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Thread: Otari MX80 remote shenanigans

  1. #1
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    Otari MX80 remote shenanigans

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    I am using an Otari CB140 remote unit with an MX80 24-track machine. It has been a little cantankerous, occasionally blowing the 2A internal fuse, especially if the unit was knocked. The other day, it threw a strop and now is blowing fuses continuously. I am still investigating, but I'm posting what I have discovered so far in case anyone else is interested or has a similar problem.

    The CB140 takes power from the MX80 via the D-sub connector. Pin 1 is the chassis ground, pins 16 and 17 are an unregulated 24v DC supply (they are connected together). The other pins are 19 for the tach signal, plus a bunch of others to set up a balanced RS422 serial interface to the deck.

    The CB140 takes the 24v supply and by some convoluted means converts it to a regulated 5v DC output to power the system logic. The guts of it is an STK772B chopper regulator of which there is very little information available, and almost the entire circuit board is taken up by support components. The regulator itself is attached to a massive slug of aluminium as a heatsink. The later output stages involve a triac, for reasons I do not comprehend.

    The PSU board is thus:
    dsc_5078-33-jpg

    When we see the reverse side, things start to get ugly. Someone has been dicking around with it, cutting tracks off the board and attaching these strange globes, which I think may be some esoteric form of zener diode, but look like some kind of alien technology, or would if they glowed.

    dsc_5085-33-jpg

    I've replaced the electrolytics. The PSU board blows fuses even when disconnected from the motherboard which simplifies things a bit - going by the schematics the only other components after the fuse are the triac, a zener diode, and a mylar capacitor. I have sourced a replacement triac, but not the other suspect parts. In any case the board has some extra weirdness not on the schematics, and I have to wonder if the tacked-on globes or the cut traces are somehow shorting - the fact that it tended to blow fuses when knocked does make me wonder.

    cb140_psu-png

    At this point I'm starting to run out of 2A fuses as well as patience, so my current plan is to do away with the entire board, and replace it with a 5V DC-DC converter off ebay. As I write this I have actually managed to get the remote to boot and operate by patching a USB charger to the 5V line on the motherboard, but this is a complete bodge.

    It may be of interest to note that if the remote is powered independently like this, with the remote booting up before the deck is powered, it will display Error 90, and then establish connection to the deck later once that has been powered up.

    One thing that disturbs me is that using a continuity tester on the motherboard, the 5v and Ground test points appear to be shorted together. This persists unless CN5 is disconnected, which is the ribbon connector for the transport buttons on the remote control. Since I am more interested in recording my song than experimenting on the fragile innards of an already sick remote control I have left this disconnected for now, but I would be interested to know if anyone else with a CB140 can take the same measurements and let me know if that's normal (i.e. the Fluke is seeing ghosts).

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    Had you asked this a few weeks back, I could have turned around, unplugged my CB-140, opened it up...done.

    Long story short....the CB-140 is packed away in storage and I'm back to using the original CB-124 that came with my deck...the CB-140 I picked up later, to have as a spare, and it had to get overhauled pretty heavily to get it fully functional.
    It was working fine after the rebuild...I just wanted to bring the CB-124 back online since it was sitting in storage for a couple of years.
    I didn't want it to get "stale".

    If you find you really need that one question answered (about the 5V and Ground shorting) before you can sort out your issues...let me know, and I will go dig the CB-140 out of storage, open it up, and see what I get with 5V and the Ground.
    It's not a big deal...I just don't have it here in the studio at the moment. I don't think comparing the guts of the CB-124 would be of any value.

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    Yeah, if you get the chance, I would really appreciate that. And I am on the lookout for a spare remote, just in case. Since without one I am left with a tape player instead of a tape recorder.

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    Quote Originally Posted by jpmorris View Post
    Yeah, if you get the chance, I would really appreciate that. And I am on the lookout for a spare remote, just in case. Since without one I am left with a tape player instead of a tape recorder.
    Hey...just getting back...I'll pull out the 140 today, and let you know what I get across the 5V and Ground.

    That's the one thing I wish the MX-80 (and other pro-deck) would have provided the ability to "manually" engage the most basic I/O options at the deck too, and not just the remote...but I guess that would have added a lot more circuitry to the whole package, and a lot more cost.

    Yeah...a spare remote is a must-have.
    I had a 3rd spare at one point, a really beat up CB-124 that I found on eBay, but I promised it to the Otari tech as part of the rebuild he did on the 140 for me...not that it was part of the payment, I just offered it to him to have as a parts unit, figuring it would be a "pay-forward" if/when I ever needed his services again...and we developed a good rapport, though I haven't been in touch with him for a couple of years now.
    One thing I learned from him...the remotes are really finicky to work on, and to get all "balanced" out so that all the functions work as expected. So I never wanted to try and fix my own, and preferred to keep a good rapport with him, just in case.
    Much of the other stuff with the deck, I can deal with.

    I'll have some info for you later on the 140....

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    Yeah, I'm glad it's just the power converter that's gone tits-up, debugging the logic is beyond my ability. (Curiously, the CPU has a manufacturing date of 1996 - looks like someone replaced it).

    Truth be told I had a nasty scare when rigging the remote up to USB power. At first it did nothing at all, some of the lamps glowed and that was it. At one point a weird line appeared on the display. Then I realised that I'd swapped two of the ribbon cables over during earlier diagnostics. After that it was happy and I have been tracking with it today. (But without the transport buttons, I don't trust CN5, especially with no fuse!)

    What would be really interesting would be attaching the MX80/remote combo to an RS422 analyser and reverse-engineering the protocol. That would allow for writing a software remote control, given a suitable USB-to-4-wire-RS422 dongle.

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    OK...here's the scoop...DO NOT mess with those funny looking jumpers/globes...my CB-140 has the exact same mods, so I am certain that was done by Otari during manufacturing to fix some issue. Maybe later models don't have it, or they all have it.
    Considering that my CB-140 was a rebuild by an Otari tech, if that stuff was some weird hack done by a previous owner, the Otari tech would have sorted all that out...but it's all in place, and looks the same as yours....here's the pics:

    studio_cb140_june_16_2018_-1-jpg

    studio_cb140_june_16_2018_-2-jpg

    AFA the +5V and Ground points off of CN1...they are not showing continuity. If I hold the tester for several seconds on the +5v and C10 + points, and juice up that cap...I'll get a continuity "beep" for a split second if I then move the probes to the 5V and Ground points, but it goes open instantly...so you should not have steady continuity between 5V and Ground.
    I'm wondering if C10 is crapped on your board...that big 1000 uF 25V cap...???

    The other thing I notice on your picture, is in the bottom right corner, with the comp side PCB right-side up...you have a large "glob" on that copper where the screw hole is. What does that connect with?
    Mine is clean.
    Of course, I doubt that's the issue, since your CB-140 was working, and then crap...so I think it's a component that went on you.

    Hope that helps.
    Good luck...and keep us posted.

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    Thanks. Definitely handy to know that that's a common mod - if only there was a schematic of the revised supply.

    As for C10, that's of the caps I've replaced so I don't think that's it. C9 is a suspect but I don't have any suitable replacements. Interesting that you don't have the solder blob. Looking at it again, what they've done is scrape away the resist so that the blob links the ground plane to the chassis via the screw.

    Just checking, did you measure continuity against the two test points in the bottom-right of the mainboard? Leaving the PSU aside (in fact, even with it disconnected) I get continuity between those two, but only while CN5 is connected. If you're not, I guess I'll have to pull the transport button PCB at some point and see what's happening.

    I don't really want to open the remote at the moment while it's usable, so the underlying picture was borrowed from RevoxRemotes:

    cb140_from_revoxremotes-png

    Note how the first two ribbon cables cross over. They are not keyed, so if anyone out there is working on a CB140, be careful to ensure they're put back right. Mine have labels on them, be sure they match the number on the silkscreen.

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    Quote Originally Posted by jpmorris View Post
    Just checking, did you measure continuity against the two test points in the bottom-right of the mainboard? Leaving the PSU aside (in fact, even with it disconnected) I get continuity between those two, but only while CN5 is connected.
    No...I only tested on the PSU, after I removed it. I thought that's what/where you meant about the 5V and Ground.

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    Quote Originally Posted by miroslav View Post
    No...I only tested on the PSU, after I removed it. I thought that's what/where you meant about the 5V and Ground.
    To be fair, the description was a bit rambling. I stayed up late to record while I knew the machine was working. But yes, there are two problems. Firstly, something is shorting in the power converter PCB itself, even when disconnected from the motherboard. But I'm not so concerned about that because I'm going to replace the PSU with a modern equivalent.

    The second problem is that on the motherboard, even with the PSU disconnected, I appear to be seeing a dead short between the +5v test point and the 0v test point in the bottom-right. This only happens when CN5 - the transport control buttons - is plugged in. This is a bit more concerning because it would be nice to be able to use the transport controls on the remote. I don't know if this is a real problem (e.g. possibly the reason the PSU blew up in the first place) or merely a glitch on the fluke meter, like the capacitor effect you mentioned earlier.
    If a working remote also does this, I know it's a glitch. If other remotes do NOT do this, it's probably not a good idea for me to reconnect CN5.

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    Quote Originally Posted by jpmorris View Post
    If a working remote also does this, I know it's a glitch. If other remotes do NOT do this, it's probably not a good idea for me to reconnect CN5.
    Gimme about a 30-45 minutes...I'll pull out my 140 and check that. I originally assumed you were doing all the testing just on the PSU...and that's all I checked.
    I'm just heading into the studio now...been watching the World Cup and doing other stuff today.

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