Where was I? Oh yes, I had polished the legs of all analog ICs in channel 8, reinstalled them, and then observed function. Success!

So naturally at this point I disinter all the patina laden ICs in channels 5 through 7, clean them up, reseat chips, reinstall channel cards and apply power.

Nothing. Well, unless you count the full meter deflection of 5-8; audio certainly isn't passing through any of these.

That includes channel 8, of course, which, by my own observation was just passing the first tier of functional test and literally has not been touched since.

Pull card 5 (again, if I was thinking clearly and weighing out all the evidence this step would have been reduced to one selection, but I was at this point a touch flustered). Power up, 6-8 pulse the VU. Power down.

Leave 5 out, pull 6 & power up; 7 & 8 pulse the VU, power down.

Pull 7 & power up, channel 8 shows a minor bump on the VU and now passes sound. Power down, reinstall 5.

Power up, channel 5 shows a minor bump on the VU whereas channel 8 does not deflect at all, both channels pass audio. Power down and install 6, which completes the pattern as expected.

Channel 7 contains a problem that was pulling down channels 5-8. Having my quarry fixed, I now begin to puzzle this out logically, and based on the power rail observation made previous I begin with measuring the rails for shorts. -15 volts to common measures 0.8 ohms.

Ah, the dogs, they have your scent. All integrated circuits are pulled, the short remains. Of course it wasn't going to be that easy. I pore over the schematic for all points tying directly to the negative rail, and choosing for now to ignore resistors narrow my focus to a dozen or so parts and begin to pull them out, one at a time to determine which part is failed.

Next time, I'm going to start with the caps. The dense two sided PCB has made me out for a bungling idiot, and before I actually pull any traces that are important (this was just an insert) I shift my method to cutting.

Which, while destructive, limits the destruction to parts at hand. I'm still exacting far more destruction than I feel I should on this simple task. This particular component was just fine before I came along, WOOF.

Having ruled out transistors I then focus on capacitors. C4 here is the problem, a Matsushita 1 uf @ 35 volts, it's marked as being polarized. Tantalum? Odd package, I can get those values in a similar dimension in ceramic, which is my plan.

Of course, I threaded up a reel of tape and learned my record levels have been killed (previously 3 & 4 had recorded well), now everything reads high to tape and plays back low, with the exception of previously recorded material. So now I chase a common point in all channel recording. I'm thinking bias as a starting point, have mercy.

Having temporarily ruled out a logic problem (which seems logical in retrospect, since I have observed the status LEDs appear to be driven from the state of the channel card itself and not the logic commanding the card), it was back to troubleshooting the machine as a complete unit.

While I had all the cards pulled I naturally brushed out cobwebs and hit the edge connectors with contact cleaner, so it was only logical to fully reassemble the unit and see if a good cleaning did the trick. No change.

Next step was to narrow down point of focus. For brevity sake I'm just going to observe the behaviour of the VU meter on power up and power down, since in consulting a friend who has a very functional version of the same machine I was informed that meter deflection of any sort in response to the on/off switch was unusual. So I swapped cards 4 & 5.

No change in 5-8 deflection at power up, though now #4 was pegging the channel 5 meter as a steady state while the natives along 6-8 returned to zero. Quickly power down & confirm card was seated correctly. I pull channel 4 card & channel 6 card to compare, they are, for all intents & purposes identical (beyond the fact that they carry factory stickers indicating their home channel).

I then install the channel 4 card into channel 6 slot, placing channel 6 card into channel 5 slot. 5-8 full deflection tap and now channel 6 is pegged. Power down.

While channels 1-4 have not been considered suspect, due to them actually functioning (and bolstered by the fact that channel 4 has in fact recorded to tape and passed the listening test, which I would like to think is pretty stringent). I then swap Channel 3 with #4 in 6. 5-8 full deflection tap and channel 6 remains pegged. Power down.

At this point I overlook a step that could have saved me a little time and effort. It should be obvious that I am not a factory trained technician, nor am I intimately familiar with the workings of this machine (getting there). I'm pretty much working in the dark with my empirical approach and a set of freely downloaded schematics.

Make no mistake, I am brimming over with a sense of gratitude toward the good people at AnalogRules.com, for without these highly useful documents I would quite simply be fucked. However, there came a point at which deep circuit analysis turned into a hide and seek sort of affair due to a lack of clarity in the scan. So yeah, I'm pretty much just making this process up as I go along.

So, my next step was to remove all the cards, set 1-4 aside as known functional and install 5-8 into the 1-4 slots. Anyone care to guess what happened?

Nothing, unless you count 1-4 meters deflecting fully on power up. I run signal test to confirm a complete lack of function.

Based on this observation (and brownie points to whoever catches my mistakes here) I shift focus on channel cards 5-8 themselves, and ignore for the moment the steady pegging of the low number cards in high number slots (another hint). I had deemed the probability of discrete failures on the four cards, right next to each other, as highly unlikely, but it's worth a look anyway.

All my analog ICs are looking a touched charred (this batch is among the cleaner looking). Granted, there's patina on the analog ICs in channel 1-4 too, but I'm not paying them much scrutiny, as they are not problematic. Incidentally, all the digital ICs are looking quite fresh & nickel plated, so it's a metallurgy thing, perhaps there's a high content of silver in the legs. Whatever.

So, channel 8 is my guinea pig, I've plucked the chips and go to work.

600 grit sandpaper and a little time, before and after shot here.

The ICs cleaned up pretty well, flushed the sockets with contact cleaner, reseated the ICs into channel 8 card and sink that channel into channel 8 slot - with no other cards with the exception of the logic control. Power the machine up. Minor deflection on channel 8. The card passes sound!

It is at this point that, flushed with a sense of progress and ready for a break I happily report to my magnificent and wonderfully patient wife that I've finally secured what appears to be a solid lead amidst a convoluted mess of red herrings. HAHAHAHA, oh the humanity.

So back to the Otari MX-5050 mkIII 8. Symptoms as follows:
On power up meters 5-8 deflect fully then return to zero.
On power down meters 1-4 deflect fully then return to zero.
Setting monitor to all input, audio passes through channels 1-4 while 5-8 has nothing.

So, my thinking is naturally focused on the split point between channels 4 & 5. With that in mind, and based on the observation that a lot of the CMOS gates used in logic direction are four per IC I started my analysis there.

As you can plainly see, that angle was not fruitful. There is not a single instance of component pairs straddling the 1-4/5-8 configuration. With that, I cracked the case and began to pull cards.

Here's the control card to go along with the (partial) schematic above. The fact of the matter is that this card does not have the same part number as the schematic I've been working from. I settle into the idea that I may have to resort to a trace by trace comparison to see if I can narrow down a culprit IC, but since I'm pulling cards now I may as well go for broke and thoroughly give everything a visual inspection.

So far all the cards & traces look good. I know, that just rules out the obvious...

...having been bit in the ass by the obvious after chasing the esoteric a few times too many that's where I like to begin.

While I have the motherboard exposed like this I do a continuity check on my pin 2/3 swap on the inputs and outputs of both channels. Everything measures up well.

Without really getting into photography it's difficult to demonstrate the look of backlit traces. Let's just say that evidence of pulls, shears or cracks do not exist. Everything passes on the visual level. One thing I do notice, is that the +/-15 volt supply rails (brightened in the picture) just span 1-4 and 5-8 (with a contiguous ground reference). This is the first point of a well defined 1-4 versus 5-8 I have observed on this machine, and it may prove useful later. But for now I simply apply power and confirm that the supply rails are delivering at all locations.

Next up, chasing a series of red herrings..

Okay, I've known about the Otari XLR pin 3 hot configuration for quite some time now, but in practice I'd never run into a catastrophic failure because of it (that is to say, I was able to record and playback audio on the 2 track machine with little issue). Therefore, that bit of information sunk into the dusty corners of my mind where it was wholly ignored. Of course, all that free riding came to a screeching halt once the Autogram AC-8 was planted in the audio control trunk. See, in my wiring up the output I included unbalanced 1/4" phone plugs which were at one point normalled to the monitor amplifier.

So, when I connected the pin 3 hot, pin 2 ground configuration of the Otari to the pin 2 hot pin 3 neutral (actually earth referenced thanks to my inclusion of unbalanced 1/4") configuration of the mixer output, a whole lot of nothing happened. Nothing to the tune of even killing the monitor feed to the amplifier.

I had entered into really easy diagnostics, unplug cable = sound, plug it in = zilch. So, with a nod to the widely published pin 2/3 issue, I built a pair of XLR cables with pins 2 & 3 swapped. Solved. Of course, at this point I recall a few months previous while I am working on the basket case MX5050-8 and can't get any channel response out of it, which, at that point was a step backward but given the hard life the machine has had I place it closer to the parts bin. Why yes, I had been feeding it audio from the Autogram while failing to get signal, it was just prior to the monitor amp feed modification.

So, the 2/3 swap XLR cables worked here too. I was able to commit signal to tape and playback (on conventional XLR), and it sounded good. So, my options now are to make another 6 pin swap XLR cables, or just modify the machine to reflect pin 2 hot. Ready to cut traces and jumper stuff at board level, I was met with this magnificent sight. Build six special purpose cables, or simply rewire 16 connectors?

Easy decision.

I did this, from poor placement in a room (that gets gear hauled in and out of)...

..and this. Very preliminary internet searches do not favor the notion that this is an easy & standard current production off the shelf part. Since the switch still works I think I may drill a small pilot hole in the lever portion and turn down a bit of small bar stock to have an end pin, then epoxy the new arm in place. Worse case I'll still have to find a replacement switch.

Since this machine got hammered the interior finds include stuff like this, from the side panels.

After a short session of soldering I decided to tackle some of the mangled sheet metal on the Otari 8 track.

I still haven't conjured up a sequence of events that explains this condition while supporting the integrity of the surrounding machinery. Must've been a one in a million shot.

Here's another angle on the vent cover distortion, after extraction for hammering. Naturally, most of the hammering occurred while the sheet metal was not installed.

I popped the meter bridge face off to facilitate removal of the vent metal, only to discover that it too had taken on some stress.

Since a wood floor isn't a precision surface, here's a shot with the piece reversed proving the bend is in the metal. The repair for this was gently bending it back over my knee. Very little force was required.

Installation was not as easy as place & bolt, there was a mild interference fit at the butt joint between the vent shape and the transport shell. I'm not overly concerned about the skin of the machine though, so it was hammered into place.

While not minty fresh, this is a vast improvement over the former condition. Surface distortion, while very apparent to the touch, does not jump out and grab me by the eyes. However, while everything was disassembled a knob for input level struck out on the path of freedom, and remains at large. While I'm certainly not going to overlook a possible bout of personal absent mindedness in this, I'm hinging my investigations around my helper monkey who may have swiped it as a trophy.

We're preparing for a move. This will absolutely disrupt an already shaky path of progress. How long? Who knows. Onward and upward, I'm fully looking forward to upgrading space. In the meantime, I'm going to endeavor to ramp up my posting here, so as to not lose sight. I think this is going to turn into more small posts and a few one liners.


Threaded tape on the Otari again, pictured here is a destructive alignment issue. Machine appears to be keeping speed (the first tape I played appears to have been recorded on a machine with SERIOUS issues, dishing up a heap of red herrings).

Once I have the tape path free of obstructions I plan on buying a proper calibration tape.

A little over a year and a half ago I happened into an Otari MX5050 mkIII-8 half inch eight track in as-is condition for a price I couldn't pass up.


It looked like it had taken a bit of a tumble while in storage, so I'm approaching it as if it's inoperable but remain cautiously optimistic.


That'll hammer right out...


I can image people recoiling in horror at the condition. I think some proper wood panels may be in order once electronics and mechanicals prove serviceable.


I am at present awaiting the arrival of tape, by the time it arrives I will have exacted a brute force fix to this take up side idler arm.


The (presumably) original roller was about the consistency of Play-Doh™, and absolutely useless for anything beyond making a mess. I bagged it up and sent it to Terry's Rubber Pinch Rollers and Wheels for rebuilding.


Terry does good work. Here's the returned Otari roller posing with the Tascam 34 roller it travelled with in April of 2010.


Here's the roller installed on the machine fresh out of storage, I will be pulling it and washing it in mild soap and water to eliminate the storage film.. along with a rigorous cleaning of the rest of the deck.


The headstack carries a JRF Magnetics relap sticker. I've seen a thread on gearslutz in which someone is questioning the condition of a similar looking stack, also carrying a JRF sticker. See, the contact edge to tape appears to be a flat as opposed to convex plane, and at first impression it looks like the tape has cut a saddle and the heads are shot.


However, the polished "saddle" extends above and below actual tape path as seen here indicating intentional machine and polish. So with luck I've got a few more hours on the heads.


I expect to pull and recap these channels, once I'm through recapping the mixers (one of which was partially disassembled last night, which is a post for another day). The tape and take up reel that are in transit will arrive prior to electronic work, so I expect I'll get an earful of the electronics here prior to recap. Which, better judgement aside, will factor in to prioritizations..

The logic circuits, front panel switching and transport all appear to be in good condition.


The 3 & 6 input connectors may receive a little of the aforementioned brute force repair as well.

Not striving for minty fresh here, just functionality.