control Module ever since I’ve had it. I’ll have to start investigating.

And this is how it goes. It’s a prototype.

 

[sweetbeats]

Fixed it…sort of.

So with the first problem with the bleed-through in the right channel of the master buss, I figured either it’s reaching the outputs in some way outside of the fader and fader booster amp, or it’s somehow a problem with the master fader. So I figured one way to test that is just remove the master fader. If the problem stops it’s either upstream of the fader or a problem with the fader. So I did that and the problem was gone. Okay. So then I figured I could test if the problem is the fader itself by swapping the L and R connections…if the problem was upstream of the fader it wouldn’t matter which fader element was connected to which amplifier PCB. Conversely if the problem followed the swap (if there was now bleed in the left channel), then the issue resides in the fader assembly. The problem followed the swap. So I pulled the fader assembly out again to open it up and take a look. These faders are older ALPS assemblies, like pre-K fader type…tall vertical conductive plastic element board, wipers are very fine filaments, polished slide rails, etc. Pretty nice…I couldn’t guess what the problem might be. And you know what? I’m still not sure. The wiper carriage is an assembly of cast aluminum and molded nylon. The nylon tends to shrink over time and crack, so I did see that, but I couldn’t identify what or how that was causing an issue. I did install some mini zip ties to help hold the carriage together. So, I still don’t get it…but everything is now working correctly. I don’t usually sit comfortably with “I don’t know how or why, but it’s fixed”, but in this case I’m pretty okay with it because at least I know it’s the fader. If the problem resurfaces I’ll pull it back out and dig deeper. The good news is with the modular plugin construction, I can have the fader accessed and removed in about 20 seconds.

Now for the second problem, the wierd phase cancellation between the control room and studio monitor banks, well, I chose the very easy path…I did nothing. But this was after taking some time to just mentally deliberate the issue…time to tease out a question I had when I first discovered the issue. The problem is this…the console is all analog and unbalanced internally right? So it’s not like I have hot and cold conductors to swap as in a balanced line phase inversion…and that was my question when I was just discovering the problem…if there is only a single AC signal conductor, how could it possibly be inverted when sourced both in the control room monitor and studio monitor switchracks and summed? Short answer is it can’t, at least not at the signal conductor level…if I’ve lost my mind on this and am overlooking audio signal 101 somebody please tell me…anyway that led me to realize it is likely an issue with signal paths in those two monitoring banks, and a design error where there is a misalignment in the number of inverting and non-inverting opamp stages. For those that understand opamp theory hopefully what I’m saying makes. I know enough to be dangerous. But opamps have inverting and non-inverting inputs, and care has to be taken so the outputs of each circuit block are aligned either inverted or non-inverted, or you run into issues just like I’m experiencing. So I think that’s the issue. And I’m not going to dive in and try to identify the specific issue needing redesigned, because it is highly likely there will be a domino effect from that. I’m going to need to chalk it up to prototype “charm”…I could even probably make lemons out of lemonade and find a way to use the flaw to my advantage when using the null technique to set tape machine azimuth. But the reality is the problem only arises when monitoring the same exact source between both monitoring banks, and then summing them…and there is never a functional reason to do that…for instance to source the main buss in the control room monitor bank and also in the studio monitor bank and then sum those together and listen to them both in the headphones or whatever. No. Certainly might be sourcing the same thing in both, but for the purpose of sending playback to the recording room as well as listening to playback on the control room speakers, but not summed. And there’s not a general functional problem with one being inverted compared to the other because the outputs of those monitoring banks are not used for tracking. They are used for monitoring. But I think I want to do a little further testing to verify my theory and check if the problem exists at the output jacks as well without summing using the “CR” source switch in the studio monitor switchrack. It would matter to me if it was the control room out inverted compared to the rest of the console vs the studio monitor out, and I think I know how to test that. Sorry this all probably doesn’t make a lot of sense…it would be easier to demonstrate in a video I think…selfishly part of the reason I’m info-dumping this here is for my own reference down the road.

I had one more little thing happen…the LED indicator for the global REMOTE source switching for the MONI buss on the Control Module wasn’t making good contact with its socket anymore…wasn’t lighting up. Nothing I was doing as far as tweaking the pins was working. I looked closer and the position of the little daughter board the socket housing is soldered to is further away from the control panel, so the LED when flush with the control panel isn’t making good contact. To fix it right the entire Control Module assembly would have to be completely disassembled in order to desolder, reposition, and resolder the daughter board to the Phono Amp PCB. Not doing it. I consider the Control Module assembly to be relatively fragile. The phenolic resin PCBs don’t help, but neither does all the prototyping activities, cut traces, jumpers, resoldering and probably countless insertions and extractions, both preceding my acquisition of the console as well as all the repairs and discovery I’ve done since I’ve had it. Every time I have to take it apart I worry what else is going to break. When I do discover something else that’s failed I re-do it in a way that will last longer, but it’s increasingly important I think to be mindful of minimizing disassembly and handling. So instead I was able to just desolder the socket housing itself, tweak the position, and resolder. Its good now. Not “right” as much as I’d normally like to accomplish, but in this case weighing the pros and cons I felt it better to avoid full disassembly for something I could address more locally without any disassembly.

SO…now…maybe…after a few more tests regarding the control room and studio monitor bank phase inversion issue, I can get to capturing that video series detailing the features, functions and construction of the Tascam M-__.