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Thread: Otari MX80 remote shenanigans

  1. #11
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    OK...this is not going to clear things up for you...well, maybe a bit.

    With everything connected...I get a Short/continuity across +5V and Ground
    With CN5 disconnected...I get an Open.

    With CN5 connected and then removing one by one the three connectors on the PSU...I still get Short/Continuity for at each step....so it doesn't look like the PSU has any impact on the continuity when CN5 is connected.

    I also tried it with both CN5 and the PSU disconnected, and it was Open, which I expected, since it's tied to the CN5 being connected or not.'

    So...does that help you sort out why your 140 crapped out...mmm...probably not much, but at least it's not what you were thinking.

    If it's not powering up and you're not getting any of the logic working...I'm still thinking it's something in the PSU...and that +5V and Ground thing on the other PCB and CN5 has nothing to do with it since mine does the same thing, and mine works OK. Then again, if you kinda replaced the PSU temporarily...things were still wonky, right?...but then you probably didn't have all the connections that are created by the three PSU connectors...plus, you have that "mod" that mod on the PSU to also consider.

    You said you replace the big cap on the PSU...yes?...so why/when did you do that...have you had previous issues with this 140?

    [EDIT]

    You also said that the PSU fuse was blowing any time the 140 got knocked about...so could it be a loose/open connection somewhere that was shorting enough to blow the fuse, and then ended up in a permanent short with the fuses now blowing upon power up?
    Last edited by miroslav; 06-17-2018 at 18:06.

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  3. #12
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    Quote Originally Posted by miroslav View Post
    OK...this is not going to clear things up for you...well, maybe a bit.
    With everything connected...I get a Short/continuity across +5V and Ground
    With CN5 disconnected...I get an Open.

    With CN5 connected and then removing one by one the three connectors on the PSU...I still get Short/Continuity for at each step....so it doesn't look like the PSU has any impact on the continuity when CN5 is connected.

    I also tried it with both CN5 and the PSU disconnected, and it was Open, which I expected, since it's tied to the CN5 being connected or not.'

    So...does that help you sort out why your 140 crapped out...mmm...probably not much, but at least it's not what you were thinking.
    Thanks, that's extremely helpful, since it implies it's normal behaviour (probably the Fluke being over-sensitive) and I can risk plugging the transport control panel back in without causing further damage. Right now the remote is running happily off USB, but because the fuse is on the PSU board (and the PSU board is sitting in the parts cupboard), the only short-circuit protection I have is whatever's built into the USB power unit. Just being cautious, really.

    Right now it's working well enough to record on, but needs a more permanent fix since it's kind of held together with string, so to speak.

    You said you replace the big cap on the PSU...yes?...so why/when did you do that...have you had previous issues with this 140?
    You also said that the PSU fuse was blowing any time the 140 got knocked about...so could it be a loose/open connection somewhere that was shorting enough to blow the fuse, and then ended up in a permanent short with the fuses now blowing upon power up?
    I replaced the electrolytics the other day to see if that helped the fuse-blowing problem at all, and also because they're several decades old. And yes, that would be my guess on the cause of the short, but I've so far been unable to track it down.

  4. #13
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    Can't help with the specifics but I have had quite a bit of experience with SMPSUs some years ago in TVs, VCRs and other kit. They ALWAYS have "afterthought" bits and mods on the back of the PCB! Most often CR 'snubbers' .

    The modern digital test meter is a wonderful device (I have a much prized Fluke 83) but they can give weird readings, especially for continuity. Worth investing a few quid/$ on a 20k/volt fully analogue meter for this eventuality. "Avo" were dogs whatsits here but Simpson meters were pretty good.

    Quite honestly, in your position I would buy a conservatively rated 'line lump' 5V SMPSU, crack it open and incorporate it in the chassis. You could look around for a 24> 5V SMPSU but the price might shock you.

    Oh! And BTW,, the triac is a 'crowbar' protection device. You would not want 24V punching through to the 5V rail!

    Dave.

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    Quote Originally Posted by ecc83 View Post
    The modern digital test meter is a wonderful device (I have a much prized Fluke 83) but they can give weird readings, especially for continuity. Worth investing a few quid/$ on a 20k/volt fully analogue meter for this eventuality. "Avo" were dogs whatsits here but Simpson meters were pretty good.
    I may do that. Might ask what Dad is using these days, I remember him always using a Simpson when I was a kid.

    Quite honestly, in your position I would buy a conservatively rated 'line lump' 5V SMPSU, crack it open and incorporate it in the chassis. You could look around for a 24> 5V SMPSU but the price might shock you.
    I did wonder about that, to be honest - what a USB charger would do if it was fed 24V DC instead of 240V AC. Funnily enough there is no information about this online.

    Oh! And BTW,, the triac is a 'crowbar' protection device. You would not want 24V punching through to the 5V rail!
    Dave.
    Ohh. That answers two questions. I've heard of them used for AC switching, but that's a new one on me. I was also wondering what I could do in terms of protecting the circuitry in case the DC-DC converter ever went rogue. Now I have something to look into.

    EDIT: That also raises the question of whether the STK772B has gone bizarre and the fuses are blowing because the crowbar has kicked in.

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    A mains rated 5V SMPSU 'lump' will not work at 24V dc. I have had them work down to about 70V AC.

    Crowbar circuits were common in PSUs years ago but I found THEY caused more trouble than they were worth due to spurious firing! Another option would be a nice efficient 60Hz 20VA toroid of about 8V and an L785S05CV linear regulator. You can go stupid with the bridge, 400V PIV+ and 15 amps, still cheap as chips and will never fail. 78 reg chips DO fail of course but extremely rarely and even there it is often due to penny-pinched heat sink design.

    Dave.

  8. #16
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    Or,
    https://www.mouser.co.uk/datasheet/2...-17w-53711.pdf

    About $50 but probably comparable to building a linear alternative?

    Dave.

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    Quote Originally Posted by jpmorris View Post
    EDIT: That also raises the question of whether the STK772B has gone bizarre and the fuses are blowing because the crowbar has kicked in.
    Yeah...I was wondering too. There are some out there to be found:
    STK772B - Google Search

    AFA replacing the PSU...do you see any problems with rest of the connections. There seems to be a bit more there going on than just a 5V feed to the 140.
    Do you see any other issues with that?

  10. #18
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    I am an old idiot! The feedback network R5/VR1. This is notorious for going high resistance/oc in such circuits (got one in 'our' HT pedals)

    If VR1 wiper loses momentary contact..WOOOPS! Crowbar fires. Replace VR1 with a cermet trimmer and set Vout on a suitable load with the triac disconnected. Once set to 5V you can decide whether to put it back or not.

    Dave.

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    Quote Originally Posted by miroslav View Post
    Yeah...I was wondering too. There are some out there to be found:
    STK772B - Google Search

    AFA replacing the PSU...do you see any problems with rest of the connections. There seems to be a bit more there going on than just a 5V feed to the 140.
    Do you see any other issues with that?
    I assumed that at first, but having studied the schematics and got the remote working with the PSU board completely removed, no. Connection-wise, it does three things: Converts 24v to 5v for the logic board, grounds the chassis, and passes 24v back out to the external power connector on the back of the remote. I'm not even sure if there are any peripherals that can use the external power connector.

    Literally everything else coming in from the D-sub port is routed around the power supply PCB with a cable loom, they go directly to the motherboard.

    Quote Originally Posted by ecc83 View Post
    I am an old idiot! The feedback network R5/VR1. This is notorious for going high resistance/oc in such circuits (got one in 'our' HT pedals)
    If VR1 wiper loses momentary contact..WOOOPS! Crowbar fires. Replace VR1 with a cermet trimmer and set Vout on a suitable load with the triac disconnected. Once set to 5V you can decide whether to put it back or not.
    Dave.
    That is a very good point. It might even explain how the unit seems to have blown fuses after being bumped while in operation. I'll have to take a look. I don't have any suitable replacements to hand, but it makes a lot of sense.

  12. #20
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    Quote Originally Posted by jpmorris View Post
    I assumed that at first, but having studied the schematics and got the remote working with the PSU board completely removed, no. Connection-wise, it does three things: Converts 24v to 5v for the logic board, grounds the chassis, and passes 24v back out to the external power connector on the back of the remote. I'm not even sure if there are any peripherals that can use the external power connector.

    Literally everything else coming in from the D-sub port is routed around the power supply PCB with a cable loom, they go directly to the motherboard.



    That is a very good point. It might even explain how the unit seems to have blown fuses after being bumped while in operation. I'll have to take a look. I don't have any suitable replacements to hand, but it makes a lot of sense.
    Just short out the trimmer. The voltage will be low but you can 'tack in' various values until you hit 5V +-say 5%. In fact the only reason a trimmer was used I guess was to save this process in production? Much better to find a fixed value. Try 3 values, do a graph!

    Dave.

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